As temperature increases, pressure increases. 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, like that of an ideal gas at constant volume. With increasing temperature, the pressure of real saturated vapor increases faster than the pressure of an ideal gas ( Fig.11.1, part of the curve AB). This becomes obvious if we draw isochores of an ideal gas through the points A And IN(dashed lines). Why is this happening?

When a liquid is heated in a closed container, some of the liquid turns into steam. 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 steam in a closed vessel changes (or when the volume changes at a constant temperature), the mass of the steam changes. The liquid partially turns into vapor, or, on the contrary, 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 a constant volume will increase in direct proportion to the absolute temperature (see. Fig.11.1, part 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 are formed throughout the entire volume of the liquid, which float to the surface. The boiling point of the liquid remains constant. This happens because all the energy supplied to the liquid is spent on turning it into vapor. Under what conditions does boiling begin?
A liquid always contains dissolved gases, released at the bottom and walls of the vessel, as well as on dust particles suspended in the liquid, which are centers of vaporization. The liquid vapors inside the bubbles are saturated. As the temperature increases, the saturated vapor pressure increases and the bubbles increase in size. Under the influence of buoyant force they float upward. If the upper layers of the liquid have a lower temperature, then vapor condensation occurs in bubbles in these layers. The pressure drops rapidly and the bubbles collapse. The collapse occurs so quickly that the walls of the bubble collide and produce something like an explosion. Many such micro-explosions create a characteristic noise. When the liquid warms up enough, the bubbles will 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 it boils.
The dependence of saturated 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, vapor formation occurs throughout the entire volume of the liquid.
Boiling begins at the temperature at which the saturated vapor pressure in the bubbles is equal to the pressure in the liquid.
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 in hermetically sealed vessels - autoclaves ( Fig.11.2) boiling of water also occurs 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, by reducing external pressure, we thereby lower the boiling point. By pumping air and water vapor out of the flask, you can make the water boil at room temperature ( Fig.11.3). 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 under these conditions.

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

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

Evaporation- vaporization that occurs at any temperature from the free surface of a liquid. The uneven distribution of the kinetic energy of molecules during thermal motion leads to the fact that at any temperature the kinetic energy of some molecules of a liquid or solid may exceed the potential energy of their connection with other molecules. Molecules with greater speed have greater kinetic energy, and the temperature of a body depends on the speed of movement of its molecules, therefore, evaporation is accompanied by cooling of the liquid. The rate of evaporation depends on: the open surface area, temperature, and the 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 a value at which the number of molecules returning to the liquid becomes equal to the number of molecules leaving the liquid during the same time. A dynamic equilibrium is established between the processes of evaporation and condensation of the substance. A substance in a gaseous state that is in dynamic equilibrium with a liquid is called saturated vapor. (Vapor is the collection of molecules that leave the liquid during the process of evaporation.) Vapor at a pressure below saturated 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 contained 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, saturated vapor pressure does not depend on volume, but depends on temperature. This dependence cannot be expressed by a simple formula, therefore, based on an experimental study of the dependence of saturated vapor pressure on temperature, tables have been compiled from which its pressure can be determined at various temperatures.

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

Most of the phenomena observed in nature, for example, the rate of evaporation, drying out of various substances, and wilting of plants, depend not on the amount of water vapor in the air, but on how close this amount is to saturation, i.e., on relative humidity, which characterizes the degree of saturation air with water vapor. At low temperatures and high humidity, heat transfer increases and a person becomes hypothermic. 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 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 the opposite of the daily variation of temperature. During the day, with increasing temperature and, consequently, with increasing saturation pressure, 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 air temperature, the steam in it can be brought to saturation. The dew point is the temperature at which 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 air humidity, instruments called hygrometers and psychrometers are used.

Take a closed vessel into which we pour water. Hydrogen molecules, which have a large amount of energy, are able to escape from the surface of the water into the gas phase. Some of them may return back to the water. Over time, an equilibrium is established between the number of molecules released into vapor and those returned to liquid.

