Vaporization pressure. Dependence of saturated vapor pressure on temperature. Boiling - Hypermarket of knowledge. Calculation of steam pressure in special cases

Using the phenomenon of cooling a liquid as it evaporates; dependence of the boiling point of water on pressure.

During vaporization, a substance passes from a liquid state to a gaseous state (steam). There are two types of vaporization: evaporation and boiling.

Evaporation- This is vaporization occurring from the free surface of a liquid.

How does evaporation occur? We know that the molecules of any liquid are in continuous and random motion, some of them moving faster, others slower. They are prevented from flying out by the forces of attraction towards each other. If, however, there is a molecule with a sufficiently high kinetic energy at the surface of the liquid, then it will be able to overcome the forces of intermolecular attraction and fly out of the liquid. The same thing will be repeated with another fast molecule, with the second, third, etc. Flying out, these molecules form vapor above the liquid. The formation of this steam is evaporation.

Since the fastest molecules fly out of a liquid during evaporation, the average kinetic energy of the molecules remaining in the liquid becomes less and less. As a result the temperature of the evaporating liquid decreases: The liquid is cooled. This is why, in particular, a person in wet clothes feels colder than in dry clothes (especially in the wind).

At the same time, everyone knows that if you pour water into a glass and leave it on the table, then, despite evaporation, it will not cool continuously, becoming colder and colder until it freezes. What's stopping this? The answer is very simple: heat exchange between water and the warm air surrounding the glass.

Cooling of a liquid during evaporation is more noticeable in the case when evaporation occurs quickly enough (so that the liquid does not have time to restore its temperature due to heat exchange with environment). Volatile liquids with weak intermolecular attractive forces, such as ether, alcohol, and gasoline, evaporate quickly. If you drop such a liquid on your hand, you will feel cold. Evaporating from the surface of the hand, such a liquid will cool and take away some heat from it.

Rapidly evaporating substances are widely used in technology. For example, in space technology, descent vehicles are coated with such substances. When passing through the planet's atmosphere, the body of the apparatus heats up as a result of friction, and the substance covering it begins to evaporate. As it evaporates, it cools spacecraft, thereby saving it from overheating.

Cooling of water during its evaporation is also used in instruments used to measure air humidity - psychrometers(from the Greek “psychros” - cold). The psychrometer consists of two thermometers. One of them (dry) shows the air temperature, and the other (the reservoir of which is tied with cambric, lowered into water) shows a lower temperature, due to the intensity of evaporation from the wet cambric. The drier the air whose humidity is measured, the greater the evaporation and therefore the lower the wet-bulb reading. And vice versa, the higher the air humidity, the less intense evaporation occurs and therefore the more high temperature this thermometer shows. Based on the readings of dry and humidified thermometers, air humidity, expressed as a percentage, is determined using a special (psychrometric) table. The highest humidity is 100% (at this air humidity, dew appears on objects). For humans, the most favorable humidity is considered to be between 40 and 60%.

By using simple experiments It is easy to establish that the rate of evaporation increases with increasing temperature of the liquid, as well as with increasing area of its free surface and in the presence of wind.

Why does liquid evaporate faster when there is wind? The fact is that simultaneously with evaporation on the surface of the liquid, the reverse process also occurs - condensation. Condensation occurs due to the fact that some of the vapor molecules, moving randomly over the liquid, return to it again. The wind carries away the molecules that fly out of the liquid and does not allow them to return back.

Condensation can also occur when the vapor is not in contact with the liquid. It is condensation, for example, that explains the formation of clouds: molecules of water vapor rising above the ground in the colder layers of the atmosphere are grouped into tiny droplets of water, the accumulations of which constitute clouds. The condensation of water vapor in the atmosphere also results in rain and dew.

Dependence of boiling temperature on pressure

The boiling point of water is 100°C; one might think that this is an inherent property of water, that water, no matter where and in what conditions it is, will always boil at 100°C.

But this is not so, and residents of high mountain villages are well aware of this.

Near the top of Elbrus there is a house for tourists and a scientific station. Beginners are sometimes surprised at “how difficult it is to boil an egg in boiling water” or “why doesn’t boiling water burn.” Under these conditions, they are told that water boils at the top of Elbrus already at 82°C.

What's the matter? What physical factor interferes with the boiling phenomenon? What is the significance of altitude above sea level?

This physical factor is the pressure acting on the surface of the liquid. You don't need to climb to the top of a mountain to verify the truth of what has been said.

By placing heated water under a bell and pumping or pumping out air from there, you can make sure that the boiling point rises as the pressure increases and falls as it decreases.

Water boils at 100°C only at a certain pressure - 760 mm Hg. Art. (or 1 atm).

The boiling point versus pressure curve is shown in Fig. 4.2. At the top of Elbrus the pressure is 0.5 atm, and this pressure corresponds to a boiling point of 82°C.

Rice. 4.2

But water boiling at 10-15 mm Hg. Art., you can cool down in hot weather. At this pressure the boiling point will drop to 10-15°C.

You can even get “boiling water”, which has the temperature of freezing water. To do this, you will have to reduce the pressure to 4.6 mm Hg. Art.

An interesting picture can be observed if you place an open vessel with water under the bell and pump out the air. Pumping will cause the water to boil, but boiling requires heat. There is nowhere to take it from, and the water will have to give up its energy. The temperature of the boiling water will begin to drop, but as pumping continues, the pressure will also drop. Therefore, the boiling will not stop, the water will continue to cool and eventually freeze.

Such a boil cold water occurs not only when pumping air. For example, when a ship's propeller rotates, the pressure in a rapidly moving approx. metal surface The layer of water drops strongly and the water in this layer boils, i.e., numerous bubbles filled with steam appear in it. This phenomenon is called cavitation (from the Latin word cavitas - cavity).

By reducing the pressure, we lower the boiling point. And by increasing it? A graph like ours answers this question. A pressure of 15 atm can delay the boiling of water, it will begin only at 200°C, and a pressure of 80 atm will cause water to boil only at 300°C.

So, a certain external pressure corresponds to a certain boiling point. But this statement can be “turned around” by saying this: each boiling point of water corresponds to its own specific pressure. This pressure is called vapor pressure.

The curve depicting the boiling point as a function of pressure is also a curve of vapor pressure as a function of temperature.

The numbers plotted on a boiling point graph (or on a vapor pressure graph) show that vapor pressure changes very sharply with temperature. At 0°C (i.e. 273 K) the vapor pressure is 4.6 mmHg. Art., at 100°C (373 K) it is equal to 760 mm Hg. Art., i.e. increases 165 times. When the temperature doubles (from 0°C, i.e. 273 K, to 273°C, i.e. 546 K), the vapor pressure increases from 4.6 mm Hg. Art. almost up to 60 atm, i.e. approximately 10,000 times.

Therefore, on the contrary, the boiling point changes with pressure rather slowly. When the pressure changes twice from 0.5 atm to 1 atm, the boiling point increases from 82°C (355 K) to 100°C (373 K) and when the pressure doubles from 1 to 2 atm - from 100°C (373 K) to 120°C (393 K).

The same curve that we are now considering also controls the condensation (condensation) of steam into water.

Steam can be converted into water either by compression or cooling.

Both during boiling and during condensation, the point will not move from the curve until the conversion of steam into water or water into steam is complete. This can also be formulated this way: under the conditions of our curve and only under these conditions, the coexistence of liquid and vapor is possible. If you do not add or remove heat, then the amounts of steam and liquid in a closed vessel will remain unchanged. Such vapor and liquid are said to be in equilibrium, and vapor that is in equilibrium with its liquid is called saturated.

The boiling and condensation curve, as we see, has another meaning: it is the equilibrium curve of liquid and vapor. The equilibrium curve divides the diagram field into two parts. To the left and up (toward higher temperatures and lower pressures) is the region of stable state of steam. To the right and down is the region of the stable state of the liquid.

The vapor-liquid equilibrium curve, i.e. the curve of the dependence of boiling point on pressure or, which is the same, vapor pressure on temperature, is approximately the same for all liquids. In some cases the change may be somewhat more abrupt, in others somewhat slower, but the vapor pressure always increases rapidly with increasing temperature.

We have already used the words “gas” and “steam” many times. These two words are pretty equal. We can say: water gas is water vapor, oxygen gas is oxygen liquid vapor. Nevertheless, a certain habit has developed when using these two words. Since we are accustomed to a certain relatively small temperature range, we usually apply the word “gas” to those substances whose vapor elasticity at ordinary temperatures is higher than atmospheric pressure. On the contrary, we talk about vapor when, at room temperature and atmospheric pressure, the substance is more stable in the form of a liquid.

From the above considerations it is clear that the boiling point of a liquid must depend on the external pressure. Observations confirm this.

The greater the external pressure, the higher the boiling point. Thus, in a steam boiler at a pressure reaching 1.6 × 10 6 Pa, water does not boil even at a temperature of 200 °C. In medical institutions, water boiling in hermetically sealed vessels - autoclaves (Fig. 6.11) also occurs at elevated pressure. Therefore, the boiling point is significantly higher than 100 °C. Autoclaves are used to sterilize surgical instruments, dressings, etc.

