How does the boiling point of a liquid depend on pressure? Boiling and evaporation of water. The dependence of the boiling point on pressure. Evaporation of solids

Using the phenomenon of liquid cooling during its evaporation; 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 Vaporization occurs from the free surface of a liquid.

How does evaporation take place? We know that the molecules of any liquid are in continuous and chaotic motion, with some moving faster and others slower. Forces of attraction to each other prevent them from flying out. If, however, a molecule with a sufficiently large kinetic energy appears near the surface of the liquid, then it can 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 vapor is evaporation.

Since the fastest molecules escape from the liquid during evaporation, the average kinetic energy of the molecules remaining in the liquid becomes smaller and smaller. As a result the temperature of the evaporating liquid decreases: liquid is cooled. That is why, in particular, a person in wet clothes feels colder than in dry clothes (especially when it is windy).

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 continuously cool, becoming more and more cold until it freezes. What prevents this? The answer is very simple: the heat exchange of water with warm air surrounding the glass.

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

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

The 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) - a lower temperature due to the intensity of evaporation from wet batiste. The drier the air whose humidity is being measured, the stronger the evaporation and therefore the lower the wet-bulb reading. And vice versa, the higher the humidity of the air, the less intense the evaporation and therefore the higher the temperature this thermometer shows. Based on the readings of dry and wetted thermometers, using a special (psychrometric) table, the air humidity, expressed as a percentage, is determined. The highest humidity is 100% (at this humidity, dew appears on objects). For a person, the most favorable humidity is considered to be in the range from 40 to 60%.

With the help of simple experiments, it is easy to establish that the evaporation rate increases with an increase in the temperature of the liquid, as well as with an increase in its free surface area and in the presence of wind.

Why does liquid evaporate faster in the presence of wind? The fact is that simultaneously with evaporation on the surface of the liquid, the reverse process occurs - condensation. Condensation occurs due to the fact that part of the vapor molecules, randomly moving above the liquid, returns to it again. The wind carries away the molecules that have flown 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: the molecules of water vapor rising above the earth in the colder layers of the atmosphere are grouped into tiny droplets of water, the accumulations of which are clouds. The condensation of water vapor in the atmosphere also causes rain and dew.

Boiling temperature versus pressure

The boiling point of water is 100°C; one might think that this is an inherent property of water, that water, wherever and under what conditions it is, will always boil at 100 ° C.

But this is not so, and the inhabitants 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 sometimes wonder "how difficult it is to boil an egg in boiling water" or "why boiling water does not burn." Under these conditions, they are told that water boils at the top of Elbrus already at 82°C.

What is the matter here? What physical factor interferes with the phenomenon of boiling? What is the significance of altitude?

This physical factor is the pressure acting on the surface of the liquid. You do not need to climb to the top of the mountain to check the validity of what has been said.

By placing heated water under the bell and pumping air in or out of it, one can be convinced that the boiling point rises with increasing pressure and falls with decreasing pressure.

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 freshen up 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 make the water 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 the pumping continues, so will the pressure. Therefore, the boiling will not stop, the water will continue to cool and eventually freeze.

Such boiling of cold water occurs not only when air is pumped out. For example, when a ship's propeller rotates, the pressure in a layer of water rapidly moving near a metal surface drops sharply 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 lowering the pressure, we lower the boiling point. What about increasing it? A graph like ours answers this question. A pressure of 15 atm can delay the boiling of water, it will only start at 200°C, and a pressure of 80 atm will make water boil only at 300°C.

So, a certain external pressure corresponds to a certain boiling point. But this statement can also be "turned over", 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 the curve of vapor pressure as a function of temperature.

Figures plotted on a boiling point graph (or vapor pressure graph) show that vapor pressure changes very rapidly with temperature. At 0°C (i.e., 273 K), the vapor pressure is 4.6 mm Hg. Art., at 100 ° C (373 K) it is equal to 760 mm Hg. Art., i.e. increases by 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. up to almost 60 atm, i.e., about 10,000 times.

Therefore, on the contrary, the boiling point changes rather slowly with pressure. When the pressure is doubled 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 is doubled 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 (thickening) of steam into water.