Vapor that is in equilibrium with a liquid is called rich, and the pressure it exerts is called saturated steam pressure ( P°A).

P°A- saturated vapor pressure on a pure solvent.

Now let’s take the same closed vessel and pour a solution containing substances A + B (non-volatile) molecules of the dissolved substance do not come out, solvent molecules come out. Fewer solvent molecules come out because there are fewer of them in solution than in a pure solvent. Therefore, equilibrium will be established at lower pressure.

P A- saturated vapor pressure of the solvent above the solution. This pressure is always less than the saturated vapor pressure of a pure solvent ( P A< P° A ).

Based on these experiences Raoul deduced his law, which has two forms of notation, and therefore two formulations:

1) the saturated vapor pressure of the solvent above the solution is directly proportional to the molar fraction of the solvent. P A = P° A *N A

2) instead of the molar fraction of the solvent, it is necessary to enter the molar fraction of the solute

N A =1-N B

P A = P° A *(1-N B)

N B =(P° A -P A)/ P° A

P° A -P A characterizes the decrease in the saturated vapor pressure of the solvent above the solution.

The relative decrease in the saturated vapor pressure of the solvent above the solution is equal to the molar fraction of the solution.

2) boiling point of the solution is the t at which the saturated vapor pressure of the solvent above the solution becomes equal to the external pressure.

AB characterizes the change in saturated vapor pressure in a pure solvent with t

CD characterizes the change in the saturated vapor pressure of the solvent above a solution with concentration C m 1, c t

C’D’ characterizes the change in the saturated vapor pressure of the solvent above a solution with a concentration of C m 2, C m 2 > C m 1

Conclusions:

1) all solutions boil at a temperature higher than the pure solvent

2) the increase in boiling point is directly proportional to the molal concentration of the solution.

∆T to =T to solution -T to solution

∆T to-increase in boiling point

∆T k =E*C m(E - ebullioscopic constant)

Physical meaning of the value E:

The ebullioscopic constant characterizes the increase in boiling t that would be observed if C

If C m =1 mol/kg*H 2 O, then E=∆T k

The value of E depends only on the nature of the solvent and does not depend on the nature of the reactant

E Н2о =0.51 degrees*kg/mol

When calculating ∆ T to temperatures are taken in ºС!!!

3) freezing point of the solution is t at which the saturated vapor pressure of the solvent above the solution becomes equal to the saturated vapor pressure above the ice.

MN characterizes the change in saturated vapor pressure above the ice with t.

1) all solutions freeze at a temperature lower than the pure solvent.

2) a decrease in freezing temperature is directly proportional to the solution.

∆T z = T z r-la - T z r-ra(∆T=0)

∆T z = K*C m

TO– crystalloscopic constant

If C m =1 mol/kg*H 2 O, then K= ∆ T z

K Н2о =1.86 degrees*kg/mol

Practical use of the properties of solutions to freeze at lower t:

1) for preparing cooling mixtures

2) in exchange for icy conditions, the road is sprinkled with ice and salt.

3) cryoscopic method for determining the molar mass of a dissolved substance:

Take a sample of the solvent, cool it with a mixture of ice and salt, and determine the freezing temperature using a special thermometer, which is called Beckmann thermometer . After this, the solvent is melted and a portion of the dissolved substance is added to it and the freezing point is also determined. Then calculate

∆T z = T z r-la - T z r-ra

∆T k =(K*m B *1000)/(M B *m A) and from this formula they calculate M B .

M B =(K*m B *1000)/(∆T to *m A)

4) osmosis and osmotic pressure

Let's consider the device of a simple osmometer. An osmometric cell is placed in a glass of water, which is closed from below with a semi-permeable membrane so that the sugar level and the water level are at the same level. The water rises and the level rises

Osmosis - This is the one-way spontaneous diffusion of solvent molecules through a semi-permeable membrane from a solution with a lower concentration to a solution with a higher concentration.

Osmotic pressure is equal to the hydrostatic pressure of a column of liquid of height h, which must be applied to the solution in order to delay osmosis.

