Rough Science Colorado: Inside the science - clean

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When the air temperature reaches 25 ^oC or more we feel hot and begin to sweat. This is one way in which animals, including humans, lose water. But the real purpose of sweating, however, is not to lose water, but to cool the body. Sweating itself doesn’t produce cooling. If you wrap up in tight clothes in a hot place, then sweating doesn’t cool you down. Sweat needs to be able to evaporate, and it’s the evaporation of the water - the change from a liquid to a vapour - that produces the cooling effect.

In order to understand this cooling effect, let’s first think about what happens when you heat water. You can do this in all kinds of ways: in an electric kettle or using various fuels - for example, gas, paraffin or wood. These all provide energy, in one form or another, and the energy they supply will raise the temperature of a kettle of water. But let’s focus on an electric kettle: the temperature of the water rises steadily as the heating element in the kettle provides energy to the water. When the water reaches its normal boiling temperature, 100 ^oC, the temperature stops rising; it stays at 100^oC while the water boils merrily away. Energy is still being supplied to the water - your electricity meter shows this - but now, rather than raising the temperature of the water, the energy supplied is being used to convert the water to vapour, that is to evaporate it. This demonstrates that it takes energy to raise the temperature of a liquid (or a solid) and it also takes energy to evaporate a liquid, and if we’re using an electric kettle then the energy comes from the electricity supply.

So what is the connection between an electric kettle of boiling water and the process of sweating? Sweat on the skin will evaporate into the air. Unlike the boiling water in the kettle, the skin isn’t at 100 ^oC, but energy is still required in order for the water to evaporate. The energy to evaporate the sweat comes from the body. We usually refer to the energy that is supplied in this way as heat, so the body has to provide heat to evaporate the sweat from the skin. The loss of heat from the surface of the body to the water vapour means that the skin cools down. This cooling effect is even more noticeable when you put perfume or aftershave on the skin; your skin immediately feels cool because it supplies heat to evaporate the liquid.

You may be aware of applications of the principle that evaporation of water produces a cooling effect. Suppose you don’t have a refrigerator and are expecting friends for a meal. You want to cool a bottle of wine or fruit juice; how would you do it? A very effective way is to put a wet towel round the bottle and put it on an outdoor window ledge. The water evaporates from the towel, and the heat required to evaporate the water comes from the bottle. So the evaporation of the water from the towel cools the wine or fruit juice in the same way that evaporation of your sweat cools you down on a hot day.

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