When water turns from a liquid into a vapor, the molecules actually spread out and move farther apart.
We can actually use a process of elimination here, to take out the answers that couldn't be correct.
The molecules don't become smaller, because when water turns from a liquid to a vapor, it goes through a physical change, not a chemical change.
The molecules don't become heavier. Water vapor is lighter and less dense than liquid water.
The molecules don't increase in number either. Because we know that molecules can't just appear out of nowhere. There really is no form of matter that can just appear out of nowhere