R osm. =C M *R*T

CM=1 mol/m3

mol/l * 1000 = mol/m3

P osm. -> Pa (N/m 2)

T->K

Determining the amount by which the boiling point of solutions increases is called ebulliometry.

Determining the magnitude of the decrease in freezing t of a pure solvent and solution is called cryometry .

Van't Hoff's law: osmotic pressure ( R osm) is directly proportional to the molar concentration (c) and the absolute temperature of the solution (T):

R osm. =C M *R*T

Solutions that have the same osmotic pressure are called isotonic.

If two solutions have different osmotic pressures, then the solution with the higher osmotic pressure is hypertensive in relation to the second, and the second – hypotonic in relation to the first.

Electrolyte solutions have greater values ​​of all colligative properties than non-electrolytes.

Colligative properties of electrolyte solutions:

1) isotonic coefficient (i) – a value showing how many times the property of an electrolyte solution is greater than the property of a non-electrolyte solution of the same concentration:

i = c*R*T el. / c*R*T neel. =∆T deputy el. /∆T dep.neel. = ∆T boil.el. /∆T boil.neel.

Meaning i depends on the degree of dissociation ( α ) of a given electrolyte and the number of ions (v) formed during the dissociation of one molecule:

i = 1 + α (v – 1)

2) activity(s) is a quantity whose substitution instead of concentration in equations valid for ideal systems makes them applicable to solutions of strong electrolytes. It can be represented as the product of concentration (c) by some variable factor (f), called activity coefficient . those. a = f*c

The activity coefficient, which includes correction for interaction forces, is related to the ionic strength of the solution ( μ ) by the following relation: log f = -0.5Z*square root of μ.

where Z is the charge of the ion.

3) ionic strength electrolyte solution is equal to half the sum of the products of the concentrations (c) of each of the ions present in the solution by the square of their charge, i.e.

μ=1/2∑C 1 Z 1 2 =1/2(C 1 Z 1 2 + C 2 Z 2 2 +…+ C n Z n 2)

4) dissociation constant

In solutions of weak electrolytes, along with ions, there are undissociated molecules, i.e. equilibrium is observed: NA↔H + +A -

A characteristic of the strength of an electrolyte is the dissociation constant: To diss. =[A - ]/

The relationship between the dissociation constant and the electrolyte concentration and the degree of dissociation was established by Ostwald. Ostwald's dilution law: To diss. = cα 2 /(1-α)

For weak electrolytes it is very small and its value can be neglected.

Then: To diss. =α 2 c

2.10,11,12

Diffusion – a spontaneous process of leveling a substance in a solution.

From the point of view of thermodynamics, the cause of diffusion is the movement of a substance from a higher chemical potential to a lower one: μ(from 1)> μ(from 2), at с 1 >c 2

Diffusion stops when the concentration at all points of the solution becomes the same. In this case, the chemical potential at different points of the system becomes the same.

The rate of diffusion of a substance depends on the mass and shape of its molecules, as well as on the difference in the concentrations of this substance in different layers.

In 1855 Fick, While studying diffusion processes, he established a law: the rate of diffusion of a substance is proportional to the surface area through which the substance is transferred and the concentration gradient of this substance.

∆n/∆t= -D*S*∆c/∆x

∆n/∆t - diffusion rate, mol/s

S - surface area, m 2

∆c/∆x - concentration gradient, mol/m 2

D - proportionality coefficient or diffusion coefficient of the substance, m 2 / s

Einstein and regardless of it Smoluchowski deduced the following equation for diffusion coefficient: D=(RT/N A)*(1/6πηr)

R - universal gas constant, 8.31 J/(mol K)

T - absolute temperature, K

N A - Avogadro's constant, equal to 6.02 * 10 23 1/mol

r - radius of diffusing particles, m

D - diffusion coefficient, m 2 /s

η - medium viscosity, N*s/m 2

Squirrels (proteins, polypeptides) - high-molecular organic substances consisting of amino acids connected in a chain by peptide bonds.