And vice versa, by reducing external pressure, we thereby lower the boiling point. Under the bell of an air pump, you can make water boil at room temperature (Fig. 6.12). As you climb mountains, the atmospheric pressure decreases, therefore the boiling point decreases. At an altitude of 7134 m (Lenin Peak in the Pamirs) the pressure is approximately 4 · 10 4 Pa (300 mm Hg). Water boils there at about 70 °C. It is impossible to cook meat, for example, under these conditions.

Figure 6.13 shows a curve of the boiling point of water versus external pressure. It is easy to understand that this curve is also a curve expressing the dependence of saturated water vapor pressure on temperature.

Differences in boiling points of liquids

Each liquid has its own boiling point. The difference in boiling points of liquids is determined by the difference in the pressure of their saturated vapors at the same temperature. For example, ether vapors already at room temperature have a pressure greater than half atmospheric. Therefore, in order for the ether vapor pressure to become equal to atmospheric pressure, a slight increase in temperature (up to 35 ° C) is necessary. In mercury, saturated vapors have a very negligible pressure at room temperature. The pressure of mercury vapor becomes equal to atmospheric pressure only with a significant increase in temperature (up to 357 ° C). It is at this temperature, if the external pressure is 105 Pa, that mercury boils.

The difference in boiling points of substances is widely used in technology, for example, in the separation of petroleum products. When oil is heated, its most valuable, volatile parts (gasoline) evaporate first, which can thus be separated from “heavy” residues (oils, fuel oil).

A liquid boils when its saturated vapor pressure equals the pressure inside the liquid.

§ 6.6. Heat of vaporization

Is energy required to change liquid into vapor? Probably yes! Is not it?

We noted (see § 6.1) that the evaporation of a liquid is accompanied by its cooling. To maintain the temperature of the evaporating liquid unchanged, it is necessary to supply heat from outside. Of course, heat itself can be transferred to the liquid from surrounding bodies. Thus, the water in the glass evaporates, but the temperature of the water, slightly lower than the ambient temperature, remains unchanged. Heat is transferred from air to water until all the water has evaporated.

To maintain the boiling of water (or other liquid), heat must also be continuously supplied to it, for example, by heating it with a burner. In this case, the temperature of the water and the vessel does not increase, but a certain amount of steam is produced every second.

Thus, to convert a liquid into vapor by evaporation or by boiling, an input of heat is required. The amount of heat required to convert a given mass of liquid into vapor at the same temperature is called the heat of vaporization of this liquid.

What is the energy supplied to the body spent on? First of all, to increase its internal energy during the transition from a liquid to a gaseous state: after all, this increases the volume of the substance from the volume of liquid to the volume of saturated vapor. Consequently, the average distance between molecules increases, and hence their potential energy.

In addition, as the volume of a substance increases, work is done against external pressure forces. This part of the heat of vaporization at room temperature is usually several percent of the total heat of vaporization.

The heat of vaporization depends on the type of liquid, its mass and temperature. The dependence of the heat of vaporization on the type of liquid is characterized by a value called the specific heat of vaporization.

The specific heat of vaporization of a given liquid is the ratio of the heat of vaporization of a liquid to its mass:

(6.6.1)

Where r- specific heat of liquid vaporization; T- mass of liquid; Q n- its heat of vaporization. The SI unit of specific heat of vaporization is joule per kilogram (J/kg).

The specific heat of vaporization of water is very high: 2.256·10 6 J/kg at a temperature of 100 °C. For other liquids (alcohol, ether, mercury, kerosene, etc.) the specific heat of vaporization is 3-10 times less.

To regulate the evaporation pressure, use the KVP regulator, installed on the suction line downstream from the evaporator (Fig. 6.13).

In addition to its main function, the evaporation pressure regulator provides protection in the event of a strong drop in evaporation pressure in order to avoid freezing of the cooled water in the heat exchange path of the evaporators of water cooling units.

The regulator works as follows: when the pressure increases above the set pressure, the regulator opens, and when the pressure drops below the set value, it closes. The control signal is only the pressure at the inlet to the regulator.

In installations that include several evaporators and operate at different evaporation pressures, the regulator is installed behind the evaporator, the pressure in which is the highest. To avoid condensation of the refrigerant during shutdowns, a check valve is installed on the suction line immediately after the evaporator with minimal pressure. In installations with parallel evaporators and a common compressor, the regulator is installed on the suction line to maintain the same pressure in the evaporators.

In addition to this type of regulator, the evaporation pressure is stabilized using electronic control systems for one or more refrigeration chambers, cabinets, etc., providing high accuracy of maintaining the set temperature (±0.5 K) in a wide range of cooling capacity - from 10 to 100% of the nominal value.

8. Performance regulators.

Capacity regulators (Fig. 6.14) help adapt the compressor refrigeration capacity to changes in the thermal load on the evaporator in installations with a very low refrigerant charge. They avoid low suction pressure and unnecessary starts.

As the heat load on the evaporator decreases, the suction pressure drops, causing a vacuum in the circuit, which leads to the risk of moisture entering the installation. When the suction pressure drops below the set value, the regulator opens, causing a certain volume of hot gases from the discharge pipe to pass into the suction pipe. As a result, the suction pressure increases and the cooling capacity decreases. The regulator reacts only to the pressure in the suction line, i.e. at the exit from it.

9. Starting regulators.

Starting regulators allow you to avoid operating and starting the compressor when the suction pressure is too high, which occurs after a long stop of the machine or after defrosting the evaporator.

The KVL starting regulator is a type of throttle pressure regulators “after itself”. It maintains constant pressure in the suction line between the regulator and the compressor and unloads the compressor when starting.

The pressure at the inlet of the regulator acts on the bellows from below and on the valve plate from above. Since the effective area of the bellows is equivalent to the flow area, the inlet pressure is neutralized. The valve plate is acted upon from below by outlet pressure (in the crankcase), counteracting the tension force of the adjustable spring. These two forces are the acting forces of the regulator. When the regulated outlet (crankcase) pressure decreases, the valve opens, allowing refrigerant vapor to enter the compressor. For refrigeration units high performance, parallel installation of KVL starting regulators is possible. In this case, regulators are selected based on the condition of the same pressure drop in each pipeline and equivalent performance.

The regulator is adjusted to maximum values, without, however, exceeding the values recommended by the manufacturer for the compressor or compressor-condensing unit. The adjustment is carried out according to the readings of the pressure gauge on the suction line of the compressor.

The start regulator is installed on the suction line between the evaporator and the compressor (Fig. 6.15).

This regulator provides the possibility of connecting a vapor sampling line through a manometric tap on the inlet pipe having a flow diameter of 1/4". With this method of regulation, vapor sampling is carried out "after itself".

The choice of starting regulator is determined by five main indicators:

Type of refrigerant

System performance,

Design suction pressure,

Maximum design pressure,

Pressure drop in the regulator.

The difference between the design and maximum design suction pressure determines the duration of the valve opening. Regulator pressure drop is an important factor because pressure loss in the suction line affects machine performance. Therefore, the pressure drop across the regulator must be kept to a minimum. Typically in low temperature refrigeration systems the pressure drop is 3... 7 kPa. The maximum pressure drop for most refrigeration systems is 14 kPa.

When the valve is opened to its maximum, the regulator, on the one hand, provides maximum performance, and on the other hand, causes large pressure losses, which reduces the performance of the system. Therefore, the pressure drop across the regulator must be kept to a minimum.

The process of intense evaporation of a liquid begins at a temperature when the vapor pressure of the liquid exceeds the external pressure of the gas atmosphere above the liquid. At the boiling point, steam formation occurs throughout the entire mass of the liquid and flows at almost constant temperature until complete transition of liquid (single-component) and steam. By artificially lowering the pressure, you can make a liquid boil at more low temperatures ah, which is widely used in technology, since it is easier to find suitable material for equipment for working at low temperatures. Modern vacuum technology has at its disposal powerful rotary pumps capable of creating a vacuum at which the residual pressure does not exceed 0.001 mm Hg, and jet diffusion pumps that create a vacuum up to 10v-7-10v-8 mmHg. Art.
Vacuum distillation is used to obtain high-purity metals; Zn, Cd, Mg, Ca, etc. Usually operate at pressures slightly higher than the vapor pressure of the distilled metal at its melting point. Then, by distilling the liquid metal, a solid condensate is obtained, which makes it possible to use very simple design distillation device shown in Fig. 24. The device is a cylinder, in the lower part of which there is a vessel with liquid distillable metal. The vapors are condensed in the upper part of the cylinder on a special composite metal cylinder (condenser) in the form of a crystalline crust, which is removed along with the condenser after the process is completed. Before heating the metal, air is first pumped out of the device using a vacuum pump, and then from time to time the vacuum is restored, which changes due to leakage of air from outside through leaks in the equipment. If the device is sufficiently sealed, then during the distillation process, since non-condensable gases are not released, Full time job no vacuum pump needed.

