When you heat up a liquid, it evaporates and turns into a gas. But it takes energy to pull a molecule out of the liquid and turn it into a gas, since the liquid molecules stick together and it's kind of hard to pull them apart. That leaves a little less energy for keeping all the molecules jiggling around. In other words, they actually do cool down a little bit just as they evaporate. So, left to itself, the liquid would cool down as it evaporates.
This doesn't mean that steam is cooler than water, though. If you gradually raise the temperature of a pot of water, the water boils off and turns into a gas (steam). Up to a certain temperature, it is a liquid. When it reaches the boiling point, adding more heat just boils off more of the liquid, not raising the temperature. After the liquid has boiled off, as you add more heat the gas can get a lot hotter than it could as a liquid.
Evaporating liquids can cool other things, too. For instance, "evaporative cooling" is a system that is often used to cool small buildings. A fan blows the warm air in the building through a screen covered with cold water. As the warm air goes through, it heats up the water, causing it to evaporate. The heat goes from the air into the water, so the air gets cooler. A similar thing happens when a liquid sits on your skin (say as sweat). Heat will then flow from your skin into the liquid, evaporating the sweat. So you cool down.
-Tamara + Mike W.
(republished on 07/24/06)
(published on 04/16/07)
(published on 08/13/07)