Proteins are distinguished:

1) simple protein considered as a product of polycondensation of amino acids, i.e. as a specific natural polymer

2) complex proteins consist of simple protein and non-protein components - carbohydrates, nucleic acids, lipids and other compounds.

The pH value at which the protein is in isoelectric state , i.e. in a state in which the number of unlike charges in a protein particle is the same and its total charge is zero is called isoelectric point of this protein.

Salting out - this is the phenomenon of precipitation of dissolved BMC under the influence of a high concentration of electrolyte.

According to their salting out effect, all cations and anions can be arranged in lyotropic series :

The arrangement of ions in lyotropic rows is not related to the magnitude of their charge, as in the case of conventional coagulation, but to the degree of their hydration. The more an ion is able to bind a solvent, the greater its salting-out effect. The main role in salting out, as well as in swelling, belongs to anions, while cations have a lesser effect on salting out.

2.13,16,18,19,20,21

The properties of polymers change significantly with the addition of low molecular weight compounds. For example, if a cellophane film consisting of cellulose is moistened with glycerin, small molecules of glycerin penetrate into the space between the cellulose molecules and form a kind of lubricant. In this case, intermolecular bonds are weakened, and the film becomes more plastic.

Polymer plasticization - An increase in the plasticity of a polymer with a small amount of NMS is called.

Swelling and dissolution of the IUD. When the polymer (BMC) and the solvent (NMC) come into contact, swelling and then dissolution of the polymer occurs.

1)Swelling- penetration of the solvent into the polymer substance, accompanied by an increase in the volume and mass of the sample. Swelling is quantitatively measured by the degree of swelling:

The degree of swelling depends on the rigidity of the polymer chains. In rigid polymers with a large number of cross-links (cross-links) between the chains, the degree of swelling is low. For example, ebonites - highly vulcanized rubber - practically do not swell in benzene. Rubbers swell to a limited extent in gasoline. Gelatin in cold water is also characterized by limited swelling. Adding hot water to gelatin or benzene to natural rubber causes these polymers to swell indefinitely.

The influence of various factors on the degree of swelling:

1) The degree of swelling of the polymer depends on its nature and the nature of the solvent. The polymer swells better in a solvent, the molecular interactions of which with macromolecules are strong. Polar polymers swell in polar liquids (protein in water), nonpolar polymers swell in nonpolar liquids (rubber in benzene). Limited swelling is similar to limited solubility. As a result, jellies (limitedly swollen polymer) are formed.

2) In addition to the nature of the solvent, the presence of electrolytes influences the swelling of IUDs

3) pH of the environment

4) temperature.

2) The process of transition of a sol or solution of a polymer into a jelly is called gelatinization or gelling.

Factors influencing this process:

1) concentration(increasing concentration accelerates the gelatinization process)

2) nature of substances(not all hydrophobic sols can turn into gels, for example, sols of noble metals: gold, silver, platinum are not able to gel, which is explained by the peculiar structure and low concentration of their sols)

3) temperature(low temperatures promote gelation. Lowering the temperature accelerates the aggregation of particles and reduces the solubility of the substance)

4) process time(the process of gelation, even at low temperatures, requires a long time (from minutes to weeks) to form a cellular three-dimensional network. The time required for its formation is called the ripening period)

5) particle shape(gelation processes occur especially well in sols consisting of rod-shaped or ribbon-shaped particles)

6) electrolytes(variably affect the rate of gelatinization)

7) reaction of the environment(gelation occurs faster when the protein molecules have no electrical charge and are less hydrated, i.e. are in an isoelectric state)

The ability of many gels to liquefy under the influence of mechanical influences, turn into sols, and then gel again when at rest is called thixotropy .

3) IUD removal - release of IUDs from solution upon introduction of ions or non-electrolytes.

The least salting out effect will be exhibited by soft bases - anions I- and NCS- - weakly hydrated and well adsorbed on BMC molecules.

A decrease in the stability of the BMC solution is observed with a decrease in the lyophilicity of the polymer. Lyophilicity can be reduced not only by adding well-hydrated ions, but also by adding a solvent to the aqueous solution of BMC, in which the polymer is less soluble than in water. For example, ethanol has a salting-out effect on gelatin dissolved in water.

