Ray Oliver shows how evaporation causes temperatures to drop

26th May 2006 at 01:00
In the days before fridges were invented, keeping food fresh was a serious problem. One method relied on heat changes associated with changes of state, from liquid to vapour.

When liquids evaporate, their temperature falls. To keep milk fresh, the milk jug was placed in a shallow bowl of water. The jug was covered with a muslin cloth, the edges of which were dipped into the surrounding water. As the water soaked up into the cotton fabric, it evaporated more quickly, so lowering the temperature of the container and the milk.

People at remote campsites still use this cooling technique. The body uses the same strategy of evaporative cooling when you sweat after exercise.

Children can investigate evaporative cooling using two identical thermometers. Fix the thermometers vertically, side by side. Cover the bulb of one with dry cotton wool and the other with cotton wool soaked in water.

Direct a fan at the thermometers or leave in a draught. As the water evaporates from the cotton wool, the temperature falls. The energy needed to turn water into vapour is taken from its surroundings, in this case the thermometer bulb.

Most children will have noticed what happens when you splash a volatile liquid such as nail polish remover or aftershave lotion on your hand - it evaporates rapidly and the skin feels cold.

The solvents used in both products are very volatile; they boil at low temperatures, well below the boiling point of water, and the heat of the hand is sufficient to cause rapid evaporation, so the skin temperature falls.

Children can repeat the thermometer experiment, but using nail polish remover or alcohol in place of water. The more rapid evaporation should cause a larger fall in temperature.

An alternative is to stand a small container of ether in a saucer of very cold water. By blowing air through the ether - using a bicycle pump, for example - the ether evaporates fast enough for the water to turn to ice.

Be aware that ether vapour is highly flammable, so good ventilation is essential (ether was used in the 19th century for anaesthesia).

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