How does a molecule of water vapor go back to a molecule of liquid water?

Answer 1

First of all, I have to assume that by "vapor" you mean gas. Water gas is steam.

Second, you have to realize that the only difference between water as a gas, and water as a liquid, is the energy in the molecule. We measure that by measuring temperature. Temperature is a direct measurement of thermal (heat) energy.

(I know some techie is going to jump on that word, "direct" ... anyway)

Think of a running car engine (with no key). When it is on, it is vibrating, making noise, maybe even moving fast. It does that until all the energy (gas) is gone, then it stops and is still. It's the same engine, just at rest. If you add more gas/energy, and give it a spark, vroom and it's back in motion.

Just like that, a gas water molecule uses up its energy by bouncing into other molecules. Oxygen, nitrogen, and more water, if it is in the Earth's atmosphere. Eventually it doesn't have enough energy to stay a gas anymore, and the next time it hits another water molecule, it "sticks". This is what humidity is, liquid water floating around in the air, a few molecules at a time. That's why you get damp when it's humid, the water floating around in the air hits your skin and your sweat and "sticks". It doesn't have the energy it needs to bounce off and float away.

On the other hand, if a water molecule were in a vacuum, like out in space, it's likely that it would never hit another molecule of anything. Whatever energy it had, whether it was a solid, liquid, or gas... it would stay that way. On the other hand, a larger object, like a comet, would eventually hit lots of molecules, and give away most of its thermal energy. That's why Haley's Comet is made out of ice. The comet's tail is created by the solar wind when it gets close to the sun, and the ice starts to melt. That's why the tail always points away from the sun.