What Really Happens With Evaporation?

This is another of those fun bits of science that many of us think we understand until we really start to look at it, or even better, try to explain it to someone else. Then we reach a point where it becomes obvious to ourselves, and to our audience, that we don't understand it as well as we thought we did.

To try this, you will need:

• water
• two drinking glasses
• a saucer or cover for one of the glasses

Start by taking a glass of water outside. Find a nice, flat spot on your driveway or sidewalk, and pour out the water to make a big, wet spot. Now go for a walk, have a snack, read a chapter in your favorite book, have another snack, and then go back to look at the wet spot. Is it still there?

That will depend on the weather. If it is a very humid day, then it may still be there. If it is cold enough, your wet spot may still be there as a patch of ice. On the other hand, if it is a dry day, especially if it is warm or windy, then you will probably find that the water is all gone. Where did it go? It evaporated of course. But, what happens when a liquid evaporates?

When a substance evaporates, it changes from its liquid form to its gaseous form. Isn't that the same as boiling? No, water has to be hot for it to boil, yet it will evaporate even if the weather is near freezing. It will evaporate even if it is frozen into ice. How can that be? What is the difference between boiling and evaporating?

The basic process is the same. If the molecules of the liquid have enough energy, they can break away from the rest of the group, launching themselves into the air to become a gas. The difference between the boiling and evaporating is in where the molecules get that energy.

For boiling, the energy comes from heat. Put a pot of water on the stove, turn on the heat, and soon the water molecules in the pot will gain enough heat energy to let them break away in large numbers.

But, if you take that same pot of water, and put it on the table instead, it will still evaporate. Why? Those water molecules are bouncing around, bumping into each other. When they bump, energy can be transferred from one to another, just like the balls on a pool table. If the bumped molecule is in the middle of the pot, it will probably bump into another water molecule, passing along the energy, but if it happens to be at the surface, that bump could give it enough energy to break free. It evaporated!

Notice that you did not have to heat the water to the boiling point. Even if the water is very cold, you will still have molecules at the surface that get bumped hard enough to let them evaporate.

Now, for the next step, fill two glasses half-full of water (or half-empty if you happen to look at things that way.) Put them someplace where they can stay for a few days without being in the way. Cover one with the saucer, and leave the other open.

After a few days, what do you think will happen? More importantly, why will it happen?

The water in the uncovered glass will probably evaporate away, but the water in the covered glass will still be there. Why?

Does covering the glass stop the process of evaporation? No.

Does the air become saturated so that it cannot hold any more water? No.

Instead, it increases something else to balance the evaporation. Condensation.

Think back to that molecule of water that was bumped free. It is now bouncing around with the other molecules in the air. If it bounces in the right direction, it could bump back into the surface of the water. If that happens, it can stick, giving up some of its extra energy, and changing back into the liquid form of water.

In the covered glass, the water continues to evaporate just as quickly as it does in the open glass. Because the glass is closed, the number of water molecules in the air increases, meaning more and more of them will bump back into the water, changing back to a liquid. You quickly reach the point where things balance. There are just as many water molecules condensing back to water as there are evaporating into the air. So covering the glass does not stop it from evaporating. Instead, it keeps the water vapor in place, so it can bump back into the liquid again.

Now, thinking about that, why would a windy day make your water evaporate faster? If the air is still, then it is easy for a water molecule to be bumped free, rebound from an air molecule, and rejoin the water. On the other hand, if the air is moving, the water molecule may be moved away from the liquid before it bounces back. The water molecules don't leave any faster. They just have a much smaller chance of bouncing back to the liquid, so the puddle dries up faster.

Now for one of the biggest misconceptions on this topic. Many sources will say that the air in the glass becomes saturated, and cannot hold any more water, so no more water can evaporate. Thinking back to what you just read, you should see why that concept is wrong. Water molecules continue to evaporate no matter how much water vapor is in the air. Instead of becoming saturated and stopping evaporation, you reach a point where there are just as many water molecules rejoining the water as there are leaving it.

Avoiding that misconception makes it much easier to understand how evaporation and condensation can vary. If the combination of air temperature, water temperature, air pressure, and the relative humidity of the air allows more molecules to leave than return, then the net amount of liquid will decrease due to evaporation. If that combination of factors lets more water molecules join the water than are escaping through evaporation, then the net amount of liquid will increase due to condensation. If the combination of factors causes a relatively equal number of molecules to leave and rejoin the liquid, then you have a balance where the amount of liquid will remain the same.

Something that may be even more surprising is that the process keeps happening even after water is frozen. Water molecules continue to leave and return to the surface of the ice, but since cold air usually has a very low humidity, it is usual for many more molecules to leave. This is the cause of the "freezer burn" that happens to meat that is left in your freezer too long, and is also the basis for freeze drying foods. If you ever go on a long vacation, you many also see the results in any ice cubes you left in their trays in the freezer. After several weeks, you will probably find that the cubes have shrunk. Technically, the change from solid to gas is called sublimation instead of evaporation, but the basic process is the same.