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The dependence of the boiling point of the liquid from external pressure. Boiling point.

Because saturation vapor pressure is uniquely determined by the temperature, and boiling of a liquid occurs at the moment, when the saturating vapor pressure of the liquid equals the external pressure, the boiling point must depend on external pressure. Through experiments it is easy to show, what if you reduce the external pressure the boiling point decreases, if you increase the pressure increases.

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Boiling liquids at reduced pressure can be shown with the following experience. Pour in a glass of tap water I put the thermometer. A glass of water is placed under the glass cover of the vacuum Assembly and includes a pump. When the pressure under the cap enough will fall, the water in the beaker starts to boil. Since evaporation requires an energy of, the temperature of the water in the Cup when boiling begins to decrease, and with good pump water finally freezes.

The water is heated to a high temperature is carried out in boilers and autoclaves. The device of the autoclave is shown in figure. 8.6, where K—safety valve, AB — lever, pressing the valve, M — pressure gauge. At pressures more 100 ATM the water is heated to a temperature above 300°C.

The boiling point of the liquid at normal atmospheric pressure is called boiling point. From table. 8.1 and 8.2 see, the pressure of saturating vapour of ether, water and alcohol in the boiling point is equal to 1,013 *105-PA (1 ATM).

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

IMG_20140517_022003_crExperience shows, what is the boiling point of solution always higher, than the boiling point of pure solvent, and increases with increasing the concentration of the solution. However, the vapor temperature above the boiling surface of the solution equal to the boiling point of pure solvent. Therefore, to determine the boiling point of the pure liquid thermometer is better not to put in the liquid, and in pairs over the surface of the boiling liquid.

The boiling process is closely linked to the presence of dissolved gas in the liquid. If the liquid to remove dissolved gas, for example, prolonged boiling, it is possible to heat the liquid to a temperature, significantly hotter than its boiling. This liquid is called superheated. In the absence of gas bubbles to the birth of the tiny steam bubbles, which could become centers of vaporization, prevents laplacesche pressure, which with a small radius of the bubble is large. This explains the overheating of the fluid. When it boils, the boiling takes place very rapidly.

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