The described device is extremely simple; it is made of steel or heat-resistant metal alloys. What is especially important is that its lid and all sealing parts are cooled with water, i.e. they operate at room temperature, allowing the use of very advanced seals - rubber, vacuum putties, etc. The use of vacuum allows cleaning by distillation at relatively low temperatures (700 -900°) such chemically active and very aggressive metals as calcium, magnesium, barium, the distillation of which at atmospheric pressure is not feasible due to the impossibility of selecting the material for the equipment.
Let us consider the features of the evaporation process in a vacuum.
The liquid-vapor phase diagram with a decrease in pressure has the same character as the diagrams for atmospheric pressure, only the liquid and vapor lines move to the region of lower temperatures. It follows that the efficiency of separation of components during evaporation of their solution in a vacuum is approximately the same as at atmospheric pressure, but occurs at lower temperatures; The temperature is lower, the deeper the vacuum applied. A special feature of working in a vacuum is the absence of entrainment of small drops of liquid along with vapor, which is always observed when working under atmospheric pressure. When a liquid boils violently, the bursting bubbles of steam rising from the depths of the liquid produce splashes, which are carried away by the vapor into the condenser and contaminate the distillate. In a vacuum (sufficiently deep), the formation of splashes does not occur, since the boiling process is fundamentally different from boiling at atmospheric pressure. In a vacuum, steam formation occurs only on the surface of the liquid; bubbles do not form inside the liquid; the surface is calm and does not boil; therefore, splashes cannot occur. Therefore, vacuum distillation produces a purer distillate than distillation at atmospheric pressure.
Let us use an example to show the peculiarity of the boiling process in a vacuum. Let in one case water in a vessel with a layer depth of 250 mm boil at atmospheric pressure (760 mm Hg). Then the steam released from the surface of the water, in order to overcome the external pressure, must have atmospheric pressure (760 mm Hg), which develops at a water surface temperature of 100 °. The steam bubble formed at the bottom of the vessel must have greater pressure, since, in addition to atmospheric pressure, it needs to overcome hydrostatic pressure a water column with a height of 250 mm, which corresponds to an excess pressure of 18 mm Hg. Art. Thus, the steam released from the bottom of the vessel should have a pressure of 760 + 18 = 778 mm Hg. Art., which corresponds to the temperature of the water at the bottom of the vessel being 100.6°. Such a slight overheating of water at the bottom (0.6°) is quite real, and the boiling process proceeds in such a way that steam is formed throughout the entire mass of the layer. The water boils vigorously and produces splashes when the bubbles on the surface are broken.
Now consider the boiling of the same layer of water in a vacuum of 4.58 mmHg. Art. For boiling, the surface layer of water must have a temperature of 0°, at which the saturated vapor pressure is 4.58 mm Hg. Art. The bubble formed at the bottom must overcome the hydrostatic pressure of the water column of 250 mm, which corresponds to a pressure of 18 mmHg. Art., and have a total pressure of 4.58 + 18 = 22.58 mm Hg. Art. Water will have this saturated vapor pressure at a temperature of ~ 23°, i.e., in order for a steam bubble to form at the bottom of the vessel, it is necessary to have a temperature of 23° at the bottom. It is impossible to obtain such a difference between the temperatures at the bottom and at the surface, since convection currents will prevent this. Consequently, bubbles will not form deep in the liquid layer and vaporization will occur only from the surface of the liquid.
Metal melts have high thermal conductivity, which prevents local overheating of the liquid and, consequently, boiling with the formation of bubbles.
Until the pressure in the device becomes very low, molecules are exchanged between the surface of the liquid and the vapor and a mobile liquid-vapor equilibrium is established. An ordinary gas stream of steam flows to the condenser and the results of the distillation process are determined by the liquid-vapor phase diagram.
If the pressure in the device is so low that the mean free path of the molecules becomes more sizes device, the nature of the distillation process changes radically.
Under these conditions, there is no exchange of molecules between vapor and liquid, the liquid-vapor mobile equilibrium is not established, and the liquid-vapor phase diagram does not describe the evaporation process. A regular gas line between the evaporator and condenser. Not formed, the vapor molecules separated from the surface of the liquid follow a straight path, without colliding with other molecules, fall on the cold surface of the condenser and remain there - they condense; the evaporation process is completely irreversible and has the character of molecular evaporation. The result of distillation is determined by the rate of evaporation, which depends on the type of substance being evaporated and temperature and is independent of the external pressure in the system if this pressure is low enough. The evaporation rate under these conditions can be calculated using the Langmuir formula:

Taking the evaporation rate as the mass of the substance evaporating per second per unit surface, expressing the vapor pressure p in millimeters of mercury and replacing the values of R and π with their numerical values, we obtain equation (III, 13) in a different form, convenient for practical calculations:

During molecular evaporation, substances with the same vapor pressure can be separated if their molecular weights are different, as proven by isotope separation experiments.

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States of matter

Iron vapor and solid air

Isn't it a strange combination of words? However, this is not nonsense at all: both iron vapor and solid air exist in nature, but not under ordinary conditions.

What conditions are we talking about? The state of a substance is determined by two factors: temperature and pressure.

Our life takes place in relatively little changing conditions. Air pressure fluctuates within a few percent around one atmosphere; the air temperature, say, in the Moscow region ranges from -30 to +30°C; in the absolute temperature scale, in which the lowest possible temperature (-273°C) is taken as zero; this interval will look less impressive: 240-300 K, which is also only ±10% of the average value.

It is quite natural that we are accustomed to these ordinary conditions and therefore, speaking simple truths like: “iron is a solid, air is a gas,” etc., we forget to add: “under normal conditions.”

If you heat iron, it will first melt and then evaporate. If air is cooled, it will first turn into liquid and then solidify.

Even if the reader has never encountered iron vapor or solid air, he will probably easily believe that any substance, by changing the temperature, can be obtained in solid, liquid, and gaseous states, or, as they also say, in solid , liquid or gaseous phases.

It is easy to believe in this because everyone observed one substance, without which life on Earth would be impossible, both in the form of a gas, and as a liquid, and in the form of a solid. We are, of course, talking about water.

Under what conditions do transformations of matter from one state to another occur?

Boiling

If we lower the thermometer into the water that is poured into the kettle, turn on the electric stove and monitor the mercury of the thermometer, we will see the following: almost immediately the mercury level will creep up. Now it’s 90, 95, and finally 100°C. The water boils, and at the same time the rise of mercury stops. The water has been boiling for many minutes, but the mercury level has not changed. Until all the water boils away, the temperature will not change (Fig. 4.1).

Rice. 4.1

Where does the heat go if the water temperature does not change? The answer is obvious. The process of turning water into steam requires energy.

Let's compare the energy of a gram of water and a gram of steam formed from it. Steam molecules are located further apart from each other than water molecules. It is clear that because of this, the potential energy of water will differ from the potential energy of steam.

The potential energy of attracting particles decreases as they approach each other. Therefore the steam energy more energy water, and turning water into steam requires energy. This excess energy is transferred by the electric stove to the boiling water in the kettle.

Energy required to convert water into steam; called the heat of vaporization. To convert 1 g of water into steam, 539 cal are required (this is the figure for a temperature of 100 ° C).

If 539 cal is consumed per 1 g, then 18*539 = 9700 cal will be consumed per 1 mole of water. This amount of heat must be spent on breaking intermolecular bonds.

You can compare this figure with the amount of work required to break intramolecular bonds. In order to split 1 mole of water vapor into atoms, it takes about 220,000 cal, i.e. 25 times more energy. This directly proves the weakness of the forces that bind molecules together, compared to the forces that pull atoms together into a molecule.

Dependence of boiling temperature on pressure

The boiling point of water is 100°C; one might think that this is an inherent property of water, that water, no matter where and in what conditions it is, will always boil at 100°C.

But this is not so, and residents of high mountain villages are well aware of this.

What's the matter? What physical factor interferes with the boiling phenomenon? What is the significance of altitude above sea level?

This physical factor is the pressure acting on the surface of the liquid. You don't need to climb to the top of a mountain to verify the truth of what has been said.

By placing heated water under a bell and pumping or pumping out air from there, you can make sure that the boiling point rises as the pressure increases and falls as it decreases.

Water boils at 100°C only at a certain pressure - 760 mm Hg. Art. (or 1 atm).

The boiling point versus pressure curve is shown in Fig. 4.2. At the top of Elbrus the pressure is 0.5 atm, and this pressure corresponds to a boiling point of 82°C.

Rice. 4.2

But water boiling at 10-15 mm Hg. Art., you can cool down in hot weather. At this pressure the boiling point will drop to 10-15°C.

You can even get “boiling water”, which has the temperature of freezing water. To do this, you will have to reduce the pressure to 4.6 mm Hg. Art.

