Dependence of the boiling point on external factors. What determines the boiling of water. Boiling distilled water

To prepare various delicious dishes, water is often needed, and if it is heated, then sooner or later it will boil. Each educated person at the same time, he knows that water begins to boil at a temperature of one hundred degrees Celsius, and with further heating, its temperature does not change. It is this property of water that is used in cooking. However, not everyone knows that this is not always the case. Water can boil when different temperatures depending on the conditions in which it is located. Let's try to figure out what the boiling point of water depends on, and how it should be used.

When heated, the water temperature approaches the boiling point, and numerous bubbles form throughout the volume, inside which water vapor is located. The vapor density is less than the density of water, so the Archimedes force acting on the bubbles raises them to the surface. In this case, the volume of the bubbles either increases or decreases, so the boiling water emits characteristic sounds. Reaching the surface, the bubbles with water vapor burst, for this reason the boiling water gurgles intensively, releasing water vapor.

The boiling point explicitly depends on the pressure exerted on the water surface, which is explained by the dependence of the pressure saturated steam located in the bubbles on the temperature. In this case, the amount of steam inside the bubbles, and with it their volume, increases until the pressure of saturated steam exceeds the pressure of water. This pressure is the sum of the hydrostatic pressure of water due to gravitational attraction to the Earth and external atmospheric pressure. Therefore, the boiling point of water increases with an increase in atmospheric pressure and decreases with a decrease. Only in the case of normal atmospheric pressure of 760 mm Hg. (1 atm.) Water boils at 100 0 C. The graph of the dependence of the boiling point of water on atmospheric pressure is presented below:

The graph shows that if you increase Atmosphere pressure up to 1.45 atm, then the water will boil already at 110 0 C. At an air pressure of 2.0 atm. water will boil at 120 0 С and so on. Increasing the boiling point of water can be used to speed up and improve the cooking process for hot foods. For this, pressure cookers were invented - pans with a special hermetically sealed lid, equipped with special valves to regulate the boiling temperature. Due to the tightness, the pressure in them rises to 2-3 atm., Which ensures the boiling point of water 120-130 0 С. high fever... Therefore, you need to be as careful as possible so as not to get burned.

The opposite effect is observed if the atmospheric pressure decreases. In this case, the boiling point also decreases, which happens with an increase in altitude above sea level:

On average, with a rise of 300 m, the boiling point of water decreases by 1 0 C and drops quite high in the mountains to 80 0 C, which can lead to some difficulties in cooking.

If the pressure is further reduced, for example, by pumping out air from a vessel with water, then at an air pressure of 0.03 atm. water will boil at room temperature, and this is quite unusual, since the usual boiling point of water is 100 0 C.

When boiling, the liquid begins to intensively turn into vapor, vapor bubbles are formed in it, rising to the surface. When heated, at first, steam appears only on the surface of the liquid, then this process begins throughout the volume. Small bubbles appear on the bottom and walls of the dish. As the temperature rises, the pressure inside the bubbles increases, they increase and rise upward.

When the temperature reaches the so-called boiling point, bubbles begin to form violently, there are a lot of them, the liquid boils. Steam is formed, the temperature of which remains constant until all the water is present. If vaporization occurs under normal conditions, at a standard pressure of 100 MPa, its temperature is 100 ° C. If the pressure is artificially increased, you can get superheated steam... Scientists managed to heat water vapor to a temperature of 1227 ° C, with further heating, the dissociation of ions turns the vapor into plasma.

At a given composition and constant pressure, the boiling point of any liquid is constant. In textbooks and manuals, you can see tables indicating the boiling points of various liquids and even metals. For example, water boils at a temperature of 100 ° C, at 78.3 ° C, ether at 34.6 ° C, gold at 2600 ° C, and silver at 1950 ° C. This data is for a standard pressure of 100 MPa and is calculated at sea level.

How to change the boiling point

If the pressure decreases, the boiling point decreases, even if the composition remains the same. This means that if you climb a mountain 4000 meters high with a pot of water and put it on a fire, the water will boil at 85 ° C, which will require much less firewood than below.

Housewives will be interested in the comparison with a pressure cooker, in which the pressure is artificially increased. The boiling point of the water also increases, due to which food is cooked much faster. Modern pressure cookers allow you to smoothly change the boiling point from 115 to 130 ° C and more.

Another secret of the boiling point of water is its composition. Hard water, which contains various salts, takes longer to boil and requires more energy to heat up. If you add two tablespoons of salt to a liter of water, its boiling point will increase by 10 ° C. The same can be said for sugar, 10% sugar syrup boils at a temperature of 100.1 ° C.

It is clear from the above reasoning that the boiling point of a liquid should depend on the external pressure. Observations confirm this.

The higher 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, boiling of water in hermetically sealed vessels - autoclaves (Fig. 6.11) also occurs when high blood pressure... Therefore, the boiling point is significantly higher than 100 ° C. Autoclaves are used to sterilize surgical instruments, dressings, etc.

Conversely, by decreasing the external pressure, we thereby lower the boiling point. Under the bell of an air pump, you can make water boil at room temperature (fig. 6.12). When climbing 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). The water boils there at about 70 ° C. For example, it is impossible to cook meat under these conditions.

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

Difference in boiling points of liquids

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

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

A liquid boils when the pressure of its saturated vapor is equal to the pressure inside the liquid.

§ 6.6. Heat of vaporization

Does it take energy to convert a liquid to vapor? Probably yes! Is not it?

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

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

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

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

In addition, with an increase in the volume of a substance, work is performed against the forces of external pressure. This part of the heat of vaporization at room temperature is usually a few percent of the total heat of vaporization.

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

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

(6.6.1)

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

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

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), 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.