Vaporization pressure. Boiling

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

Process

The very process of converting the state of aggregation from liquid to gaseous is complex. And although people don't 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 in the container where the water is heated.
2. In the second stage, the volume of the bubbles increases. All of them begin to tear to the surface, since they contain saturated steam, which is lighter than water. With an increase in the heating temperature, the pressure of the bubbles increases, and they are pushed out to the surface due to the known force of Archimedes. At the same time, you can hear the characteristic boiling sound, which is formed due to the constant expansion and decrease 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 a cloudy water. This process is popularly called "boiling with a white key", and it lasts for 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, splashing means 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, it, like other gases, puts a certain pressure on it. During vaporization, the temperatures of steam and water remain constant until all the 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 evaporation of all the liquid, becomes dry. If its temperature begins to exceed the temperature of water, then such steam is superheated, and by its characteristics it will be closer to gas.

Salt water boiling

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 a region between water molecules. This is how the chemical composition of water with salt differs from ordinary 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 the boiling of a liquid with dissolved salt will take longer. As the temperature rises, the molecules in the salt-containing water move faster, but there are fewer of them, which makes collisions between them less frequent. As a result, less steam is generated, and its pressure is therefore lower than the steam pressure of fresh water. Consequently, more energy (temperature) is required for full-fledged vaporization. On average, to boil one liter of water containing 60 grams of salt, it is necessary to raise the boiling degree of water by 10% (that is, by 10 C).

Boiling pressure versus pressure

It is known that in the mountains, regardless of the chemical composition of the water, the boiling point will be lower. This is due to the fact that the atmospheric pressure is lower at altitude. Pressure with a value of 101.325 kPa is considered normal. With it, the boiling point of water is 100 degrees Celsius. But if you go up the 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 have to spend almost half the time. For thermal processing of products, a certain temperature is required.

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 temperature will be 90 C.

Note that this law also works in the opposite direction. If you place a liquid in a closed flask through which steam cannot pass, then with an increase in temperature and the formation of steam, the pressure in this flask will increase, and boiling at an increased 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 the teapot

Often people are interested in the temperature at which water boils in a kettle, since it is these devices that they use to boil liquid. Taking into account the fact that the atmospheric pressure in the apartment is equal to the standard, and the water used does not contain salts and other impurities that should not be there, then the boiling temperature 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 fluid composition affect this process. There is nothing difficult in this, and children receive such information at school. The main thing is to remember 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, which 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 at institutes.

Boiling -it is vaporization that occurs in the volume of the entire liquid at a constant temperature.

The evaporation process can occur not only from the surface of the liquid, but also inside the liquid. Vapor bubbles inside the liquid expand and float to the surface if the saturated vapor pressure is equal to or higher than the external pressure. This process is called boiling. While the liquid is boiling, its temperature remains constant.

At a temperature of 100 ° C, the pressure of saturated water vapor is equal to normal atmospheric pressure, therefore, at normal pressure, water boils at 100 ° C. At 80 ° C, the saturated vapor pressure is about half the normal atmospheric pressure. Therefore, water boils at 80 ° C, if the pressure above it is reduced to 0.5 normal atmospheric pressure (figure).

With a decrease in external pressure, the boiling point of the liquid decreases; with an increase in pressure, the boiling point rises.

Boiling point of liquid- This is the temperature at which the pressure of saturated vapor in the liquid bubbles is equal to the external pressure on its surface.

Critical temperature.

B1861 DI Mendeleev established that for each liquid there must be such a temperature at which the difference between the liquid and its vapor disappears. Mendeleev named her temperature of absolute boiling (critical temperature). There is no fundamental difference between gas and steam. Usually gas called a substance in a gaseous state when its temperature is higher than the critical one, and ferry- when the temperature is below critical.

The critical temperature of a substance is the temperature at which the density of the liquid and the density of its saturated vapor become the same.

Any substance in a gaseous state can turn into a liquid. However, each substance can undergo such a transformation only at temperatures less than a certain value, specific for each substance, called the critical temperature T k. At temperatures higher than the critical temperature, the substance does not turn into a liquid at any pressure.

The ideal gas model is applicable to describe the properties of gases actually existing in nature in a limited range of temperatures and pressures. When the temperature drops below the critical value for a given gas, the action of the forces of attraction between the molecules can no longer be neglected, and at a sufficiently high pressure, the molecules of the substance combine with each other.

If a substance is at a critical temperature and critical pressure, then its state is called a critical state.

