This is another of those fun bits of science that many of us think we understand until we really start to look at it. 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? But, water has to be hot for it to boil, yet it will evaporate even if the weather is near freezing. 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? 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. 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.

Have a wonder-filled week.

This week's experiment got its start while I was reading Craig F. Bohren's "What Light Through Yonder Window Breaks." It is a book on atmospheric physics, and is written so that you don't have to be a physics professor (or even a physics student) to understand and enjoy it. He writes about the dew that forms on your house windows in the winter, which made me think of other questions about dew drops.

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