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Boiling by cooling

The normal way to bring water to the boil is to 'put the kettle on', meaning: add heat to the water to raise the temperature to the boiling point. In this experiment water will be boiling not by heating it, but, on the contrary, by cooling it.

Water is poured into a vessel and is, in the conventional way, heated to a fairly high temperature (but under the normal boiling point). Then the heating is stopped and the opening of the vessel is shut off with a plug.

The vessel is cooled down by pooring cold water over it. Observe how the water starts to boil.

Explanation 1

By heating the vessel, the space above the fluid is filled with hot water vapour. After sealing and cooling, the vapour pressure above the fluid drops quickly, while the fluid stays hot. Because of this low pressure above the fluid, the water starts to boil at a lower temperature than 100 °C.

Explanation 2

Boiling of a fluid means that the pressure of the vapour is equal to (or slightly greater than) the pressure of the surroundings. In the case of boiling water, the pressure of the water-vapour should be at least atmospheric pressure. Under normal atmospheric circumstances the temperature of the water must then be 100 °C.

When the pressure of the air above the fluid is lower than normal atmospheric pressure (like high up in the mountains), the pressure of the water-vapour needs also be lower for the fluid to boil. This lower pressure of the vapour is achieved at likewise lower temperatures.

By heating the water in the vessel, the air in the vessel is superseded by water-vapour of fairly high temperature, say about 95 °C. The water is not boiling at this moment.

When the vessel is taken away from the heat source and when it is sealed, no air or vapour can come in or out. The cooling of the vessel causes the pressure of the vapour to drop quickly to a low value (the vapour pressure drops much faster with decreasing temperature than a gas would do, i.e. much faster than according to a linear dependence). Because water-fluid has a greater specific heat than water-vapour, the temperature of the water is scarcely lowered.

Because of this temperature of the water, the vapour pressure in the water is higher than the pressure of the vapour above the water. This makes the fluid boil, although the boiling temperature is well below 100 °C.

In a pressure cooker the opposite occurs. By closing and sealing the lid, and by continuously heating, the vapour pressure can rise to a higher value, permitting a higher boiling temperature than 100 °C for the water. (The safety valve sets a limit for the pressure and the temperature).