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Have you ever wondered why vases dry up and then had someone tell you it is from evaporation? Was your next question ‘what is evaporation?’ Evaporation is the vaporization of a liquid, but it only occurs on the surface of the liquid. Evaporation is an important part of the water cycle.
For of a liquid to evaporate, it molecules must have three things going on: be located near the surface, be moving in the proper direction, and have sufficient kinetic energy to break free. Since only a small number of molecules meet these criteria, the rate of evaporation is limited. Evaporation happens faster at higher temperatures. As evaporation occurs the liquid cools, so evaporation occurs in cycles. That is how evaporating sweat cools your body. Heat, humidity and air movement affect the rate of evaporation. Even at lower temperatures, individual molecules of a liquid can evaporate if they have more than the minimum amount of kinetic energy required for vaporization.
Evaporation is an essential part of the water cycle here on Earth. Solar energy(heat) drives evaporation from oceans, lakes, soil moisture, and any other exposed source of water. This water is drawn up into the atmosphere. Once the clouds are saturated, the water is returned to the surface in the form of rain, ice, or snow.
When evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapor. Much of the vapor will return to the liquid, the return happens more quickly as the density and pressure of the vapor increases. When the escape and return is at equilibrium the vapor is saturated and no more change in vapor pressure and density will occur.
Liquids that do not evaporate visibly at certain temperatures(cooking oil at room temperature) have molecules that tend to transfer energy in a way that is insufficient to frequently give a molecule the energy to turn into vapor. These liquids are evaporating, but the process is much slower and less visible.
You can see evidence of evaporation everywhere around you. Humans were using it to their advantage long before they understood it. You can see the ultimate result of it when it rains and the cycle starts anew.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.