Boiling

Because we know bubbling water is at a temperature of 100°C without using a thermometer we need to differentiate between this state and very hot water when describing cooking in water, hence words like ‘simmering’ and ‘poaching’.

To be accurate, it is only right to say that water boils at 100°C if you qualify the statement with the words ‘at sea level’. Every 300 metres (1,000 feet) you go up, the boiling point reduces by 1°C because the atmospheric pressure on the surface of the water gets less. Mountaineers know that over about 1,500 metres (5,000 ft) their cup of tea is going to be insipid, rather like the cup which fails to cheer abroad when hot water is brought to the table for you to dunk a tea bag. The higher the pressure, the more energy needed by water molecules to vaporize and escape from the surface. It is this principle which is utilized in pressure cookers, which boil water in an airtight container, preventing steam from escaping and thereby increasing the cooking temperature.