Molecular physics. Saturated and unsaturated vapors

Ticket number 1

Saturated steam.

If the vessel with liquid is tightly closed, then the amount of liquid will first decrease, and then will remain constant. At a constant temperature, the liquid - vapor system will come to a state of thermal equilibrium and will remain in it for an arbitrarily long time. Simultaneously with the evaporation process, condensation also occurs, both processes, on average, compensate each other.

At the first moment, after the liquid is poured into the vessel and closed, the liquid will evaporate and the vapor density above it will increase. However, at the same time, the number of molecules returning to the liquid will also increase. The higher the vapor density, the more its molecules are returned to the liquid. As a result, a dynamic (mobile) equilibrium between liquid and vapor will be established in a closed vessel at a constant temperature, i.e., the number of molecules leaving the surface of the liquid over a certain period of time will be equal, on average, to the number of vapor molecules returning to the liquid in the same time.

Steam in dynamic equilibrium with its liquid is called saturated steam. This definition emphasizes that given volume no more steam can be present at a given temperature.

Saturated steam pressure.

What will happen to saturated steam if the volume occupied by it is reduced? For example, if you compress vapor that is in equilibrium with a liquid in a cylinder under a piston, keeping the temperature of the contents of the cylinder constant.

When the vapor is compressed, the equilibrium will begin to be disturbed. The vapor density at the first moment will increase slightly, and more molecules will begin to pass from gas to liquid than from liquid to gas. After all, the number of molecules leaving the liquid per unit time depends only on the temperature, and the compression of the vapor does not change this number. The process continues until the dynamic equilibrium and vapor density are again established, and hence the concentration of its molecules will not take their previous values. Consequently, the concentration of saturated vapor molecules at a constant temperature does not depend on its volume.

Since the pressure is proportional to the concentration of molecules (p=nkT), it follows from this definition that the pressure of saturated vapor does not depend on the volume it occupies.

Pressure p n.p. the vapor at which the liquid is in equilibrium with its vapor is called the saturation vapor pressure.

Saturated vapor pressure versus temperature

The state of saturated steam, as experience shows, is approximately described by the equation of state ideal gas, and its pressure is determined by the formula

As the temperature rises, the pressure rises. Since the saturation vapor pressure does not depend on volume, it therefore depends only on temperature.

However, the dependence of рn.p. from T, found experimentally, is not directly proportional, as in an ideal gas at constant volume. With increasing temperature, the pressure of real saturated steam increases faster than the pressure of an ideal gas (Fig. section of curve 12). Why is this happening?

When a liquid is heated in a closed vessel, part of the liquid turns into vapor. As a result, according to the formula Р = nкТ, the saturated vapor pressure increases not only due to an increase in the temperature of the liquid, but but also due to an increase in the concentration of molecules (density) of the vapor. Basically, the increase in pressure with increasing temperature is determined precisely by the increase in concentration.

(The main difference in the behavior of an ideal gas and saturated vapor is that when the temperature of the vapor in a closed vessel changes (or when the volume changes at a constant temperature), the mass of the vapor changes. The liquid partially turns into vapor, or, conversely, the vapor partially condenses. C Nothing like this happens in an ideal gas.

When all the liquid has evaporated, the vapor will cease to be saturated upon further heating, and its pressure at constant volume will increase in direct proportion to the absolute temperature (see Fig., curve section 23).

Boiling.

Boiling is an intense transition of a substance from a liquid state to a gaseous state, occurring throughout the entire volume of the liquid (and not just from its surface). (Condensation is the reverse process.)

As the temperature of the liquid increases, the rate of evaporation increases. Finally, the liquid begins to boil. When boiling, rapidly growing vapor bubbles form throughout the volume of the liquid, which float to the surface. The boiling point of a liquid remains constant. This is because all the energy supplied to the liquid is spent on turning it into steam.

Under what conditions does boiling begin?

