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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When water vaporizes it takes energy – in this case, heat – from the surface it’s evaporating from. When this happens, the temperature of the surface decreases, thus cooling it. In the case of a heat stroke, it cools the affected person’s body when the water vaporizes from their skin.
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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.
When water vaporizes from a surface, it absorbs heat from that surface in order to undergo the change from a liquid to a gas. This transfer of heat from the surface to the water molecules causes the surface to cool.
This process is known as evaporative cooling, and it is the same principle that allows your body to cool down when you sweat. As the sweat on your skin evaporates, it absorbs heat from your body, causing you to feel cooler.
Temperature is a description of the average molecular kinetic energy of a substance (water in this case).
The “average” aspect is very important in this case because not all of the molecules are moving with the same speed. Some are faster, some are slower.
The faster molecules are usually prevented from escaping the rest of the water because they bump into other molecules along the way, transfering their energy. However, occassionally, a molecule gets enough velocity and is close enough to the surface to escape the rest of the water and become a molecule of water vapor. That molecule has a higher portion of the energy of the total volume of the water, and takes that energy with it, resulting in the average energy (and thus temperature) of the water falling.
Now, one molecule isn’t going to take a whole lot of energy with it, but there are a *lot* of molecules of water in a drop of water, so you have millions of molecules escaping this way and taking their energy with them, resulting in a noticable drop in the temperature of the remaining liquid water.
The short answer is that the water molecules that leave the liquid into the gas (the vaporized molecules) are the ones with the most energy (that’s why they are able to escape the liquid). You take the most energetic molecules out, and the remaining molecules are on average less energetic (i.e. colder).
I don’t think anyone here is answering the heart of your question. The important thing about treating someone from heat stroke is to cool them down. The reason getting them wet isn’t enough is the same reason that being sweaty is either refreshing or makes your hot situation worse. Moisture conducts heat, it can trap heat in, or conduct heat away, the difference is whether there is wind or ventilation. If there is no wind, you’re just trapping heat in. If there is wind, the wind takes the heat from the water on the skin, and the water quickly takes the heat from the skin. Rinse and repeat this and you have a rapidly cooling cycle that is a much more efficient heat transfer than either just blowing air over skin or dumping water on skin.
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