I’ve read that when someone is having a heat stroke just making them wet isn’t enough, you have to ventilate them so the water vaporazes, cooling them. But why does water vaporizing cools something?
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> But why does water vaporizing cools something?
There is an amount of energy required for substances to change state. In the case of a liquid turning to a gas it is called the “**enthalpy of vaporization**”, or with a solid turning to a liquid the “enthalpy of fusion”.
This amount of heat required to transition the liquid to vapor doesn’t change the temperature of the water, it just enables the phase change. In the case of water that amount of energy is quite large, and as it transitions it sucks this necessary heat up from its surroundings. As a result a person with a thin layer of evaporating water on them will be cooled as it pulls heat from them in order to evaporate.
Remember that water doesn’t need to reach the boiling point in order to evaporate. It just needs to be exposed to dry enough air so it is below the vapor pressure equilibrium. All liquids have some amount of tendency for particles to escape from them and evaporate into gas, and the vapor pressure is the pressure exerted by this vapor when at the same temperature as its liquid. You can basically think of this as a measure of the tendency of the liquid to evaporate (alcohol has a higher vapor pressure than mercury for example). So if there isn’t enough water in the air to reach the vapor pressure of water at the given temperature then it will evaporate, and that evaporation necessarily sucks a significant amount of heat out of the surrounding material, namely the person with heat stroke.
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