4) Coacervation - if the stability of the IUD solution is disrupted, the formation of coacervate - a new liquid phase enriched with polymer. The coacervate may be present in the original solution in the form of drops or form a continuous layer (delamination);

Coacervation occurs when the temperature or composition of the solution changes and is caused by a decrease in the mutual solubility of the solution components. The most studied is the coacervation of proteins and polysaccharides in aqueous solutions. According to one of the theories of the origin of life on Earth (A.I. Oparin), coacervates are the embryos of ancient life forms.

Use: for microencapsulation of drugs. To do this, the drug substance is dispersed in a polymer solution, and then, by changing the temperature or pH of the medium, evaporating part of the solvent or introducing a salting out agent, a phase enriched in the polymer is separated from the solution. Small droplets of this phase are deposited on the surface of capsules of dispersed particles, forming a continuous shell. Microencapsulation of drugs provides stability, prolongs the effect, and masks the unpleasant taste of drugs.

2.24,25,26,27

Viscosity– a measure of the medium’s resistance to movement. This value is characterized by the viscosity coefficient.

During evaporation, simultaneously with the transition of molecules from liquid to vapor, the reverse process also occurs. Moving randomly over the surface of the liquid, some of the molecules that left it return to the liquid again.

Saturated vapor pressure.

When saturated vapor is compressed, the temperature of which is maintained constant, the equilibrium will first begin to be disturbed: the density of the vapor will increase, and as a result, more molecules will pass from gas to liquid than from liquid to gas; this will continue until the vapor concentration in the new volume becomes the same, corresponding to the concentration of saturated vapor at a given temperature (and equilibrium is restored). This is explained by the fact that the number of molecules leaving the liquid per unit time depends only on temperature.

So, the concentration of molecules of saturated steam at a constant temperature does not depend on its volume.

Since the pressure of a gas is proportional to the concentration of its molecules, the pressure of saturated vapor does not depend on the volume it occupies. Pressure p 0, at which the liquid is in equilibrium with its vapor is called saturated steam pressure.

When saturated vapor is compressed, most of it turns into a liquid state. Liquid occupies less volume than vapor of the same mass. As a result, the volume of steam, while its density remains unchanged, decreases.

Dependence of saturated vapor pressure on temperature.

For an ideal gas, a linear dependence of pressure on temperature at constant volume is valid. As applied to saturated steam with pressure p 0 this dependence is expressed by the equality:

p 0 =nkT.

Since saturated vapor pressure does not depend on volume, it therefore depends only on temperature.

Experimentally determined dependence p0(T) differs from dependence ( p 0 =nkT) for an ideal gas.

With increasing temperature, the pressure of saturated vapor increases faster than the pressure of an ideal gas (section of the curve AB on the image). This becomes especially obvious if we draw an isochore through the point A(dashed line). This happens because when a liquid is heated, part of it turns into steam, and the density of the steam increases. Therefore, according to the formula ( p 0 =nkT), the saturated vapor pressure increases not only as a result of an increase in the temperature of the liquid, but also due to an increase in the concentration of molecules (density) of the vapor. The main difference in the behavior of an ideal gas and saturated vapor is the change in the mass of vapor with a change in temperature at a constant volume (in a closed vessel) or with a change in volume at a constant temperature. Nothing like this can happen with an ideal gas (the molecular kinetic theory of an ideal gas does not provide for the phase transition of gas into liquid).

After all the liquid has evaporated, the behavior of the vapor will correspond to the behavior of an ideal gas (section Sun curve in the figure above).

Unsaturated steam.

If in a space containing vapor of a liquid, further evaporation of this liquid can occur, then the vapor located in this space is unsaturated.

Vapor that is not in equilibrium with its liquid is called unsaturated.

Unsaturated vapor can be converted into liquid by simple compression. Once this transformation has begun, the vapor in equilibrium with the liquid becomes saturated.