The liquid always contains dissolved gases that are released on the bottom and walls of the vessel, as well as on dust particles suspended in the liquid, which are the centers of vaporization. The liquid vapors inside the bubbles are saturated. As the temperature increases, the vapor pressure increases and the bubbles increase in size. Under the action of the buoyant force, they float up. If the upper layers of the liquid have more low temperature, then in these layers the vapor condenses in the bubbles. The pressure drops rapidly and the bubbles collapse. The collapse is so fast that the walls of the bubble, colliding, produce something like an explosion. Many of these microexplosions create a characteristic noise. When the liquid warms up enough, the bubbles stop collapsing and float to the surface. The liquid will boil. Watch the kettle on the stove carefully. You will find that it almost stops making noise before boiling.

The dependence of saturation vapor pressure on temperature explains why the boiling point of a liquid depends on the pressure on its surface. A vapor bubble can grow when the pressure of the saturated vapor inside it slightly exceeds the pressure in the liquid, which is the sum of the air pressure on the surface of the liquid (external pressure) and the hydrostatic pressure of the liquid column.

Boiling begins at a temperature at which the saturation vapor pressure in the bubbles is equal to the pressure in the liquid.

The greater the external pressure, the higher the boiling point.

Conversely, by reducing the external pressure, we thereby lower the boiling point. By pumping out air and water vapor from the flask, you can make the water boil at room temperature.

Each liquid has its own boiling point (which remains constant until the entire liquid boils away), which depends on its saturated vapor pressure. The higher the saturation vapor pressure, the lower the boiling point of the liquid.

Specific heat of vaporization.

Boiling occurs with the absorption of heat.

Most of the heat supplied is spent on breaking the bonds between the particles of the substance, the rest - on the work done during the expansion of the steam.

As a result, the interaction energy between vapor particles becomes greater than between liquid particles, so the internal energy of the vapor is greater than the internal energy of the liquid at the same temperature.

The amount of heat required to transfer liquid to vapor during the boiling process can be calculated using the formula:

where m is the mass of liquid (kg),

L - specific heat of vaporization (J / kg)

The specific heat of vaporization shows how much heat is needed to turn 1 kg of a given substance into steam at the boiling point. The unit of specific heat of vaporization in the SI system:

[ L ] = 1 J/kg

Air humidity and its measurement.

The air around us almost always contains some amount of water vapor. The humidity of the air depends on the amount of water vapor it contains.

Raw air contains a higher percentage of water molecules than dry air.

Of great importance relative humidity air, messages about which are heard every day in the weather forecast reports.

O
Relative humidity is the ratio of the density of water vapor contained in the air to the density of saturated vapor at a given temperature, expressed as a percentage. (shows how close water vapor in the air is to saturation)

Dew point

The dryness or humidity of the air depends on how close its water vapor is to saturation.

If moist air is cooled, then the vapor in it can be brought to saturation, and then it will condense.

A sign that the steam is saturated is the appearance of the first drops of condensed liquid - dew.

The temperature at which the vapor in the air becomes saturated is called the dew point.

The dew point also characterizes the humidity of the air.

Examples: dew in the morning, fogging of cold glass if you breathe on it, the formation of a drop of water on a cold water pipe, dampness in the basements of houses.

Hygrometers are used to measure air humidity. There are several types of hygrometers, but the main ones are hair and psychrometric. Since it is difficult to directly measure the pressure of water vapor in the air, the relative humidity of the air is measured indirectly.

It is known that the rate of evaporation depends on the relative humidity of the air. The lower the air humidity, the easier it is for moisture to evaporate..

AT The psychrometer has two thermometers. One is ordinary, it is called dry. It measures the temperature of the surrounding air. The flask of another thermometer is wrapped in a fabric wick and lowered into a container of water. The second thermometer does not show the temperature of the air, but the temperature of the wet wick, hence the name wet bulb. The lower the humidity of the air, the more intensely the moisture evaporates from the wick, the more heat per unit time is removed from the wetted thermometer, the lower its readings, therefore, the greater the difference between the readings of dry and wetted thermometers. Saturation = 100 ° C and specific characteristics of the state rich liquid and dry rich pair v"=0.001 v""=1.7 ... wet saturated steam with the degree of dryness Calculate the extensive characteristics of wet rich pair on...

As the temperature rises, the pressure rises. Because Saturated vapor pressure does not depend on volume, therefore, it depends only on temperature.
However, dependence r n.p. from T, found experimentally, is not directly proportional, as in an ideal gas at constant volume. With increasing temperature, the pressure of a real saturated vapor increases faster than the pressure of an ideal gas ( fig.11.1, section of the curve AB). This becomes obvious if we draw the isochores of an ideal gas through the points BUT and AT(dashed lines). Why is this happening?