This boiling of cold water occurs not only when air is pumped out. For example, when a ship's propeller rotates, the pressure in a rapidly moving layer of water near a metal surface drops greatly and the water in this layer boils, that is, numerous steam-filled bubbles appear in it. This phenomenon is called cavitation (from the Latin word cavitas - cavity).

The curve depicting the boiling point as a function of pressure is also a curve of vapor pressure as a function of temperature.

The same curve that we are now considering also controls the condensation (condensation) of steam into water.

Steam can be converted into water either by compression or cooling.

Evaporation

Boiling is a fast process, and in a short time no trace remains of boiling water; it turns into steam.

But there is another phenomenon of turning water or other liquid into steam - this is evaporation. Evaporation occurs at any temperature, regardless of pressure, which under normal conditions is always close to 760 mm Hg. Art. Evaporation, unlike boiling, is a very slow process. A bottle of cologne that we forgot to close will be empty in a few days; o the saucer with water will stand longer, but sooner or later it will turn out to be dry.

Air plays a major role in the evaporation process. By itself, it does not prevent water from evaporating. As soon as we open the surface of the liquid, water molecules will begin to move into the nearest layer of air.

The vapor density in this layer will increase rapidly; After a short period of time, the vapor pressure will become equal to the elasticity characteristic of the temperature of the medium. In this case, the vapor pressure will be exactly the same as in the absence of air.

The transition of steam into air does not mean, of course, an increase in pressure. The total pressure in the space above the water surface does not increase, only the share of this pressure that is taken over by steam increases, and accordingly the share of air that is displaced by steam decreases.

Above the water there is steam mixed with air; above there are layers of air without steam. They will inevitably mix. Water vapor will continuously move to higher layers, and in its place, air that does not contain water molecules will enter the lower layer. Therefore, in the layer closest to the water, places will always be freed up for new water molecules. The water will continuously evaporate, maintaining the water vapor pressure at the surface equal to elasticity, and the process will continue until the water has completely evaporated.

We started with the example of cologne and water. It is well known that they evaporate at different rates. Ether evaporates extremely quickly, alcohol evaporates quite quickly, and water much more slowly. We will immediately understand what is going on here if we find in the reference book the values of the vapor pressure of these liquids, say, at room temperature. Here are the numbers: ether - 437 mm Hg. Art., alcohol - 44.5 mm Hg. Art. and water - 17.5 mm Hg. Art.

The greater the elasticity, the more vapor in the adjacent layer of air and the faster the liquid evaporates. We know that vapor pressure increases with increasing temperature. It is clear why the rate of evaporation increases with heating.

The rate of evaporation can be influenced in another way. If we want to help evaporation, we need to quickly remove the vapor from the liquid, that is, speed up the mixing of the air. That is why evaporation is greatly accelerated by blowing liquid. Water, although it has a relatively low vapor pressure, will disappear quite quickly if the saucer is placed in the wind.

It is understandable, therefore, why a swimmer who comes out of the water feels cold in the wind. The wind accelerates the mixing of air with steam and, therefore, accelerates evaporation, and the human body is forced to give up heat for evaporation.

A person’s well-being depends on whether there is a lot or little water vapor in the air. And dry and wet air unpleasant. Humidity is considered normal when it is 60%. This means that the density of water vapor is 60% of the density of saturated water vapor at the same temperature.

If moist air is cooled, eventually the water vapor pressure in it will equal the vapor pressure at that temperature. The steam will become saturated and will begin to condense into water as the temperature drops further. The morning dew that moistens the grass and leaves appears precisely due to this phenomenon.

At 20°C, the density of saturated water vapor is about 0.00002 g/cm 3 . We will feel good if there is 60% of this number of water vapor in the air - that means only a little more than one hundred thousandth of a gram per 1 cm 3.

Although this figure is small, it will lead to impressive amounts of steam for the room. It is not difficult to calculate that in a medium-sized room with an area of 12 m2 and a height of 3 m, about a kilogram of water can “fit” in the form of saturated steam.

This means that if such a room is tightly closed and an open barrel of water is placed, a liter of water will evaporate, no matter what the capacity of the barrel is.

It is interesting to compare this result for water with the corresponding figures for mercury. At the same temperature of 20°C, the density of saturated mercury vapor is 10 -8 g/cm 3 .

In the room just discussed, no more than 1 g of mercury vapor will fit.

By the way, mercury vapor is very poisonous, and 1 g of mercury vapor can seriously harm the health of any person. When working with mercury, you must ensure that even the smallest drop of mercury does not spill.

Critical temperature

How to turn gas into liquid? The boiling point chart answers this question. You can turn a gas into a liquid by either decreasing the temperature or increasing the pressure.

In the 19th century, increasing pressure seemed an easier task than lowering temperature. At the beginning of this century, the great English physicist Michael Farada managed to compress gases to vapor pressure values and in this way turn many gases (chlorine, carbon dioxide, etc.) into liquid.

However, some gases - hydrogen, nitrogen, oxygen - could not be liquefied. No matter how much pressure was increased, they did not turn into liquid. One might think that oxygen and other gases cannot be liquid. They were classified as true, or permanent, gases.

In fact, the failures were caused by a lack of understanding of one important circumstance.

Let us consider liquid and vapor in equilibrium and think about what happens to them as the boiling point increases and, of course, the corresponding increase in pressure. In other words, imagine that a point on the boiling graph moves upward along the curve. It is clear that as the temperature increases, a liquid expands and its density decreases. As for steam, does the boiling point increase? Of course, it contributes to its expansion, but, as we have already said, the saturated vapor pressure increases much faster than the boiling point. Therefore, the vapor density does not fall, but, on the contrary, quickly increases with increasing boiling temperature.

Since the density of the liquid decreases and the density of the vapor increases, then, moving “up” along the boiling curve, we will inevitably reach a point at which the densities of the liquid and vapor are equal (Fig. 4.3).

Rice. 4.3

At this remarkable point, called the critical point, the boiling curve ends. Since all the differences between gas and liquid are associated with the difference in density, at the critical point the properties of the liquid and gas become the same. Each substance has its own critical temperature and its own critical pressure. Thus, for water, the critical point corresponds to a temperature of 374 ° C and a pressure of 218.5 atm.

If you compress a gas whose temperature is below the critical temperature, then the process of its compression will be represented by an arrow crossing the boiling curve (Fig. 4.4). This means that at the moment of reaching a pressure equal to the vapor pressure (the point where the arrow intersects the boiling curve), the gas will begin to condense into a liquid. If our vessel were transparent, then at this moment we would see the beginning of the formation of a layer of liquid at the bottom of the vessel. At constant pressure, the layer of liquid will grow until finally all the gas turns into liquid. Further compression will require an increase in pressure.

Rice. 4.4

The situation is completely different when compressing a gas whose temperature is above the critical temperature. The compression process can again be depicted as an arrow going from bottom to top. But now this arrow does not cross the boiling curve. This means that when compressed, the steam will not condense, but will only be continuously compacted.

At temperatures above the critical temperature, the existence of liquid and gas separated by an interface is impossible: When compressed to any density, there will be a homogeneous substance under the piston, and it is difficult to say when it can be called a gas and when a liquid.

The presence of a critical point shows that there is no fundamental difference between the liquid and gaseous states. At first glance, it might seem that there is no such fundamental difference only when we are talking about temperatures above critical. This, however, is not the case. The existence of a critical point indicates the possibility of turning a liquid - a real liquid that can be poured into a glass - into a gaseous state without any semblance of boiling.

This transformation path is shown in Fig. 4.4. A cross marks a known liquid. If you lower the pressure a little (down arrow), it will boil, and it will also boil if you raise the temperature a little (arrow to the right). But we will do something completely different. We will compress the liquid very strongly, to a pressure above critical. The point representing the state of the liquid will go vertically upward. Then we heat the liquid - this process is depicted by a horizontal line. Now, after we find ourselves to the right of the Critical Temperature, we lower the pressure to the original one. If you now reduce the temperature, you can get real steam, which could be obtained from this liquid in a simpler and shorter way.

Thus, it is always possible, by changing pressure and temperature bypassing the critical point, to obtain steam by continuously transferring it from liquid or liquid from steam. This continuous transition does not require boiling or condensation.

Early attempts to liquefy gases such as oxygen, nitrogen, and hydrogen were unsuccessful because the existence of a critical temperature was not known. These gases have very low critical temperatures: nitrogen -147°C, oxygen -119°C, hydrogen -240°C, or 33 K. The record holder is helium, its critical temperature is 4.3 K. Convert these gases into liquid can only be used in one way - you need to reduce their temperature below the specified one.

Receiving low temperatures

Significant temperature reduction can be achieved different ways. But the idea of all methods is the same: we must force the body that we want to cool to expend its internal energy.

