John Foster, age 15, of E1 Paso, Texas, for his question:

WHAT CAUSES EVAPORATION?

Evaporation is a gradual change of a liquid into a gas without boiling. The term is sometimes applied also to the gradual change of a solid directly into a gas, as in the "evaporation" of dry ice, which is solid carbon dioxide. But this process is more properly called sublimation.

The molecules of any liquid are in a constant state of motion. In any particular liquid, the average speed of the molecules depends only on the temperature, but individual molecules may be moving at a speed far greater or far less than the average.

At temperatures below the boiling point, individual molecules approaching the surface with above average speed may have enough energy to escape from the surface and pass into the space above as gas molecules. This is evaporation. As only the fastest molecules escape, the average speed of the remaining molecules is lowered. As the temperature depends only on the average speed of the molecules, the temperature of the remaining liquid is also lowered.

If a liquid evaporates in a closed vessel, the space above the liquid rapidly becomes filled with vapor, and evaporation is soon balanced by the opposite process: condensation.