When a liquid is heated in a closed vessel, part of the liquid turns into vapor. As a result, according to formula (11.1) saturated vapor pressure increases not only due to an increase in the temperature of the liquid, but also due to an increase in the concentration of molecules (density) of the vapor. Basically, the increase in pressure with increasing temperature is determined precisely by the increase in concentration. The main difference in the behavior of an ideal gas and saturated steam is that when the temperature of the vapor in a closed vessel changes (or when the volume changes at a constant temperature), the mass of the vapor changes. The liquid partially turns into vapor, or, conversely, the vapor partially condenses. Nothing like this happens with an ideal gas.
When all the liquid has evaporated, the vapor will cease to be saturated upon further heating, and its pressure at constant volume will increase in direct proportion to the absolute temperature (see Fig. fig.11.1, section of the curve sun).
Boiling. As the temperature of the liquid increases, the rate of evaporation increases. Finally, the liquid begins to boil. When boiling, rapidly growing vapor bubbles form throughout the volume of the liquid, which float to the surface. The boiling point of a liquid remains constant. This is because all the energy supplied to the liquid is spent on turning it into steam. Under what conditions does boiling begin?
The liquid always contains dissolved gases that are released on the bottom and walls of the vessel, as well as on dust particles suspended in the liquid, which are the centers of vaporization. The liquid vapors inside the bubbles are saturated. As the temperature increases, the vapor pressure increases and the bubbles increase in size. Under the action of the buoyant force, they float up. If the upper layers of the liquid have a lower temperature, then vapor condenses in these layers in the bubbles. The pressure drops rapidly and the bubbles collapse. The collapse is so fast that the walls of the bubble, colliding, produce something like an explosion. Many of these microexplosions create a characteristic noise. When the liquid warms up enough, the bubbles stop collapsing and float to the surface. The liquid will boil. Watch the kettle on the stove carefully. You will find that it almost stops making noise before boiling.
The dependence of saturation vapor pressure on temperature explains why the boiling point of a liquid depends on the pressure on its surface. A vapor bubble can grow when the pressure of the saturated vapor inside it slightly exceeds the pressure in the liquid, which is the sum of the air pressure on the surface of the liquid (external pressure) and the hydrostatic pressure of the liquid column.
Let us pay attention to the fact that the evaporation of a liquid occurs at temperatures lower than the boiling point, and only from the surface of the liquid; during boiling, the formation of vapor occurs throughout the entire volume of the liquid.
Boiling begins at a temperature at which the saturation vapor pressure in the bubbles is equal to the pressure in the liquid.
The greater the external pressure, the higher the boiling point. So, 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 in hermetically sealed vessels - autoclaves ( fig.11.2) water also boils at elevated pressure. Therefore, the boiling point of the liquid is much higher than 100°C. Autoclaves are used to sterilize surgical instruments, etc.

And vice versa, reducing the external pressure, we thereby lower the boiling point. By pumping out air and water vapor from the flask, you can make the water boil at room temperature ( fig.11.3). When climbing mountains Atmosphere pressure decreases, so 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 in these conditions.

Each liquid has its own boiling point, which depends on the pressure of its saturated vapor. The higher the saturated vapor pressure, the lower the boiling point of the liquid, since at lower temperatures the saturated vapor pressure becomes equal to atmospheric pressure. For example, at a boiling point of 100 ° C, the pressure of saturated water vapor is 101,325 Pa (760 mm Hg), and mercury vapor is only 117 Pa (0.88 mm Hg). Mercury boils at 357°C at normal pressure.
A liquid boils when its saturated vapor pressure becomes equal to the pressure inside the liquid.

And what will happen to saturated steam if the volume occupied by it is reduced? For example, if you compress vapor that is in equilibrium with a liquid in a cylinder under a piston, keeping the temperature of the contents of the cylinder constant.