How to do this? One way is to make the liquid boil without adding heat from outside. To do this, as we know, we need to reduce the pressure - reduce it to the value of vapor pressure. The heat spent on boiling will be borrowed from the liquid and the temperature of the liquid and steam, and with it the vapor pressure will drop. Therefore, in order for the boiling to not stop and to happen faster, air must be continuously pumped out of the vessel with the liquid.

However, the temperature drop during this process reaches a limit: the elasticity of the vapor eventually becomes completely insignificant, and even the most powerful pumps cannot create the required pressure.

In order to continue lowering the temperature, it is possible, by cooling the gas with the resulting liquid, to turn it into a liquid with a lower boiling point.

Now the pumping process can be repeated with the second substance and thus obtain lower temperatures. If necessary, this “cascade” method of obtaining low temperatures can be extended.

This is exactly what they did at the end of the last century; Liquefaction of gases was carried out in stages: ethylene, oxygen, nitrogen, hydrogen - substances with boiling points of -103, -183, -196 and -253°C - were sequentially converted into liquid. With liquid hydrogen, you can get the lowest boiling liquid - helium (-269°C). The neighbor on the left helped to get the neighbor on the right.

The cascade cooling method is almost a hundred years old. In 1877, liquid air was obtained by this method.

In 1884-1885 Liquid hydrogen was produced for the first time. Finally, another twenty years later, the last fortress was taken: in 1908, Kamerlingh Onnes in the city of Leiden in Holland turned helium into liquid - a substance with the lowest critical temperature. The 70th anniversary of this important scientific achievement was recently celebrated.

For many years, the Leiden Laboratory was the only "low-temperature" laboratory. Now, in all countries, there are dozens of such laboratories, not to mention factories producing liquid air, nitrogen, oxygen and helium for technical purposes.

The cascade method of obtaining low temperatures is now rarely used. In technical installations to lower the temperature, another method is used to reduce the internal energy of the gas: they force the gas to quickly expand and produce work using internal energy.

If, for example, air compressed to several atmospheres is put into an expander, then when the work of moving the piston or rotating the turbine is performed, the air will cool so sharply that it will turn into liquid. Carbon dioxide, if quickly released from a cylinder, cools so sharply that it turns into “ice” on the fly.

Liquid gases are widely used in technology. Liquid oxygen is used in explosive technology, as a component of the fuel mixture in jet engines.

Air liquefaction is used in technology to separate the gases that make up air.

In various fields of technology it is required to work at liquid air temperature. But for many physical studies this temperature is not low enough. Indeed, if we convert degrees Celsius to an absolute scale, we will see that the temperature of liquid air is approximately 1/3 of room temperature. Much more interesting for physics are “hydrogen” temperatures, i.e. temperatures of the order of 14-20 K, and especially “helium” temperatures. The lowest temperature obtained when pumping liquid helium is 0.7 K.

Physicists have managed to get much closer to absolute zero. Temperatures have now been obtained that exceed absolute zero by only a few thousandths of a degree. However, these ultra-low temperatures are obtained in ways that are not similar to those we described above.

IN last years low-temperature physics has given rise to a special branch of industry engaged in the production of equipment that allows large volumes to be maintained at temperatures close to absolute zero; power cables have been developed whose conductive busbars operate at temperatures below 10 K.

Supercooled steam and superheated liquid

When the vapor passes its boiling point, it must condense and turn into a liquid. However,; It turns out that if the steam does not come into contact with the liquid and if the steam is very pure, then it is possible to obtain supercooled or “supersaturated steam” - steam that should have long ago become a liquid.

Supersaturated steam is very unstable. Sometimes a push or a grain of steam thrown into space is enough for the delayed condensation to begin.

Experience shows that the condensation of steam molecules is greatly facilitated by the introduction of small foreign particles into the steam. In dusty air, supersaturation of water vapor does not occur. Condensation may be caused by clouds of smoke. After all, smoke consists of small solid particles. Once in the steam, these particles collect molecules around them and become condensation centers.

So, although unstable, steam can exist in a temperature range suitable for the “life” of a liquid.

Can a liquid “live” in the vapor region under the same conditions? In other words, is it possible to overheat a liquid?

It turns out that it is possible. To do this, you need to ensure that the liquid molecules do not come off the surface. A radical remedy is to eliminate the free surface, that is, place the liquid in a vessel where it would be compressed on all sides by solid walls. In this way, it is possible to achieve overheating of the order of several degrees, i.e., move the point representing the state of liquids to the right of the boiling curve (Fig. 4.4).

Overheating is a shift of liquid into the vapor region, so overheating of the liquid can be achieved both by adding heat and reducing pressure.

The last method can achieve amazing results. Water or other liquid, carefully freed from dissolved gases (this is not easy to do), is placed in a vessel with a piston reaching the surface of the liquid. The vessel and piston must be wetted with liquid. If you now pull the piston towards you, the water adhered to the bottom of the piston will follow it. But the layer of water clinging to the piston will pull the next layer of water with it, this layer will pull the underlying one, as a result the liquid will stretch.

In the end, the column of water will break (it is the column of water, not the water, that will break away from the piston), but this will happen when the force per unit area reaches tens of kilograms. In other words, a negative pressure of tens of atmospheres is created in the liquid.

Even at low positive pressures the vapor state of the substance is stable. And the liquid can be brought to negative pressure. You can't think of a more striking example of "overheating".

Melting

There is no solid body that can withstand an increase in temperature as much as possible. Sooner or later the solid piece turns into liquid; right, in some cases we will not be able to reach the melting point - chemical decomposition may occur.

As the temperature increases, the molecules move more and more intensely. Finally, a moment comes when maintaining order among the strongly “swinging” molecules becomes impossible. The solid melts. Tungsten has the highest melting point: 3380°C. Gold melts at 1063°C, iron - at 1539°C. However, There are also low-melting metals. Mercury, as is well known, melts at a temperature of -39 ° C. Organic substances do not have high melting points. Naphthalene melts at 80 ° C, toluene - at -94.5 ° C.

Measure the melting point of a body, especially if it melts in the temperature range that is measured an ordinary thermometer, not at all difficult. It is not at all necessary to follow the melting body with your eyes. Just look at the mercury column of the thermometer. Until melting begins, body temperature rises (Fig. 4.5). Once melting begins, the temperature increase stops and the temperature will remain the same until the melting process is complete.

Rice. 4.5

Like turning a liquid into vapor, turning a solid into a liquid requires heat. The heat required for this is called the latent heat of fusion. For example, melting one kilogram of ice requires 80 kcal.

Ice is one of the bodies with a high heat of fusion. Melting ice requires, for example, 10 times more energy than melting the same mass of lead. Of course, we are talking about the melting itself; we are not saying here that before the lead begins to melt, it must be heated to +327°C. Due to the high heat of melting of ice, the melting of snow slows down. Imagine that the heat of melting would be 10 times less. Then spring floods would lead to unimaginable disasters every year.

So, the heat of melting of ice is large, but it is also small if compared with specific heat vaporization at 540 kcal/kg (seven times less). However, this difference is completely natural. When converting a liquid into vapor, we must separate molecules from one another, but when melting, we only have to destroy the order in the arrangement of the molecules, leaving them at almost the same distances. Clearly, the second case requires less work.

The presence of a certain melting point is an important feature of crystalline substances. It is by this feature that they can be easily distinguished from other solids called amorphous or glasses. Glasses are found among both inorganic and organic substances. Window glass usually made from sodium and calcium silicates; on desk Organic glass is often used (also called plexiglass).

Amorphous substances, unlike crystals, do not have a specific melting point. The glass does not melt, but softens. When heated, a piece of glass first becomes soft from hard, it can easily be bent or stretched; at a higher temperature, the piece begins to change its shape under the influence of its own gravity. As it heats up, the thick viscous mass of glass takes the shape of the vessel in which it lies. This mass is first thick, like honey, then like sour cream, and finally becomes almost the same low-viscosity liquid as water. Even if we wanted to, we cannot indicate here a specific temperature for the transition of a solid into a liquid. The reasons for this lie in the fundamental difference between the structure of glass and the structure crystalline bodies. As mentioned above, atoms in amorphous bodies arranged randomly. Glasses are similar in structure to liquids. Already in solid glass, the molecules are arranged randomly. This means that increasing the temperature of the glass only increases the range of vibrations of its molecules, giving them gradually greater and greater freedom of movement. Therefore, the glass softens gradually and does not exhibit a sharp transition from “solid” to “liquid”, characteristic of the transition from the arrangement of molecules in a strict order to a disorderly arrangement.

When we talked about the boiling curve, we said that liquid and steam can, although in an unstable state, live in foreign areas - steam can be supercooled and transferred to the left of the boiling curve, liquid can be overheated and pulled to the right of this curve.

Are similar phenomena possible in the case of a crystal with a liquid? It turns out that the analogy here is incomplete.

If you heat a crystal, it will begin to melt at its melting point. It will not be possible to overheat the crystal. On the contrary, when cooling a liquid, it is possible, if certain measures are taken, to “overshoot” the melting point relatively easily. In some liquids it is possible to achieve great hypothermia. There are even liquids that are easy to supercool, but difficult to make crystallize. As such a liquid cools, it becomes increasingly viscous and finally solidifies without crystallizing. That's what glass is like.

