The boiling point of water is the temperature at which its vapour pressure equals the ambient pressure. The result of this is that bubbles of water vapour are able to form within a body of liquid water. (This in no way distinguishes it from the boiling point of any other liquid.) Thus the boiling point depends on the ambient pressure, and is higher than normal near the Dead Sea, and lower than normal at high altitudes. It even varies slightly with the weather, for which reason eggs should be boiled for a little longer during a hurricane. (Tornadoes are unlikely to affect the cooking conditions for long enough to make an appreciable difference.)

When a body of water has been heated to the boiling point of same, the effect of any further heat energy being introduced into it is no longer an increase in its temperature, but an increase in the rate at which the water turns into water vapour. The extra energy goes into overcoming the force of attraction between the water molecules that tends to keep the water in the liquid phase, and it is referred to as 'latent heat of condensation', since it is the heat that will be given up by water vapour when it condenses. Thus the boiling point is the highest temperature at which liquid water can exist under normal circumstances.

The constancy and general predictability of the temperature of boiling water makes it a useful medium for cooking: immersing a particular kind of food in it for a given period of time will generally lead to similar results. If this period of time is such that given your schedule your guests are likely to have fainted from inanition before the food has reached the desired state, you may consider using a pressure cooker. This retains some of the water vapour produced by the water boiling inside it, leading to an increase in pressure within the pressure cooker, which leads to an increase in the boiling point of the remaining water. The food is cooked at a higher temperature and is therefore ready more quickly. An alternative use, once popular in England, is to ensure that no life or vitamins remain in vegetables intended for human consumption. Another possible use would be making a decent cup of tea at high altitudes, but mountaineers tend to object to the extra weight.

Since the creation of vapour bubbles requires a force to be exerted against the surface tension of the inner water surface of the new bubble, they, like the bubbles in champagne and Fanta, cannot form without a nucleation point – an imperfection in the container, or a particle floating in the liquid – and it is therefore possible for pure water in a perfectly smooth container to enter a 'supercritical state' in which its temperature rises above boiling point because the water is physically unable to start boiling. The addition of a teaspoon of salt to a sample of water in this state can be entertaining.1 The addition of salt or another soluble material at an earlier stage can increase the boiling point of the water: substances dissolve because of the attraction of their molecules or ions to water molecules. This attraction increases the amount of energy required to remove water molecules from the solution.

Supercritical water can also be produced by heating it in a closed system under pressure and then allowing it to escape from that system. Doing this in the general direction of a turbine attached to a dynamo can be very useful. It is a more effective way of delivering energy than allowing the water to boil and then producing a jet of water vapour because the energy density by volume of the water is greater by far than that of the equivalent mass of steam.

1. Check your accident insurance and make your will before playing with supercritical water. Apparently microwave ovens have a tendency to produce it.

No more inspiration for interesting additions will be considered. Thank-you filoraene, rootbeer277, wetperch, and mkb, who got in quickly enough.

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