When the vapor is compressed, the equilibrium will begin to be disturbed. The vapor density at the first moment will increase slightly, and more molecules will begin to pass from gas to liquid than from liquid to gas. After all, the number of molecules leaving the liquid per unit time depends only on the temperature, and the compression of the vapor does not change this number. The process continues until the dynamic equilibrium and vapor density are again established, and hence the concentration of its molecules will not take the same value. Therefore, the concentration of saturated vapor molecules at a constant temperature does not depend on its volume.

Since pressure is proportional to the concentration of molecules (p = nkT), then it follows from this definition that saturation vapor pressure is independentotons of volume it occupies.

Steam pressure, at which the liquid is in equilibrium with its vapor is called the saturation vapor pressure.

• unsaturated steam.

We used the words many times gas and steam. There is no fundamental difference between gas and steam. But if, at a constant temperature, a gas can be turned into a liquid by simple compression, then we call it vapor, more precisely, unsaturated steam.

• Dependence of pressure of saturated vapor on temperature.

The state of saturated steam, as experience says, is approximately described by the equation of state of an ideal gas, and its pressure is determined by the formula

As the temperature rises, the pressure rises. Because d pressure saturatedsteam does not depend on the volume,it only dependsfrom temperature.

However, this dependency mouth), found experimentally, is not directly proportional, as in an ideal gas at constant volume. With increasing temperature, the pressure of saturated vapor increases faster than the pressure of an ideal gas (Fig. 30, section of the curve AB). This becomes especially obvious if we draw an isochore through the point BUT(dashed line) Why is this happening?

When a liquid is heated in a closed vessel, part of the liquid turns into vapor. As a result, according to the formula
pressure saturated steam is growing not only due to temperature increase liquid, but also due to the increase concentration of molecules (raft ness) pair . Basically, the increase in pressure with increasing temperature is determined precisely by the increase in concentration. The main difference in the behavior of an ideal gas and saturated steam is that when the temperature of the vapor in a closed vessel changes (or when the volume changes at a constant temperature), the mass of the vapor changes. The liquid partially turns into vapor or, conversely, the vapor partially condenses. When all the liquid evaporates, the vapor, upon further heating, will cease to be saturated and its pressure at constant volume will increase in direct proportion to the absolute temperature (see Fig. 30, section VS).

The processes of evaporation and condensation are continuous and parallel to each other.

In an open vessel, the amount of liquid decreases over time, because. evaporation prevails over condensation.

Vapor that is above the surface of a liquid when evaporation prevails over condensation, or vapor in the absence of liquid, is called unsaturated.

In a hermetically sealed vessel, the liquid level does not change over time, because evaporation and condensation compensate each other: how many molecules fly out of the liquid, as many of them return to it in the same time, a dynamic (mobile) equilibrium occurs between the vapor and its liquid.

A vapor that is in dynamic equilibrium with its liquid is called saturated.

At a given temperature, the saturated vapor of any liquid has the highest density ( ) and creates maximum pressure ( ) that the vapor of that liquid can have at that temperature.

The pressure and density of saturated vapor at the same temperature depends on the type of substance: more pressure creates saturated vapor of the liquid that evaporates faster. For example, and

Properties of unsaturated vapors: Unsaturated vapors obey the gas laws of Boyle - Mariotte, Gay-Lussac, Charles, and the ideal gas equation of state can be applied to them.

Saturated vapor properties:1. With a constant volume, with increasing temperature, the pressure of saturated vapor increases, but not in direct proportion (Charles' law is not fulfilled), the pressure grows faster than that of an ideal gas. , with increasing temperature ( ) , the mass of vapor increases, and therefore the concentration of vapor molecules increases () and the pressure of saturated vapor will melt for two reasons (

3 1 – unsaturated steam (ideal gas);

2 2 - saturated steam; 3 - unsaturated steam,

1 obtained from saturated steam in the same

volume when heated.

2. The pressure of saturated vapor at a constant temperature does not depend on the volume it occupies.

With an increase in volume, the mass of the vapor increases, and the mass of the liquid decreases (part of the liquid passes into vapor), with a decrease in the volume of vapor, it becomes less, and the liquid becomes larger (part of the vapor passes into liquid), the density and concentration of saturated vapor molecules remain constant, therefore, and pressure remains constant ().

liquid

(sat. steam + liquid)

Unsaturated steam

Saturated vapors do not obey the gas laws of Boyle - Mariotte, Gay-Lussac, Charles, because the mass of vapor in the processes does not remain constant, and all gas laws are obtained for a constant mass. The equation of state for an ideal gas can be applied to saturated steam.

