When the Earth begins to get too hot, the water increases in temperature. As the water continues to get hotter, it begins to undergo a phase change in which water turns to gas. The water droplets turn into water vapor because the temperature has caused the individual water molecules to speed up considerably. This process of heating water up and turning it into vapor is known as evaporation.
Evaporation should not be confused with boiling because when water undergoes evaporation, only water molecules that are on the surface of the water are actually turning into water vapor. Under conditions of boiling, the entire water has reached a complete phase change and therefore, the water is being turned into gas at a much faster rate. The steam that is rising off of a pot of boiling water is water vapor evaporating.
Conditions that Affect Evaporation
While evaporation is a straight forward process of liquid water turning into water vapor, there are other conditions that can have an effect on the rate of evaporation. These conditions are:
- Pressure: The higher the pressure, the slower the evaporation because there is more pressure on the surface of the water keeping the molecules in place. If pressure is removed, the molecules can more freely launch themselves.
- Concentration: If the air is already very humid, it will take much longer for a cup of water to evaporate than if there is no water in the air at all. The more concentration, the slower the evaporation rate.
- Density: If a substance is more dense, the evaporation rate is slower. This is why alcohol evaporates faster than water, while water evaporates faster than mercury. Alcohol is less dense than water which is less dense than mercury.
- Surface Area: If all of the heat is only hitting an area 1 inch by 1 inch, there is only going to be room for 1 inch by 1 inch of molecules to make the phase change. If heat is hitting 1 mile by 1 mile, more water vapor will be formed. The more surface area, the faster evaporation.
While evaporation is always going on, it was found that there is only about 3 millimeters of water evaporated daily on average. In warmer areas, the amount might be more. In colder areas, the amount might be less. It is because of evaporation that the Earth doesn't get too hot. The heat on the Earth rises up and cools as it goes higher into the atmosphere.