# How air evaporates from wet clothes

## Why do things get dry (i.e. out of water)? [Duplicate]

Microscopically, both the water molecules in the air and the water molecules on clothing move quickly due to their thermal energy. Every now and then a molecule on clothing has enough energy to free itself. Every now and then a molecule in the air sticks to your clothes. Because the humidity in your room is less than 100%, the first operation is done more often, causing water to get into the air. On the other hand, if you put your clothes in a sauna where the humidity is higher than 100%, your clothes will get wetter over time.

Now let's look macroscopically. Why should there be evaporation at all if it costs energy? The reason for this is that there is "more space" in the air for the water molecules than there is on your clothing. Hence, things are more likely to jump in the air (which is big) than they are to land on your clothes (which is small). Formally, we say that the process is entropy-driven. (Since entropy counts the available microstates, this is exactly the same.)

Increasing entropy and decreasing energy are separate goals. In this case the energy and entropy effects face each other, but the entropy wins; In general, you can tell from the change in Helmholtz free energy which is winning. F. = U.-T.S. .

### OE1

Very insightful, thank you. I just had a follow up question reading your answer that may have nothing to do with if I'm wrong. The small particles in the air, like dust, are almost always present. Is it for the same reason? For example, does everything in my house contribute the dust particles (Idk, if there's a better word for it) in the air in the break-free process just described?

### knzhou

Not quite. Dust is much heavier than water molecules, so this is where the energy factor wins. If nothing disturbs you, almost everything will fall to the floor. But as you walk around the room, tiny currents of air take it in.