So, Saturated steam can be converted to unsaturated steam either by heating it at a constant volume or by increasing its volume at a constant temperature. Unsaturated steam can be converted to saturated steam either by cooling it at a constant volume or by compressing it at a constant temperature.

Critical situation

The presence of a free surface in a liquid makes it possible to indicate where the liquid phase of the substance is located, and where the gaseous one. The sharp difference between a liquid and its vapor is explained by the fact that the density of a liquid is many times greater than that of a vapor. If a liquid is heated in a hermetically sealed vessel, then due to expansion, its density will decrease, and the vapor density above it will increase. This means that the difference between a liquid and its saturated vapor is smoothed out and disappears altogether at a sufficiently high temperature. The temperature at which differences in physical properties between a liquid and its saturated vapor, and their densities become the same, is calledcritical temperature.

Critical point

For the formation of a liquid from a gas, the average potential energy of attraction of molecules must exceed their average kinetic energy.

Critical temperature – Maximum temperature at which the vapor is converted to liquid. The critical temperature depends on the potential energy of molecular interaction and is therefore different for different gases. Due to the strong interaction of water molecules, water vapor can be turned into water even at a temperature of . At the same time, nitrogen liquefaction occurs only at a temperature less than = -147˚, because nitrogen molecules weakly interact with each other.

Another macroscopic parameter that affects the vapor-liquid transition is pressure. With an increase in external pressure during gas compression, the average distance between particles decreases, the force of attraction between them increases and, accordingly, the average potential energy of their interaction.

Pressuresaturated steam at its critical temperature is called critical. This is the highest possible saturation vapor pressure of a given substance.

State of matter with critical parameters is called critical(critical point) . Each substance has its own critical temperature and pressure.

In the critical state, the specific heat of vaporization and the coefficient of surface tension of the liquid vanish. At temperatures above critical, even at very high pressures, the transformation of a gas into a liquid is impossible; above the critical temperature, the liquid cannot exist. At supercritical temperatures, only the vapor state of matter is possible.

Liquefaction of gases is possible only at temperatures below the critical temperature. For liquefaction, gases are cooled to a critical temperature, for example, by adiabatic expansion, and then isothermally compressed.

Boiling

Externally, the phenomenon looks like this: from the entire volume of the liquid, rapidly growing bubbles rise to the surface, they burst on the surface, and the vapor is released into the environment.

MKT explains boiling like this: there are always air bubbles in the liquid, in which evaporation from the liquid occurs. The closed volume of bubbles turns out to be filled not only with air, but also with saturated steam. The pressure of saturated vapor in them when the liquid is heated increases faster than the air pressure. When, in a sufficiently heated liquid, the pressure of saturated vapor in the bubbles becomes greater than the external pressure, they increase in volume, and a buoyant force that exceeds their gravity lifts the bubbles to the surface. Floated bubbles begin to burst when, at a certain temperature, the pressure of saturated vapor in them exceeds the pressure above the liquid. The temperature of a liquid at which the pressure of its saturated vapor in the bubbles is equal to or greater than the external pressure on the liquid is called boiling point.

The boiling point of different liquids is different, because the pressure of saturated vapor in their bubbles is compared with the same external pressure at different temperatures. For example, the saturation vapor pressure in the bubbles is equal to the normal atmospheric pressure for water at 100°C, for mercury at 357°C, for alcohol at 78°C, for ether at 35°C.

The boiling point remains constant during the boiling process, because all the heat that is supplied to the heated liquid is spent on vaporization.

The boiling point depends on the external pressure on the liquid: with increasing pressure, the temperature rises; as the pressure decreases, the temperature decreases. For example, at an altitude of 5 km above sea level, where the pressure is 2 times lower than atmospheric pressure, the boiling point of water is 83 ° C, in the boilers of steam engines, where the steam pressure is 15 atm. (), the water temperature is about 200˚С.

Air humidity

There is always water vapor in the air, so we can talk about air humidity, which is characterized by the following values:

1.Absolute humidity is the density of water vapor in the air (or the pressure that this vapor creates ( .