Ticket No. 1

Saturated steam.

If a container with liquid is tightly closed, the amount of liquid will first decrease and then remain constant. At a constant temperature, the liquid-vapor system will come to a state of thermal equilibrium and will remain in it for as long as desired. 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 increase. The greater the density of the vapor, the greater the number of its molecules returning to the liquid. As a result, in a closed vessel at a constant temperature, a dynamic (mobile) equilibrium will be established between liquid and vapor, 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 during the same time.

Vapor that is in dynamic equilibrium with its liquid is called saturated vapor. This definition emphasizes that a larger amount of steam cannot exist in a given volume at a given temperature.

Saturated vapor pressure.

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

When the steam is compressed, the equilibrium will begin to be disturbed. At first, the vapor density will increase slightly, and a larger number of molecules will begin to move from gas to liquid than from liquid to gas. After all, the number of molecules leaving a liquid per unit time depends only on temperature, and compression of vapor does not change this number. The process continues until dynamic equilibrium and vapor density are established again, and therefore the concentration of its molecules takes on its previous values. Consequently, 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), it follows from this definition that the pressure of saturated vapor does not depend on the volume it occupies.

Pressure p n.p. vapor pressure at which a liquid is in equilibrium with its vapor is called saturated vapor pressure.

Dependence of saturated vapor pressure on temperature

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

As temperature increases, pressure increases. Since saturated vapor pressure does not depend on volume, it therefore depends only on temperature.

However, the dependence of p.n. from T, found experimentally, is not directly proportional, as in an ideal gas at constant volume. As temperature increases, 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 container, some of the liquid turns into steam. As a result, according to the formula P = nkT, the 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 steam. 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 steam will cease to be saturated upon further heating and its pressure at a constant volume will increase in direct proportion to the absolute temperature (see Fig., section of curve 23).

Boiling.

Boiling is an intense transition of a substance from a liquid 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 are formed throughout the entire volume of the liquid, which float to the surface. The boiling point of the liquid remains constant. This happens because all the energy supplied to the liquid is spent converting it into vapor.

Under what conditions does boiling begin?

A liquid always contains dissolved gases, released at the bottom and walls of the vessel, as well as on dust particles suspended in the liquid, which are centers of vaporization. The liquid vapors inside the bubbles are saturated. As the temperature increases, the saturated vapor pressure increases and the bubbles increase in size. Under the influence of buoyant force they float upward. If the upper layers of the liquid have a lower temperature, then vapor condensation occurs in bubbles in these layers. The pressure drops rapidly and the bubbles collapse. The collapse occurs so quickly that the walls of the bubble collide and produce something like an explosion. Many such micro-explosions create a characteristic noise. When the liquid warms up enough, the bubbles will 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 it boils.

The dependence of saturated 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 the temperature at which the saturated vapor pressure in the bubbles is equal to the pressure in the liquid.

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

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

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

Specific heat of vaporization.

Boiling occurs with the absorption of heat.

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

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

The amount of heat required to convert liquid into steam during the boiling process can be calculated using the formula:

where m is the mass of the liquid (kg),

L - specific heat of vaporization (J/kg)

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

[L] = 1 J/kg

Air humidity and its measurement.

There is almost always some amount of water vapor in the air around us. Air humidity depends on the amount of water vapor contained in it.

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

Relative air humidity is of great importance, messages about which are heard every day in weather forecast reports.

ABOUT
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, the steam in it can be brought to saturation, and then it will condense.

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

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

Dew point also characterizes air humidity.

Examples: dew falling in the morning, fogging up 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.

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

IN The psychrometer has two thermometers. One is ordinary, it is called dry. It measures the ambient air temperature. The bulb of another thermometer is wrapped in a fabric wick and placed in 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 thermometer. The lower the air humidity, the more intensely the moisture evaporates from the wick, the greater the amount of heat per unit time is removed from the moistened thermometer, the lower its readings, therefore, the greater the difference in the readings of the dry and moistened 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 We calculate the extensive characteristics of wet rich pair By...