(When the water is heated, the air dissolved in it is released at the walls of the vessel and the number of bubbles continuously increases, and their volume increases. With a sufficiently large volume of the bubble, the Archimedes force acting on it tears it off the bottom surface and lifts it up, and in the place of the detached bubble, the embryo of a new one remains Since when the liquid is heated from below, its upper layers are colder than the lower ones, when the bubble rises, water vapor condenses in it, and air dissolves in water again and the volume of the bubble decreases.Many bubbles disappear before reaching the water surface, and some reach the surface There is very little air and vapor in them at this point. This happens until, due to convection, the temperature in the entire liquid becomes the same. When the temperature in the liquid is equalized, the volume of the bubbles will increase during the rise. . This is explained as follows. When the same temperature is established in the entire liquid and the bubble rises up, the pressure of saturated vapor inside the bubble remains constant, and the hydrostatic pressure (pressure of the upper layer of the liquid) decreases, so the bubble grows. The entire space inside the bubble is filled with saturated vapor as it grows. When such a bubble reaches the surface of the liquid, the pressure of the saturated vapor in it is equal to the atmospheric pressure on the surface of the liquid.)

TASKS

1.Relative air humidity at 20 ° С is 58%. What is the maximum temperature for dew to fall?

2. How much water must be evaporated in 1000 ml of air, the relative humidity of which is 40% at 283 K, in order to humidify it to 40% at 290 K?

3. Air at a temperature of 303 K has a dew point at 286 K. Determine the absolute and relative humidity of the air.

4.At 28 ° C, the relative humidity is 50%. Determine the mass of dew dropped from 1 km3 of air when the temperature drops to 12 ° С.

5. In a room with a volume of 200 m3, the relative humidity at 20 ° C is 70%. Determine the mass of water vapor in the air of the room.

“And a smart person should think sometimes” 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. Let's say that for the preparation of a product and the destruction of all dangerous living creatures, including botulism spores, we need a temperature of 120 ° C. In a simple saucepan, such a temperature cannot be obtained, the 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 an airtight container from the pan, that is, an autoclave. Using the table, we determine the pressure at which the water will boil at 120 ° C. This pressure is equal to 2 kgf / cm². But this is absolute pressure, and we need gauge pressure, most of the 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. = R beats. + P bar, then the overpressure in the autoclave must be at least P g = P abs. - R bar. = 2-1 = 1 kgf / cm 2. This is what we see in the above figure. The principle of operation is that due to the injection of an overpressure of 0.1 MPa. when heated, the sterilization temperature of canned food 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 by V.P. Vukalovich

V.P. Vukalovich's table

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

The dependence of the boiling point of water on pressure is directly proportional, that is, the higher the pressure, the higher the boiling point. For a better understanding of this dependence, 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 hot water 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 water hammer in water heating networks. Why is there a crackling sound in the local heating systems of a private house and how to avoid it?

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

Why did a person begin to boil water before using it directly? Correctly, to protect yourself from many pathogenic bacteria and viruses. This tradition came to the territory of medieval Russia even before Peter the Great, although it is believed that it was he who brought the first samovar into the country and introduced the rite of unhurried evening tea drinking. In fact, our people used a kind of samovar even in ancient Russia to make drinks from herbs, berries and roots. Boiling was required here mainly for the extraction of useful plant extracts, rather than for disinfection. Indeed, at that time it was not even known about the microcosm where these bacteria with viruses live. However, thanks to boiling, our country was bypassed by global pandemics of terrible diseases such as cholera or diphtheria.

Celsius

The great meteorologist, geologist and astronomer from Sweden originally used a value of 100 degrees to indicate the freezing point of water under normal conditions, and the boiling point of water was taken as zero degrees. And after his death in 1744, a no less famous person, botanist Karl Linnaeus and Celsius' successor Morten Stremer, turned this scale upside down for ease of use. However, according to other sources, this was done by Celsius himself shortly before his death. But in any case, the stability of the readings and clear graduation influenced the widespread use of its use among the most prestigious scientific professions at that time - chemists. And, despite the fact that in the inverted form, the scale mark of 100 degrees established the point of stable boiling of water, and not the beginning of its freezing, the scale began to bear the name of its primary creator, Celsius.

Below the atmosphere

However, not everything is as simple as it seems at first glance. Looking at any state diagram in P-T- or P-S-coordinates (entropy S is a direct function of temperature), we will see how closely temperature and pressure are related. Likewise, water, depending on the pressure, changes its values. And any climber is well aware of this property. Anyone who has at least once in his life attained an altitude of over 2000-3000 meters above sea level knows how hard it is to breathe at an altitude. This is due to the fact that the higher we go, the thinner the air becomes. Atmospheric pressure drops below one atmosphere (below normal conditions, that is, below "normal conditions"). The boiling point of water also drops. Depending on the pressure at each of the heights, it can boil at both eighty and sixty

Pressure cookers

However, it should be remembered that although the main microbes die at temperatures above sixty degrees Celsius, many can survive at eighty or more degrees. That is why we are trying to boil water, that is, we bring its temperature to 100 ° C. However, there are interesting kitchen appliances that allow you to shorten the time and heat the liquid to high temperatures, without boiling it and losing mass through evaporation. Realizing that the boiling point of water can change depending on pressure, engineers from the United States, based on a French prototype, introduced a pressure cooker to the world in the 1920s. The principle of its operation is based on the fact that the lid is tightly pressed against the walls, without the possibility of steam removal. An increased pressure is created inside, and the water boils at higher temperatures. However, such devices are quite dangerous and often lead to an explosion and serious burns to users.