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

Both during boiling and during condensation, the point will not move off the curve until the conversion of steam to water or water to steam is complete. This can also be formulated as follows: under the conditions of our curve, and only under these conditions, the coexistence of liquid and vapor is possible. If at the same time no heat is added or taken away, then the quantities of vapor and liquid in a closed vessel will remain unchanged. Such vapor and liquid are said to be in equilibrium, and a vapor in equilibrium with its liquid is said to be saturated.

The curve of boiling and condensation, 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 upwards (toward higher temperatures and lower pressures) is the region of the steady state of steam. To the right and down - the region of the stable state of the liquid.

The vapor-liquid equilibrium curve, i.e., the dependence of the boiling point on pressure or, what 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 always the vapor pressure increases rapidly with increasing temperature.

We have used the words "gas" and "steam" many times. These two words are pretty much the same. We can say: water gas is the vapor of water, gas oxygen is the vapor of an oxygen liquid. Nevertheless, some habit has developed in the use of 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 pressure at ordinary temperatures is above atmospheric pressure. On the contrary, we speak of a vapor when, at room temperature and atmospheric pressure, the substance is more stable in the form of a liquid.

Since the pressure of the saturating vapor is uniquely determined by the temperature, and the boiling of a liquid occurs at the moment when the pressure of the saturating vapors of this liquid is equal to the external pressure, the boiling temperature must depend on the external pressure. With the help of experiments, it is easy to show that with a decrease in external pressure, the boiling point decreases, and with an increase in pressure, it rises.

The boiling of a liquid under reduced pressure can be shown using the following experiment. Pour tap water into a glass and lower a thermometer into it. A glass of water is placed under the glass dome of the vacuum unit and the pump is turned on. When the pressure under the cap drops sufficiently, the water in the glass begins to boil. Since energy is expended on vaporization, the temperature of the water in the glass begins to decrease during boiling, and when the pump works well, the water finally freezes.

Water is heated to high temperatures in boilers and autoclaves. The autoclave device is shown in fig. 8.6, where K is a safety valve, is a lever pressing the valve, M is a pressure gauge. At pressures greater than 100 atm, water is heated to temperatures above 300 °C.

Table 8.2. Boiling points of some substances

The boiling point of a liquid at normal atmospheric pressure is called the boiling point. From Table. 8.1 and 8.2 it is clear that the saturated vapor pressure for ether, water and alcohol at the boiling point is 1.013 105 Pa (1 atm).

It follows from the above that in deep mines water should boil at a temperature above 100 °C, and in mountainous areas - below 100 °C. Since the boiling point of water depends on the height above sea level, on the scale of the thermometer, instead of temperature, you can indicate the height at which water boils at this temperature. Determination of height using such a thermometer is called hypsometry.

Experience shows that the boiling point of a solution is always higher than the boiling point of a pure solvent, and increases with increasing concentration of the solution. However, the vapor temperature above the surface of a boiling solution is equal to the boiling point of a pure solvent. Therefore, to determine the boiling point of a pure liquid, it is better to place a thermometer not in a liquid, but in a vapor above the surface of a boiling liquid.

The boiling process is closely related to the presence of dissolved gas in the liquid. If the gas dissolved in it is removed from the liquid, for example, by prolonged boiling, then this liquid can be heated to a temperature significantly higher than its boiling point. Such a liquid is called superheated. In the absence of gas bubbles, the formation of the smallest vapor bubbles, which could become centers of vaporization, is prevented by the Laplace pressure, which is large for a small bubble radius. This explains the overheating of the liquid. When it does boil, it boils very violently.

“And a smart person should sometimes think” Gennady Malkin

In everyday life, using the example of the operation of an autoclave, one can trace the dependence of the boiling point of water on pressure. Suppose, for the preparation of the product and the destruction of all dangerous living creatures, including botulism spores, we need a temperature of 120 ° C. In a simple saucepan, this temperature cannot be obtained; water will simply boil at 100 ° C. That's right, at an atmospheric pressure of 1 kgf / cm² (760 mm Hg), water will boil at 100 ° C. In a word, we need to make a hermetic container out of the pan, that is, an autoclave. According to the table, we determine the pressure at which water boils at 120 ° C. This pressure is 2 kgf/cm². But this is absolute pressure, and we need a gauge pressure, most gauges show excess pressure. Since the absolute pressure is equal to the sum of the excess (P g) and barometric (P bar.), i.e. R abs. = P ex. + P bar, then the overpressure in the autoclave must be at least P g = P abs. - R bar. \u003d 2-1 \u003d 1 kgf / cm 2. Which is what we see in the figure above. The principle of operation is that due to the injection of an excess pressure of 0.1 MPa. when heated, the temperature of sterilization of canned products increases to 110-120°C, and the water inside the autoclave does not boil.