You can also supercool the water. Fog droplets may not freeze even in severe frosts. If you drop a crystal of a substance - a seed - into a supercooled liquid, crystallization will immediately begin.

Finally, in many cases delayed crystallization can begin from shaking or other random events. It is known, for example, that crystalline glycerol was first obtained during transportation by railway. After standing for a long time, glass may begin to crystallize (devitify, or “collapse,” as they say in technology).

How to grow a crystal

Almost any substance can give crystals under certain conditions. Crystals can be obtained from a solution or from a melt of a given substance, as well as from its vapor (for example, black diamond-shaped crystals of iodine easily fall out of its vapor when normal pressure without intermediate transition to the liquid state).

Start dissolving table salt or sugar in water. At room temperature (20°C) you can dissolve only 70 g of salt in a faceted glass. Further additions of salt will not dissolve and will settle at the bottom in the form of sediment. A solution in which further dissolution no longer occurs is called saturated. .If you change the temperature, the degree of solubility of the substance will also change. Everyone knows that hot water dissolves most substances much more easily than cold water.

Imagine now that you have prepared a saturated solution of, say, sugar at a temperature of 30°C and begin to cool it to 20°C. At 30°C you were able to dissolve 223 g of sugar in 100 g of water, at 20°C 205 g dissolved. Then, when cooled from 30 to 20°C, 18 g will turn out to be “extra” and, as they say, will fall out of solution. So, one possible way to obtain crystals is to cool a saturated solution.

You can do it differently. Prepare a saturated salt solution and leave it in an open glass. After some time, you will notice the appearance of crystals. Why were they formed? Careful observation will show that simultaneously with the formation of crystals, another change occurred - the amount of water decreased. The water evaporated, and there was an “extra” substance in the solution. So, another possible way for crystals to form is by evaporation of the solution.

How does the formation of crystals from solution occur?

We said that crystals "fall out" of solution; Should this be understood to mean that the crystal was not there for a week, and in one instant it suddenly appeared? No, that's not the case: the crystals grow. It is, of course, impossible to detect with the eye the very initial moments of growth. At first, few of the randomly moving molecules or atoms of the solute are assembled in approximately the order needed to form crystal lattice. Such a group of atoms or molecules is called a nucleus.

Experience shows that nuclei are more often formed in the presence of any extraneous tiny dust particles in the solution. Crystallization begins most quickly and easily when a small seed crystal is placed in a saturated solution. In this case, the release of a solid substance from the solution will not consist in the formation of new crystals, but in the growth of the seed.

The growth of the embryo is, of course, no different from the growth of the seed. The point of using a seed is that it “pulls” the released substance onto itself and thus prevents the simultaneous formation of a large number of nuclei. If a lot of nuclei are formed, then they will interfere with each other during growth and will not allow us to obtain large crystals.

How are portions of atoms or molecules released from the solution distributed on the surface of the embryo?

Experience shows that the growth of an embryo or seed consists, as it were, of moving the faces parallel to themselves in a direction perpendicular to the face. In this case, the angles between the faces remain constant (we already know that the constancy of angles is the most important feature of a crystal, resulting from its lattice structure).

In Fig. Figure 4.6 shows the occurring outlines of three crystals of the same substance during their growth. Similar pictures can be observed under a microscope. In the case shown on the left, the number of faces is maintained during growth. The middle picture gives an example of a new face appearing (top right) and disappearing again.

Rice. 4.6

It is very important to note that the growth rate of the faces, i.e. the speed of their movement parallel to themselves, is not the same for different faces. In this case, it is those edges that “overgrow” (disappear) that move the fastest, for example, the lower left edge in the middle picture. On the contrary, slowly growing edges turn out to be the widest and, as they say, the most developed.

This is especially clearly visible in the last figure. A shapeless fragment acquires the same shape as other crystals precisely because of the anisotropy of the growth rate. Certain facets develop most strongly at the expense of others and give the crystal a shape characteristic of all samples of this substance.

Very beautiful transitional forms are observed when a ball is taken as a seed, and the solution is alternately slightly cooled and heated. When heated, the solution becomes unsaturated and the seed is partially dissolved. Cooling leads to saturation of the solution and growth of the seed. But the molecules settle differently, as if giving preference to certain places. The substance is thus transferred from one place of the ball to another.

First, small edges in the shape of circles appear on the surface of the ball. The circles gradually increase in size and, touching each other, merge along straight edges. The ball turns into a polyhedron. Then some faces overtake others, some of the faces become overgrown, and the crystal acquires its characteristic shape (Fig. 4.7).

Rice. 4.7

When observing the growth of crystals, one is struck by the main feature of growth - the parallel movement of the faces. It turns out that the released substance builds up the edge in layers: until one layer is completed, the next one does not begin to be built.

In Fig. Figure 4.8 shows the “unfinished” packing of atoms. In which of the lettered positions will the new atom be most firmly held when attached to the crystal? Without a doubt, in A, since here he experiences the attraction of neighbors from three sides, while in B - from two, and in C - only from one side. Therefore, first the column is completed, then the entire plane, and only then the laying of the new plane begins.

Rice. 4.8

In a number of cases, crystals are formed from a molten mass - from a melt. In nature, this happens on a huge scale: basalts, granites and many other rocks arose from fiery magma.

Let's start heating some crystalline substance, such as rock salt. Up to 804°C, the rock salt crystals will change little: they expand only slightly, and the substance remains solid. A temperature meter placed in a vessel with a substance shows a continuous increase in temperature when heated. At 804°C we will immediately discover two new, interconnected phenomena: the substance will begin to melt, and the rise in temperature will stop. Until all the substance turns into liquid; the temperature will not change; a further rise in temperature means heating of the liquid. All crystalline substances have a certain melting point. Ice melts at 0°C, iron - at 1527°C, mercury - at -39°C, etc.

As we already know, in each crystal the atoms or molecules of the substance form an ordered G packing and perform small vibrations around their average positions. As the body heats up, the speed of the oscillating particles increases along with the amplitude of the oscillations. This increase in the speed of particle movement with increasing temperature constitutes one of the fundamental laws of nature, which applies to matter in any state - solid, liquid or gas.

When a certain, sufficiently high temperature of the crystal is reached, the vibrations of its particles become so energetic that a neat arrangement of particles becomes impossible - the crystal melts. With the onset of melting, the heat supplied is no longer used to increase the speed of particles, but to destroy the crystal lattice. Therefore, the rise in temperature stops. Subsequent heating is an increase in the speed of liquid particles.

In the case of crystallization from a melt that interests us, the above described phenomena are observed in reverse order: as the liquid cools, its particles slow down their chaotic movement; upon reaching a certain, sufficiently low temperature, the speed of the particles is already so low that some of them, under the influence of attractive forces, begin to attach to one another, forming crystalline nuclei. Until all the substance crystallizes, the temperature remains constant. This temperature is usually the same as the melting point.

If special measures are not taken, crystallization from the melt will begin in many places at once. The crystals will grow in the form of regular, characteristic polyhedrons in exactly the same way as we described above. However, free growth does not last long: as the crystals grow, they collide with each other, at the points of contact, growth stops, and the solidified body acquires a granular structure. Each grain is a separate crystal that failed to take its correct shape.

Depending on many conditions, and primarily on the rate of cooling, a solid may have more or less large grains: the slower the cooling, the larger the grains. The grain sizes of crystalline bodies range from a millionth of a centimeter to several millimeters. In most cases, the granular crystalline structure can be observed under a microscope. Solids usually have just such a fine-crystalline structure.

The process of solidification of metals is of great interest to technology. Physicists have studied the events that occur during casting and during the solidification of metal in molds in extremely detail.

For the most part, when solidified, tree-like single crystals grow, called dendrites. In other cases, the dendrites are oriented at random, in other cases - parallel to each other.

In Fig. Figure 4.9 shows the stages of growth of one dendrite. With this behavior, a dendrite can become overgrown before it meets another similar one. Then we will not find dendrites in the casting. Events can also develop differently: dendrites can meet and grow into each other (the branches of one into the spaces between the branches of the other) while they are still “young”.

Rice. 4.9

Thus, castings can arise whose grains (shown in Fig. 2.22) have very different structures. And the properties of metals significantly depend on the nature of this structure. You can control the behavior of the metal during solidification by changing the cooling rate and the heat removal system.

Now let's talk about how to grow a large single crystal. It is clear that measures must be taken to ensure that the crystal grows from one place. And if several crystals have already begun to grow, then in any case it is necessary to ensure that the growth conditions are favorable for only one of them.