Perfectly

Let's look at how the process itself comes and goes. Imagine a perfectly smooth and infinitely large heating surface, where the distribution of heat occurs evenly (the same amount of heat energy is supplied to each square millimeter of the surface), and the surface roughness coefficient tends to zero. In this case, with n. at. boiling in a laminar boundary layer will begin simultaneously over the entire surface area and occur instantly, immediately evaporating the entire unit volume of liquid on its surface. These are ideal conditions, in real life this does not happen.

In the reality

Let's find out what the initial boiling point of water is. Depending on the pressure, it also changes its values, but the main point here is this. Even if we take the smoothest, in our opinion, pan and bring it under a microscope, then in its eyepiece we will see uneven edges and sharp frequent peaks protruding above the main surface. We assume that heat is supplied evenly to the surface of the pan, although in reality this is also not entirely true. Even when the pan is on the largest burner, the temperature gradient on the stove is unevenly distributed, and there are always local overheating zones responsible for the early boiling of water. How many degrees are there at the peaks of the surface and in its lowlands? The peaks of the surface with an uninterrupted supply of heat warm up faster than lowlands and so-called depressions. Moreover, surrounded on all sides by low-temperature water, they better give energy to water molecules. The thermal diffusivity of the peaks is one and a half to two times higher than that of the lowlands.

Temperatures

That is why the initial boiling point of water is about eighty degrees Celsius. At this value, the peaks of the surface bring in a sufficient amount necessary for the instantaneous boiling of the liquid and the formation of the first bubbles visible to the eye, which timidly begin to rise to the surface. And what is the boiling point of water at normal pressure - many ask. The answer to this question can be easily found in the tables. At atmospheric pressure, stable boiling is established at 99.9839 ° C.

Boiling- This is vaporization that occurs simultaneously both from the surface and throughout the volume of the liquid. It consists in the fact that numerous bubbles rise and burst, causing a characteristic seething.

Experience shows that boiling of a liquid at a given external pressure begins at a quite definite temperature that does not change during boiling and can only occur when energy is supplied from the outside as a result of heat exchange (Fig. 1):

where L is the specific heat of vaporization at the boiling point.

Boiling mechanism: there is always a dissolved gas in a liquid, the degree of dissolution of which decreases with increasing temperature. In addition, there is adsorbed gas on the walls of the vessel. When the liquid is heated from below (Fig. 2), the gas begins to evolve in the form of bubbles at the walls of the vessel. The liquid evaporates into these bubbles. Therefore, in addition to air, they contain saturated steam, the pressure of which rapidly increases with increasing temperature, and the bubbles grow in volume, and therefore, the Archimedes forces acting on them increase. When the buoyancy force becomes greater than the gravity of the bubble, it begins to float. But until the liquid is evenly heated, as the bubble ascends, the volume of the bubble decreases (the saturated vapor pressure decreases with decreasing temperature) and, not reaching the free surface, the bubbles disappear (collapse) (Fig. 2, a), which is why we hear a characteristic noise in front of boiling. When the temperature of the liquid is equalized, the volume of the bubble will increase as it rises, since the saturated vapor pressure does not change, and the external pressure on the bubble, which is the sum of the hydrostatic pressure of the liquid above the bubble and the atmospheric pressure, decreases. The bubble reaches the free surface of the liquid, bursts, and the saturated vapor comes out (Fig. 2, b) - the liquid boils. The saturated vapor pressure in the bubbles is practically equal to the external pressure.

The temperature at which the pressure of the saturated vapor of a liquid is equal to the external pressure on its free surface is called boiling point liquids.

Since the saturated vapor pressure increases with increasing temperature, and during boiling it should be equal to the external one, then with an increase in external pressure, the boiling point increases.

The boiling point also depends on the presence of impurities, usually increasing with increasing concentration of impurities.

If you first free the liquid from the gas dissolved in it, then it can be overheated, i.e. heat above boiling point. This is an unstable fluid state. A little shaking is enough and the liquid boils, and its temperature immediately drops to the boiling point.