The dependence of the boiling point of water on pressure is presented in the table of V.P. Vukalovich

Table V.P. Vukalovich

R	t	i /	i //	r
0,010	6,7	6,7	600,2	593,5
0,050	32,6	32,6	611,5	578,9
0,10	45,5	45,5	617,0	571,6
0,20	59,7	59,7	623,1	563,4
0,30	68,7	68,7	626,8	558,1
0,40	75,4	75,4	629,5	554,1
0,50	80,9	80,9	631,6	550,7
0,60	85,5	85,5	633,5	548,0
0,70	89,5	89,5	635,1	545,6
0,80	93,0	93.1	636,4	543,3
0,90	96,2	96,3	637,6	541,3
1,0	99,1	99,2	638,8	539,6
1,5	110,8	111,0	643,1	532,1
2,0	119,6	120,0	646,3	526,4
2,5	126,8	127,2	648,7	521,5
3,0	132,9	133,4	650,7	517,3
3,5	138,2	138,9	652,4	513,5
4,0	142,9	143,7	653,9	510,2
4,5	147,2	148,1	655,2	507,1
5,0	151,1	152,1	656,3	504,2
6,0	158,1	159,3	658,3	498,9
7,0	164,2	165,7	659,9	494,2
8,0	169,6	171,4	661,2	489,8

P - absolute pressure in atm, kgf / cm 2; t is the temperature in o C; i / – enthalpy of boiling water, kcal/kg; i // – enthalpy of dry saturated steam, kcal/kg; r is the latent heat of vaporization, kcal/kg.

The dependence of the boiling point of water on pressure is directly proportional, that is, the greater the pressure, the greater the boiling point. To better understand this dependency, you are invited to answer the following questions:

1. What is superheated water? What is the maximum water temperature possible in your boiler room?

2. What determines the pressure at which your boiler operates?

3. Give examples of using the dependence of the boiling point of water on the pressure in your boiler room.

4. Causes of hydraulic shocks in water heating networks. Why is crackling heard in the local heating systems of a private house and how to avoid it?

5. And finally, what is the latent heat of vaporization? Why do we experience, under certain conditions, unbearable heat in the Russian bath and leave the steam room. Although the temperature in the steam room is not more than 60 ° C.

>>Physics: Dependence of saturation vapor pressure on temperature. Boiling

The liquid doesn't just evaporate. It boils at a certain temperature.
Saturated vapor pressure versus temperature. The state of saturated steam, as experience shows (we talked about this in the previous paragraph), is approximately described by the equation of state of an ideal gas (10.4), and its pressure is determined by the formula

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).
. 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). As you climb mountains, atmospheric 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.

???
1. Why does the boiling point increase with increasing pressure?
2. Why is it essential for boiling to increase the pressure of saturated vapor in the bubbles, and not to increase the pressure of the air present in them?
3. How to make a liquid boil by cooling the vessel? (This is a tricky question.)

G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics Grade 10

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Boiling is the process of changing the aggregate state of a substance. When we talk about water, we mean the change from liquid to vapor. It is important to note that boiling is not evaporation, which can occur even at room temperature. Also, do not confuse with boiling, which is the process of heating water to a certain temperature. Now that we have understood the concepts, we can determine at what temperature water boils.

