41. Boiling

As the temperature of the liquid increases, the evaporation rate increases. Finally, at a certain temperature, the liquid begins to boil. When boiling, inside the liquid a vapour bubbles appear throughout the entire volume, floating upwards. The liquid temperature at boiling point remains constant. A thermometer placed in boiling water will show approximately \(100^0C\), even though the heat is continuously supplied to the water. The temperature at which the liquid boils is called the boiling temperature.

Find out the conditions under which the liquid begins to boil. There are always dissolved gases in the liquid, which are released in the form of small bubbles on the bottom and walls of the container and on the suspended in the liquid dust. Liquid vapours, which are also present inside the bubbles, are saturated. As the temperature increases, the pressure of saturated vapours in the liquid increases and the bubbles increase in size. Under the influence of the ejection force, they float upwards. If the upper layers of the liquid have a lower temperature, the vapour in the bubbles condenses in these layers. The pressure drops rapidly and the bubbles collapse. The process of collapsing is so fast that the bubble walls collide and produce something like a small explosion. Many of these micro-explosions create an unique noise. When the liquid warms up enough, the bubbles stop collapsing and float to the surface. The liquid boils. Observe the kettle on the stove carefully. You will immediately find that it stops making noise before boiling.

Obviously, a vapour bubble can only grow when the pressure of the saturated vapour inside is greater than the external pressure. Boiling starts at a temperature at which the pressure of saturated vapour in the bubbles is compared to the external pressure. Therefore, the boiling of a liquid is a process of evaporation that takes place not only from the surface of the liquid, but also inside the bubbles throughout the liquid.

The higher the external pressure, the higher the boiling temperature. For example, at the pressure in the steam boiler, which reaches 16 atmosphere, the water does not boil at the temperature of \(200^0C\). In medical facilities, boiling water in hermetically closed containers also occurs at high pressure. Therefore, the boiling temperature is much higher than \(100^0C\). It allows to kill all microbes.

On the contrary, by reducing the pressure, we reduce the boiling temperature. Under the air pump bell, water can be forced to boil at room temperature. Atmospheric pressure decreases when climbing into the mountains. Therefore, the boiling temperature decreases. At the height of \(7134 ~m\) the pressure is approximately equal to \(300 ~mm ~Hg\) (millimetre of mercury). The boiling temperature of water is about \(70^0C\). For example, meat cannot be boiled under these conditions.

The difference in boiling temperatures of different liquids is determined by the difference in pressure of their saturated vapours at the same temperature. The higher the pressure of saturated vapour, the lower the boiling temperature of the corresponding liquid. Thus, at \(100^0C\) the pressure of saturated vapor of water is equal to \(760 ~mm ~Hg\), and mercury is only \(0.88 ~mm ~Hg\). Mercury boils at \(357^0C\) if the external pressure is equal to one atmosphere.

The table shows the heat of vaporization of some liquids at boiling temperatures at normal atmospheric pressure. The heat of water vaporization is particularly high, as can be seen from the table.

Substance	Specific heat of vaporization, kJ/kg
Water	\(2260\)
Alcohol (ethyl)	\(860\)
Nitric acid	\(480\)
Ether	\(360\)
Mercury	\(290\)
Kerosene	\(210\)

Liquid can also exist at temperatures above boiling point. This liquid is called overheated. For this purpose, the liquid must be thoroughly cleaned beforehand and free from gas. In such a liquid, there are almost no vaporization centers - the tiniest air bubbles, which are germs of vapor bubbles.

The overheated liquid is in an unstable state. It is enough to add, for example, chalk powder containing a large amount of air in its pores, as the liquid boils so rapidly that it partially spills out of the container. The temperature of the liquid drops to a boiling point, as the conversion of the liquid into vapour requires heat.