In a gas the molecules are moving around with lots of space between them without much interaction. Some quickly, some slowly, all in different directions. Occas

In a gas the molecules are moving around with lots of space between them without much interaction. Some quickly, some slowly, all in different directions. Occasionally they collide and bounce off each other, or more rarely they may temporarily form weak (liquid) bonds with each other until a fast molecule hits them and breaks them apart. In a liquid the molecules (which are indistinguishable from gas molecules of course if you looked at them individually) are moving much slower and form and re-form weak (liquid) bonds with the molecules near them. But still there are some moving faster and others more slowly and they are going in random directions. Now think about the surface where the liquid meets the gas. There are some gas molecules that plunge into the liquid and become part of the liquid and there are some liquid molecules moving fast enough to escape from the liquid, breaking all their weak (liquid) bonds, and become part of the gas. The liquid surface at a molecular scale will not be a nice flat surface. It will be a complex seething bubbling thing with droplets and molecules leaving and rejoining the liquid. If we heat the liquid, make the molecules move faster, more of them will be moving fast enough to escape from the liquid. Anyway, if there are more molecules going from the gas to the liquid we say the gas is condensing, and if there are more going the other way the liquid is evaporating. Now think what would happen if we take away all the gas molecules - put the liquid into a vacuum. There will no molecules returning to the liquid, only liquid molecules becoming gas. Conversely, if we compressed the gas above the liquid we would get more gas molecules going into the liquid. I think that's how it works.