Evaporation

2013-12-23_waterToday we have a question posed by an 8th grade science class.

Water goes from a liquid to a gas at its boiling point, so why does water turn into a gas even at room temperature?

To understand what’s going on here, a few topics need to be covered including energy, evaporation, and boiling.

Energy in this context is measured as heat, and temperature is the based on the average energy of whatever is being measured.

Evaporation is the process of a liquid turning into a gas. This happens with every liquid, from water to mercury — as long as there is not too much of it already in the air. When too much of it accumulates in the air in one place, the process runs in reverse and you get condensation. Some solids turn directly into gas as well in a process called sublimation. You’ve probably heard of this with solid/frozen carbon dioxide, also known as dry ice. The opposite of sublimation is deposition.

Boiling happens when enough the energy in a liquid is high enough to cause it to suddenly turn into a gas and evaporate, sometimes violently. Barring some unique cases of superheating and supercooling, a liquid can never go above or below a certain temperature at a certain pressure (I’ll explain the pressure caveat in a moment). A cup of water typically cannot go above 212°F (or 100°C) and it cannot go below 32°F (or 0°C). Once the boiling point is reached, the water will stay at that temperature as the high energy molecules leave (sometimes rapidly). A high pressure can allow a liquid to reach an even higher temperature, such as in a car’s radiator, which is why when a radiator is opened and the pressure drops, the liquid inside instantly evaporates, causing a dangerous explosion of water vapor.

Getting back to our question, how does water evaporate when it is not boiling?

The highest energy water molecules (remember that the temperature is an average of all of the molecules) are constantly escaping as water vapor or gas. The only way to stop this from happening is to freeze the water, trapping most of the molecules in ice.

Even when a liquid does not have enough energy to boil, the few molecules with enough energy (or heat) can escape the liquid, leaving behind their lower energy companions (see the graphic). The loss of these high energy molecules causes the average energy of the liquid to fall, lowering the temperature. This is why evaporation causes cooling — the molecules with the most energy leave, taking that energy with them. This is what happens anywhere there is water, whether it is in the ocean or the sweat on your skin.

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