The concept of boiling point

The boiling point is the temperature when the saturated vapor pressure of a liquid is equal to the external pressure, and the boiling points of different liquids are different. The boiling point changes with the change of external pressure, and the pressure is low, so is the boiling point.

Basic definition

When the liquid boils, the saturated vapor pressure in the bubbles formed inside it must be equal to the external pressure, so the bubbles can grow and rise. So the boiling point is the temperature when the saturated vapor pressure of the liquid is equal to the external pressure. The boiling point of a liquid is related to the external pressure. When the pressure on a liquid increases, its boiling point rises; When the pressure decreases; Boiling point decreases. For example, the steam pressure in a steam boiler is about tens of atmospheres, and the boiling point of water in the boiler can be above 200℃. Another example is cooking in the mountains. Water is easy to cook, but rice is not easy to cook. This is because the atmospheric pressure decreases with the elevation of the terrain, and the boiling point of water gradually decreases with the elevation. (elevation1900m, atmospheric pressure is about 79,800 Pa (600 mm Hg), and the boiling point of water is 93.5℃). Generally, those with low boiling point are vaporized first, and those with high boiling point are generally difficult to vaporize.

Different liquids

At the same atmospheric pressure, different kinds of liquids have different boiling points. This is because the saturation pressure of water vapor is related to the type of liquid. At a certain temperature, the water vapor saturation pressure of various liquids is also certain. For example, the saturated pressure of ether at 20℃ is 5865.2 Pa (44 cm Hg) lower than the atmospheric pressure, and the water vapor saturated pressure of ether is equal to the atmospheric pressure with a slight increase in temperature. When ether is heated to 35℃, it will boil. If the liquid contains impurities, it will also affect the boiling point of the liquid. The boiling point of liquid containing solute is higher than that of pure liquid. This is because after the solute exists, the attraction between liquid molecules increases, the liquid is not easy to vaporize, and the water vapor saturation pressure is small. To make the saturated pressure of water vapor equal to atmospheric pressure, the boiling point must be raised. Different liquids have different boiling points under the same external pressure. The relationship between boiling point and pressure can be obtained by clausius equation.

Boiling point classification

Boiling point/degree Celsius of some different liquids (at standard atmospheric pressure)

Boiling points of various elements

(The previous serial number is the atomic number)

74- liquid tungsten: 5927

76- liquid osmium: 5300

6- liquid carbon: 4827

92- liquid uranium: 38 18

22- liquid titanium: 3260

27- liquid cobalt: 2870

79- liquid gold: 2807

26- liquid iron: 2750