Absolute humidity does not give an idea of ​​the degree of saturation of the air with water vapor. The same amount of water vapor different temperature creates a different feeling of moisture.

2.Relative Humidity is the ratio of the density (pressure) of water vapor contained in air at a given temperature to the density (pressure) of saturated vapor at the same temperature : or

is the absolute humidity at a given temperature; - density, saturated vapor pressure at the same temperature. The density and pressure of saturated water vapor at any temperature can be found in the table. The table shows that the higher the air temperature, the greater the density and pressure of water vapor in the air must be in order for it to be saturated.

Knowing the relative humidity, you can understand how many percent of the water vapor in the air at a given temperature is far from saturation. If the vapor in the air is saturated, then . If a , then there is not enough vapor in the air to a state of saturation.

The fact that the vapor in the air becomes saturated is judged by the appearance of moisture in the form of fog, dew. The temperature at which water vapor in the air becomes saturated is called dew point.

The vapor in the air can be made saturated by adding vapor due to additional evaporation of the liquid without changing the temperature of the air, or by lowering its temperature with the amount of vapor in the air.

Normal relative humidity, the most favorable for humans, is 40 - 60%. Great importance has knowledge of humidity in meteorology for weather forecasting. In weaving, confectionery production, a certain humidity is necessary for the normal course of the process. Storing works of art and books requires maintaining the humidity at the required level.

Humidity instruments:

1. Condensation hygrometer (allows you to determine the dew point).

2. The hair hygrometer (based on the length of the fat-free hair versus humidity) measures the relative humidity in percent.

3. The psychrometer consists of two dry and wet thermometers. The wet bulb bulb is wrapped in a cloth dipped in water. Due to evaporation from the fabric, the temperature of the moistened is lower than that of the dry. The difference in thermometer readings depends on the humidity of the surrounding air: the drier the air, the more intense the evaporation from the fabric, the greater the difference in thermometer readings and vice versa. If the air humidity is 100%, then the readings of the thermometers are the same, i.e. the difference in readings is 0. To determine the humidity using a psychrometer, a psychrometric table is used.

Melting and crystallization

When melting of a solid body, the distance between the particles forming the crystal lattice increases, and the lattice itself is destroyed. The melting process requires energy. When a solid body is heated, the kinetic energy of vibrating molecules increases and, accordingly, the amplitude of their oscillations. At a certain temperature, called melting point, the order in the arrangement of particles in crystals is disturbed, the crystals lose their shape. A substance melts, changing from a solid state to a liquid state.

During crystallization there is a convergence of molecules that form a crystal lattice. Crystallization can only occur when the liquid releases energy. When the molten substance is cooled, the average kinetic energy and the speed of the molecules decrease. Attractive forces can keep particles near the equilibrium position. At a certain temperature, called solidification (crystallization) temperature, all molecules are in a position of stable equilibrium, their arrangement becomes ordered - a crystal is formed.

The melting of a solid occurs at the same temperature at which the substance solidifies.

Each substance has its own melting point. For example, the melting points for helium are -269.6˚С, for mercury -38.9˚С, for copper 1083˚С.

During the melting process, the temperature remains constant. The amount of heat supplied from outside goes to the destruction of the crystal lattice.

During the curing process, although heat is removed, the temperature does not change. The energy released during crystallization is used to maintain a constant temperature.

Until all the substance melts or all the substance solidifies, i.e. as long as the solid and liquid phases of a substance exist together, the temperature does not change.

TV+liquid liquid + tv

, where is the amount of heat, - the amount of heat required to melt a substance released during the crystallization of a substance by mass by mass

- specific heat of fusion– the amount of heat required to melt a 1 kg substance at its melting point.

What amount of heat is spent during the melting of a certain mass of a substance, the same amount of heat is released during the crystallization of this mass.

Also called specific heat of crystallization.

At the melting point, the internal energy of a substance in the liquid state is greater than the internal energy of the same mass of substance in the solid state.

For a large number of substances, the volume increases during melting, and the density decreases. On hardening, on the contrary, the volume decreases, and the density increases. For example, solid naphthalene crystals sink in liquid naphthalene.

