Why does water evaporate at room temperature?

01.04.2015 |

Episode #7 of the course “Science questions everyone should know how to answer”

We all know that water boils at 212ºF (100ºC). When water boils, steam rises into the air. We all also know that this is called evaporation. So that would lead us to believe that water evaporates at 212ºF. We should then be able to conclude that if I spill a drop of water on the kitchen counter or the floor and it evaporates, then it must be 212ºF (100ºC) in my house, right? Wrong. Clearly, water does evaporate at 212ºF, but it also evaporates at room temperature. Don’t worry, you don’t have to live in a house that’s a scalding 212ºF just for water to disappear on its own.

When answering this question, it’s helpful to think of temperature as kinetic energy—that is, energy that is transferred. While the average water molecule doesn’t have enough energy to break the intermolecular bonds until 212ºF (100ºC), a non-average water molecule does. These non-average molecules exist at the surface of water. In many cases, the very top layer of water ends up receiving enough kinetic energy to break free from the dipole attraction that water molecules have with each other. This is simply due to the random nature of molecular motion. Of course, as the surface layer of water evaporates at room temperature, it exposes a new layer, which in turn becomes the new surface layer. This process continues until every last drop of water has had its turn as the surface layer and is exposed to random kinetic energy from its surroundings.

Interestingly, the reverse of this is also true—water vapors can exist at 32ºF (0ºC). The surface of the water moving in towards the center will convert to ice at 32ºF, but at the very core, water vapors can still exist.


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