Process

The very process of transforming the state of aggregation from liquid to gaseous is complex. And although people do not see it, there are 4 stages:

1. In the first stage, small bubbles form at the bottom of the heated container. They can also be seen on the sides or on the surface of the water. They are formed due to the expansion of air bubbles, which are always present in the cracks of the tank, where the water is heated.
2. In the second stage, the volume of the bubbles increases. All of them begin to rush to the surface, as there is saturated steam inside them, which is lighter than water. With an increase in the heating temperature, the pressure of the bubbles increases, and they are pushed to the surface due to the well-known Archimedes force. In this case, you can hear the characteristic sound of boiling, which is formed due to the constant expansion and reduction in the size of the bubbles.
3. In the third stage, a large number of bubbles can be seen on the surface. This initially creates cloudiness in the water. This process is popularly called "boiling with a white key", and it lasts a short period of time.
4. At the fourth stage, the water boils intensively, large bursting bubbles appear on the surface, and splashes may appear. Most often, splashes mean that the liquid has reached its maximum temperature. Steam will start to come out of the water.

It is known that water boils at a temperature of 100 degrees, which is possible only at the fourth stage.

Steam temperature

Steam is one of the states of water. When it enters the air, then, like other gases, it exerts a certain pressure on it. During vaporization, the temperature of steam and water remains constant until the entire liquid changes its state of aggregation. This phenomenon can be explained by the fact that during boiling all the energy is spent on converting water into steam.

At the very beginning of boiling, moist saturated steam is formed, which, after the evaporation of all the liquid, becomes dry. If its temperature begins to exceed the temperature of water, then such steam is superheated, and in terms of its characteristics it will be closer to gas.

Boiling salt water

It is interesting enough to know at what temperature water with a high salt content boils. It is known that it should be higher due to the content of Na+ and Cl- ions in the composition, which occupy an area between water molecules. This chemical composition of water with salt differs from the usual fresh liquid.

The fact is that in salt water a hydration reaction takes place - the process of attaching water molecules to salt ions. The bond between fresh water molecules is weaker than those formed during hydration, so boiling liquid with dissolved salt will take longer. As the temperature rises, the molecules in water containing salt move faster, but there are fewer of them, which is why collisions between them occur less frequently. As a result, less steam is produced and its pressure is therefore lower than the steam head of fresh water. Therefore, more energy (temperature) is required for full vaporization. On average, to boil one liter of water containing 60 grams of salt, it is necessary to raise the boiling point of water by 10% (that is, by 10 C).

Boiling pressure dependences

It is known that in the mountains, regardless of the chemical composition of water, the boiling point will be lower. This is because the atmospheric pressure is lower at altitude. Normal pressure is considered to be 101.325 kPa. With it, the boiling point of water is 100 degrees Celsius. But if you climb a mountain, where the pressure is on average 40 kPa, then the water will boil there at 75.88 C. But this does not mean that cooking in the mountains will take almost half the time. For heat treatment of products, a certain temperature is needed.

It is believed that at an altitude of 500 meters above sea level, water will boil at 98.3 C, and at an altitude of 3000 meters, the boiling point will be 90 C.

Note that this law also works in the opposite direction. If a liquid is placed in a closed flask through which vapor cannot pass, then as the temperature rises and steam is formed, the pressure in this flask will increase, and boiling at elevated pressure will occur at a higher temperature. For example, at a pressure of 490.3 kPa, the boiling point of water will be 151 C.

Boiling distilled water

Distilled water is purified water without any impurities. It is often used for medical or technical purposes. Given that there are no impurities in such water, it is not used for cooking. It is interesting to note that distilled water boils faster than ordinary fresh water, but the boiling point remains the same - 100 degrees. However, the difference in boiling time will be minimal - only a fraction of a second.

in a teapot

Often people are interested in what temperature water boils in a kettle, since it is these devices that they use to boil liquids. Taking into account the fact that the atmospheric pressure in the apartment is equal to the standard one, and the water used does not contain salts and other impurities that should not be there, then the boiling point will also be standard - 100 degrees. But if the water contains salt, then the boiling point, as we already know, will be higher.

Conclusion

Now you know at what temperature water boils, and how atmospheric pressure and the composition of the liquid affect this process. There is nothing complicated in this, and children receive such information at school. The main thing to remember is that with a decrease in pressure, the boiling point of the liquid also decreases, and with its increase, it also increases.

On the Internet, you can find many different tables that indicate the dependence of the boiling point of a liquid on atmospheric pressure. They are available to everyone and are actively used by schoolchildren, students and even teachers in institutes.