Here, for example, is what one does when growing crystals of low-melting metals. The metal is melted in a glass test tube with the end pulled out. A test tube suspended on a thread inside a vertical cylindrical furnace is slowly lowered down. The drawn end gradually leaves the oven and cools. Crystallization begins. At first, several crystals form, but those that grow sideways rest against the wall of the test tube and their growth slows down. Only the crystal that grows along the axis of the test tube, i.e., deep into the melt, will be in favorable conditions. As the test tube descends, new portions of the melt entering the low temperature region will “feed” this single crystal. Therefore, of all the crystals, it is the only one that survives; as the test tube descends, it continues to grow along its axis. Eventually all the molten metal solidifies into a single crystal.

The same idea underlies the cultivation of refractory ruby crystals. Fine powder of the substance is sprayed through the flame. The powders melt; tiny drops fall onto a refractory support of a very small area, forming many crystals. As the drops continue to fall onto the stand, all the crystals grow, but again only the one that is in the most favorable position to “receive” the falling drops grows.

What are large crystals needed for?

Industry and science often need large single crystals. Great importance for technology they have crystals of Rochelle salt and quartz, which have the remarkable property of converting mechanical actions (for example, pressure) into electrical voltage.

The optical industry needs large crystals of calcite, rock salt, fluorite, etc.

The watch industry needs crystals of rubies, sapphires and some other precious stones. The fact is that the individual moving parts of an ordinary watch make up to 20,000 vibrations per hour. Such a large load places unusually high demands on the quality of the axle tips and bearings. Abrasion will be the least when the bearing for the tip of the axle with a diameter of 0.07-0.15 mm is ruby or sapphire. The artificial crystals of these substances are very durable and are very little abraded by steel. It's great that artificial stones In this case, they turn out to be better than the same natural stones.

However, the greatest importance for industry is the growing of semiconductor single crystals - silicon and germanium.

Effect of pressure on melting point

If you change the pressure, the melting point will also change. We encountered the same pattern when we talked about boiling. The higher the pressure; the higher the boiling point. This is generally true for melting as well. However, there are a small number of substances that behave anomalously: their melting point decreases with increasing pressure.

The fact is that the vast majority of solids are denser than their liquid counterparts. The exception to this rule is precisely those substances whose melting point changes with a change in pressure in an unusual way, for example water. Ice is lighter than water, and the melting point of ice decreases as pressure increases.

Compression promotes the formation of a denser state. If a solid is denser than a liquid, compression helps solidify and prevents melting. But if melting is made difficult by compression, this means that the substance remains solid, whereas previously at this temperature it would have already melted, i.e., with increasing pressure, the melting temperature increases. In the anomalous case, the liquid is denser than the solid, and pressure helps the formation of the liquid, i.e., lowers the melting point.

The effect of pressure on the melting point is much less than the similar effect on boiling. An increase in pressure by more than 100 kgf/cm2 lowers the melting point of ice by 1°C.

Why do skates glide only on ice, but not on equally smooth parquet? Apparently, the only explanation is the formation of water, which lubricates the skate. To understand the contradiction that has arisen, you need to remember the following: stupid skates glide on ice very poorly. Skates need to be sharpened so they can cut ice. In this case, only the tip of the skate edge presses on the ice. The pressure on the ice reaches tens of thousands of atmospheres, but the ice still melts.

Evaporation of solids

When they say “a substance evaporates,” they usually mean that a liquid evaporates. But solids can also evaporate. Sometimes the evaporation of solids is called sublimation.

An evaporating solid is, for example, naphthalene. Naphthalene melts at 80°C and evaporates at room temperature. It is this property of naphthalene that allows it to be used to exterminate moths.

A fur coat covered with mothballs is saturated with naphthalene vapors and creates an atmosphere that moths cannot tolerate. Anything that smells solid sublimes to a large extent. After all, the smell is created by molecules that break away from the substance and reach our nose. However, more frequent cases are when a substance sublimes to a small degree, sometimes to a degree that cannot be detected even by very careful research. In principle, any solid substance (and that is any solid substance, even iron or copper) evaporates. If we do not detect sublimation, this only means that the density of the saturating vapor is very insignificant.

You can verify that a number of substances that have a pungent odor at room temperature lose it at low temperatures.

The density of saturated vapor in equilibrium with a solid increases rapidly with increasing temperature. We illustrate this behavior with the ice curve shown in Fig. 4.10. It's true that ice doesn't smell...

Rice. 4.10

In most cases, it is impossible to significantly increase the saturated vapor density of a solid body for a simple reason - the substance will melt earlier.

The ice also evaporates. This is well known to housewives who hang wet laundry out to dry in cold weather." The water first freezes, and then the ice evaporates, and the laundry turns out to be dry.

Triple point

So, there are conditions under which vapor, liquid and crystal can exist in pairs in equilibrium. Can all three states be in equilibrium? Such a point on the pressure-temperature diagram exists; it is called triple. Where is it?

If you place water with floating ice in a closed vessel at zero degrees, then free space water (and “ice”) vapor will begin to flow. At a vapor pressure of 4.6 mm Hg. Art. evaporation will stop and saturation will begin. Now the three phases - ice, water and steam - will be in a state of equilibrium. This is the triple point.

The relationships between different states are clearly and clearly shown by the diagram for water shown in Fig. 4.11.

Rice. 4.11

Such a diagram can be constructed for any body.

The curves in the figure are familiar to us - these are the equilibrium curves between ice and steam, ice and water, water and steam. Pressure is plotted vertically, as usual, temperature is plotted horizontally.

The three curves intersect at the triple point and divide the diagram into three regions - the living spaces of ice, water and water vapor.

A state diagram is a condensed reference. Its goal is to answer the question of what state of the body is stable at such and such pressure and such and such temperature.

If water or steam is placed in the conditions of the “left region”, they will become ice. If you add a liquid or a solid to the “lower region,” you get steam. In the “right region” the steam will condense and the ice will melt.

The phase existence diagram allows you to immediately answer what will happen to a substance when heated or compressed. Heating at constant pressure is represented on the diagram by a horizontal line. A point representing the state of the body moves along this line from left to right.

The figure shows two such lines, one of them is heating at normal pressure. The line lies above triple point. Therefore, it will first intersect the melting curve, and then, outside the drawing, the evaporation curve. Ice at normal pressure will melt at a temperature of 0°C, and the resulting water will boil at 100°C.

The situation will be different for ice heated at very low pressure, say just below 5 mmHg. Art. The heating process is depicted by a line going below the triple point. The melting and boiling curves do not intersect with this line. At such a low pressure, heating will lead to a direct transition of ice into steam.

In Fig. 4.12, the same diagram shows what interesting phenomenon will occur when water vapor is compressed in the state marked with a cross in the figure. The steam will first turn into ice and then melt. The drawing allows you to immediately tell at what pressure the crystal will begin to grow and when melting will occur.

Rice. 4.12

The phase diagrams of all substances are similar to each other. Large, from an everyday point of view, differences arise due to the fact that the location of the triple point on the diagram can be very different for different substances.

After all, we exist near “normal conditions,” that is, primarily at a pressure close to one atmosphere. How the triple point of a substance is located in relation to the line of normal pressure is very important for us.

If the pressure at the triple point is less than atmospheric, then for us, living in “normal” conditions, the substance is classified as melting. As the temperature increases, it first turns into liquid and then boils.

In the opposite case - when the pressure at the triple point is higher than atmospheric - we will not see liquid when heated, the solid will directly turn into vapor. This is how “dry ice” behaves, which is very convenient for ice cream sellers. Ice cream briquettes can be transferred with pieces of “dry ice” and not be afraid that the ice cream will become wet. "Dry ice" is solid carbon dioxide C0 2. The triple point of this substance lies at 73 atm. Therefore, when solid CO 2 is heated, the point representing its state moves horizontally, intersecting only the evaporation curve of the solid (the same as for regular ice at a pressure of about 5 mm Hg. Art.).

We have already told the reader how one degree of temperature is determined on the Kelvin scale, or, as the SI system now requires us to say, one kelvin. However, we were talking about the principle of determining temperature. Not all metrology institutes have ideal gas thermometers. Therefore, the temperature scale is built using equilibrium points fixed by nature between different states of matter.

The triple point of water plays a special role in this. A degree Kelvin is now defined as the 273.16th part of the thermodynamic temperature of the triple point of water. The triple point of oxygen is taken to be 54.361 K. The solidification temperature of gold is set to be 1337.58 K. Using these reference points, any thermometer can be accurately calibrated.

The same atoms, but... different crystals

Black matte soft graphite with which we write, and shiny transparent, hard, cutting glass Diamonds are built from the same carbon atoms. Why are the properties of these two identical substances so different?

Consider the lattice of layered graphite, each atom of which has three nearest neighbors, and the lattice of diamond, whose atom has four nearest neighbors. This example clearly shows that the properties of crystals are determined by the relative arrangement of atoms. Fireproof crucibles are made from graphite that can withstand temperatures up to two to three thousand degrees, and diamond burns at temperatures above 700°C; the density of diamond is 3.5, and graphite - 2.3; graphite conducts electricity, diamond - does not conduct, etc.

