EVAPORATION OF LIQUIDS

Sometimes a liquid can be sitting in one place (maybe a puddle) and its molecules will become a gas. That's the process called evaporation. It can happen when liquids are cold or when they are warm. It happens more often with warmer liquids. Evaporation is all about the energy in individual molecules, not about the average energy of a system. The average energy can be low and the evaporation still continues.

You might be wondering how that can happen when the temperature is low. It turns out that all liquids can evaporate at room temperature and normal air pressure. Evaporation happens when atoms or molecules escape from the liquid and turn into a vapor. Not all of the molecules in a liquid actually have the same energy.

The energy you can measure with a thermometer is really an average of all the molecules in the system. There are always a few molecules with a lot of energy and some with barely any energy at all. The molecules with a lot of energy are able to build up enough power to become a gas. Once they reach that energy level, they can leave the liquid. When the molecule leaves, it has evaporated.

The rate of evaporation can also increase with a decrease in the gas pressure around a liquid. Molecules like to move from areas of higher pressure to lower pressure. The molecules are basically sucked into the surrounding area to even out the pressure. Once the vapor pressure of the area increases to a specific level, the rate of evaporation will slow down.