Some substances, for example, bismuth, ice, gallium, cast iron, etc., shrink when melted, and expand when solidified. These deviations from general rule explained by the structure crystal lattices. Therefore, the water is denser than ice ice floats in water. The expansion of water during freezing leads to the destruction of rocks.

The change in the volume of metals during melting and solidification is essential in foundry business.

Experience shows that a change in external pressure on a solid is reflected in the melting point of that substance. For those substances that expand during melting, an increase in external pressure leads to an increase in the melting point, because. hinders the melting process. If substances are compressed during melting, then for them an increase in external pressure leads to a decrease in the melting temperature, because helps the melting process. Only very big increase pressure significantly changes the melting point. For example, to lower the melting point of ice by 1˚C, the pressure must be increased by 130 atm. The melting point of a substance at normal atmospheric pressure is called the melting point of the substance.

Evaporation of liquids. Saturated and unsaturated pairs. Saturated steam pressure. Air humidity.

Evaporation- vaporization occurring at any temperature from the free surface of the liquid. Uneven distribution of the kinetic energy of molecules at thermal motion leads to the fact that at any temperature the kinetic energy of some liquid molecules or solid body may exceed potential energy their bonds with other molecules. Greater kinetic energy have molecules that have a high speed, and the temperature of the body depends on the speed of movement of its molecules, therefore, evaporation is accompanied by cooling of the liquid. Evaporation rate depends on: open surface area, temperature, concentration of molecules near the liquid.

Condensation- the process of transition of a substance from a gaseous state to a liquid state.

The evaporation of a liquid in a closed vessel at a constant temperature leads to a gradual increase in the concentration of molecules of the evaporating substance in the gaseous state. Some time after the start of evaporation, the concentration of the substance in the gaseous state will reach such a value at which the number of molecules returning to the liquid becomes equal to the number of molecules leaving the liquid in the same time. A dynamic equilibrium is established between the processes of evaporation and condensation of matter. A substance in a gaseous state that is in dynamic equilibrium with a liquid is called saturated vapor. (Vapor is a collection of molecules that have left the liquid in the process of evaporation.) Steam at a pressure below saturation is called unsaturated.

Due to the constant evaporation of water from the surfaces of reservoirs, soil and vegetation, as well as the respiration of humans and animals, the atmosphere always contains water vapor. Therefore, atmospheric pressure is the sum of the pressure of dry air and the water vapor in it. The water vapor pressure will be maximum when the air is saturated with steam. Saturated steam, unlike unsaturated steam, does not obey the laws of an ideal gas. Thus, the saturation vapor pressure does not depend on volume, but depends on temperature. This dependence cannot be expressed by a simple formula, therefore, on the basis of an experimental study of the dependence of saturated vapor pressure on temperature, tables have been compiled from which it is possible to determine its pressure at various temperatures.

The pressure of water vapor in air at a given temperature is called absolute humidity, or water vapor pressure. Since the vapor pressure is proportional to the concentration of molecules, one can determine absolute humidity as the density of water vapor in the air at a given temperature, expressed in kilograms per cubic meter (p).

Most of the phenomena observed in nature, such as the rate of evaporation, drying various substances, the wilting of plants, does not depend on the amount of water vapor in the air, but on how close this amount is to saturation, that is, on relative humidity, which characterizes the degree of saturation of air with water vapor. At low temperature and high humidity heat transfer increases and the person is exposed to hypothermia. At high temperatures and humidity, heat transfer, on the contrary, is sharply reduced, which leads to overheating of the body. The most favorable for humans in the middle climatic latitudes is a relative humidity of 40-60%. Relative humidity is the ratio of the density of water vapor (or pressure) in the air at a given temperature to the density (or pressure) of water vapor at the same temperature, expressed as a percentage, i.e.

Relative humidity varies widely. Moreover, the daily variation of relative humidity is reversed daily course temperature. During the day, with an increase in temperature and, consequently, with an increase in saturation pressure, the relative humidity decreases, and at night it increases. The same amount of water vapor can either saturate or not saturate the air. By lowering the temperature of the air, it is possible to bring the vapor in it to saturation. The dew point is the temperature at which the vapor in the air becomes saturated. When the dew point is reached in the air or on objects with which it comes into contact, water vapor begins to condense. To determine the humidity of the air, devices called hygrometers and psychrometers are used.