It is not only carbon that has this property of producing different crystals. Almost every chemical element, and not only an element, but also any Chemical substance, can exist in several varieties. There are six varieties of ice, nine varieties of sulfur, and four varieties of iron.

When discussing the phase diagram, we did not talk about different types of crystals and drew a single region of the solid. And for many substances this region is divided into sections, each of which corresponds to a certain “type” of a solid or, as they say, a certain solid phase (a certain crystalline modification).

Each crystalline phase has its own region of stable state, limited by a certain range of pressures and temperatures. The laws of transformation of one crystalline variety into another are the same as the laws of melting and evaporation.

For each pressure, you can specify the temperature at which both types of crystals will peacefully coexist. If you increase the temperature, a crystal of one type will turn into a crystal of the second type. If you lower the temperature, the reverse transformation will occur.

For red sulfur to turn yellow at normal pressure, a temperature below 110°C is needed. Above this temperature, up to the melting point, the order of arrangement of atoms characteristic of red sulfur is stable. The temperature drops, the vibrations of atoms decrease, and, starting from 110°C, nature finds a more convenient arrangement of atoms. There is a transformation of one crystal into another.

Six different ices no one came up with names. That's what they say: ice one, ice two, ...., ice seven. How about seven if there are only six varieties? The fact is that ice four was not detected during repeated experiments.

If you compress water at a temperature near zero, then at a pressure of about 2000 atm ice five is formed, and at a pressure of about 6000 atm ice six is formed.

Ice two and ice three are stable at temperatures below zero degrees.

Ice seven is hot ice; it occurs during compression hot water up to pressures of about 20,000 atm.

All ice, except ordinary ice, is heavier than water. Ice produced under normal conditions behaves abnormally; on the contrary, ice obtained under conditions different from the norm behaves normally.

We say that each crystalline modification is characterized by a certain region of existence. But if so, then how do graphite and diamond exist under the same conditions?

Such “lawlessness” occurs very often in the world of crystals. The ability to live in “foreign” conditions is almost a rule for crystals. If in order to transfer vapor or liquid into foreign areas of existence one has to resort to various tricks, then a crystal, on the contrary, almost never can be forced to remain within the boundaries allocated to it by nature.

Overheating and overcooling of crystals is explained by the difficulty of converting one order to another under extremely crowded conditions. Yellow sulfur should turn into red at 95.5°C. With more or less rapid heating, we will “overshoot” this transformation point and bring the temperature to the sulfur melting point of 113°C.

The true transformation temperature is easiest to detect when the crystals come into contact. If they are closely placed one on top of the other and the temperature is maintained at 96°C, then the yellow will be eaten by the red, and at 95°C the yellow will absorb the red. In contrast to the “crystal-liquid” transition, “crystal-crystal” transformations are usually delayed both during supercooling and overheating.

In some cases, we are dealing with states of matter that should live at completely different temperatures.

White tin should turn gray when the temperature drops to +13°C. We usually deal with white tin and know that nothing is done with it in winter. It perfectly withstands hypothermia of 20-30 degrees. However, in harsh winter conditions, white tin turns into gray. Ignorance of this fact was one of the circumstances that ruined Scott's expedition to the South Pole (1912). The liquid fuel taken by the expedition was in vessels soldered with tin. In extreme cold, white tin turned into gray powder - the vessels were unsoldered; and the fuel spilled out. It is not for nothing that the appearance of gray spots on white tin is called tin plague.

As with sulphur, white tin can be converted to gray at temperatures just below 13°C; unless a tiny grain of the gray variety falls on a tin object.

The existence of several varieties of the same substance and delays in their mutual transformations have great value for technology.

At room temperature, iron atoms form a body-centered cubic lattice, in which the atoms occupy positions at the vertices and in the center of the cube. Each atom has 8 neighbors. At high temperatures, iron atoms form a denser “packing” - each atom has 12 neighbors. Iron with 8 neighbors is soft, iron with 12 neighbors is hard. It turns out that it is possible to obtain iron of the second type at room temperature. This method - hardening - is widely used in metallurgy.

Hardening is done very simply - a metal object is heated red-hot and then thrown into water or oil. Cooling occurs so quickly that the transformation of a structure that is stable at high temperatures does not have time to occur. Thus, the high-temperature structure will exist indefinitely for a long time in conditions unusual for it: recrystallization into a stable structure occurs so slowly that it is practically unnoticeable.

When talking about hardening iron, we were not entirely accurate. Steel is hardened, i.e. iron containing fractions of a percent of carbon. The presence of very small carbon impurities delays the transformation of hard iron into soft iron and allows for hardening. As for completely pure iron, it is not possible to harden it - the transformation of the structure manages to occur even with the most rapid cooling.

Depending on the type of state diagram, changing pressure or temperature, one or another transformation is achieved.

Many crystal-to-crystal transformations are observed with changes in pressure alone. Black phosphorus was obtained in this way.

Rice. 4.13

It was possible to transform graphite into diamond only by using both high temperature and high pressure at the same time. In Fig. Figure 4.13 shows the phase diagram of carbon. At pressures below ten thousand atmospheres and at temperatures below 4000 K, graphite is a stable modification. Thus, the diamond lives in “alien” conditions, so without it special labor can be turned into graphite. But the inverse problem is of practical interest. It is not possible to transform graphite into diamond by increasing pressure alone. The phase transformation in the solid state is apparently too slow. The appearance of the state diagram suggests correct solution: Increase pressure and heat at the same time. Then we get (right corner of the diagram) molten carbon. Cooling it at high blood pressure, we must get into the diamond area.

The practical possibility of such a process was proven in 1955, and the problem is now considered technically solved.

Amazing liquid

If you lower the temperature of a body, sooner or later it will harden and acquire a crystalline structure. It does not matter at what pressure the cooling occurs. This circumstance seems completely natural and understandable from the point of view of the laws of physics, with which we have already become acquainted. Indeed, by lowering the temperature, we reduce the intensity of thermal movement. When the movement of molecules becomes so weak that it no longer interferes with the forces of interaction between them, the molecules will line up in a neat order - they will form a crystal. Further cooling will take away all the energy of their movement from the molecules, and at absolute zero the substance must exist in the form of resting molecules arranged in a regular lattice.

Experience shows that all substances behave this way. All except for one thing: helium is such a “monster”.

We have already provided the reader with some information about helium. Helium holds the record for its critical temperature. No substance has a critical temperature lower than 4.3 K. However, this record in itself does not mean anything surprising. Another thing is striking: cooling helium below the critical temperature, reaching almost absolute zero, we will not get solid helium. Helium remains liquid even at absolute zero.

The behavior of helium is completely inexplicable from the point of view of the laws of motion we have outlined and is one of the signs of the limited validity of such laws of nature that seemed universal.

If a body is liquid, then its atoms are in motion. But by cooling the body to absolute zero, we have taken away all the energy of movement from it. We have to admit that helium has such energy of motion that cannot be taken away. This conclusion is incompatible with the mechanics we have been studying so far. According to this mechanics we have studied, the movement of a body can always be slowed down to a complete stop, taking away all its kinetic energy; in the same way, you can stop the movement of molecules by taking away their energy when they collide with the walls of a cooled vessel. For helium, such mechanics are clearly not suitable.

The "strange" behavior of helium is an indication of a fact of great importance. For the first time we encountered the impossibility of applying in the world of atoms the basic laws of mechanics established by direct study of the movement of visible objects. bodies, - laws, which seemed to be the unshakable foundation of physics.

The fact that at absolute zero helium “refuses” to crystallize cannot in any way be reconciled with the mechanics we have studied so far. The contradiction that we encountered for the first time - the non-subordination of the world of atoms to the laws of mechanics - is only the first link in a chain of even more acute and drastic contradictions in physics.

These contradictions lead to the need to revise the fundamentals of mechanics atomic world. This revision is very profound and leads to a change in our entire understanding of nature.

The need for a radical revision of the mechanics of the atomic world does not mean that we need to put an end to the laws of mechanics that we have studied. It would be unfair to force the reader to study unnecessary things. The old mechanics is completely valid in the world of large bodies. This alone is enough to treat the relevant chapters of physics with complete respect. However, it is also important that a number of laws of “old” mechanics pass into “new” mechanics. This includes, in particular, the law of conservation of energy.

The presence of “irremovable” energy at absolute zero is not a special property of helium. Turns out; All substances have “zero” energy.

Only in helium is this energy sufficient to prevent the atoms from forming a regular crystal lattice.

Do not think that helium cannot be in crystalline state. To crystallize helium, you only need to increase the pressure to about 25 atm. Cooling carried out at higher pressure will result in the formation of solid crystalline helium with completely normal properties. Helium forms a face-centered cubic lattice.

In Fig. Figure 4.14 shows the phase diagram of helium. It differs sharply from the diagrams of all other substances in the absence of a triple point. Melting and boiling curves do not intersect.

Rice. 4.14

And this unique state diagram has one more feature: there are two different helium liquids. You will find out what their difference is a little later.