Why is water is a liquid at 20 degrees C but nitrogen and oxygen are gases?

There are any number of reasons why some things are liquids at a given temperature and others not, too many to casually list here.

However, water is famously a liquid at this temperature when other similar compounds are not because of hydrogen bonding, the process whereby the electro-negative oxygen in one water molecule is attracted to the electro-positive hydrogen(s) on another water molecule, creating a temporary dipole (a "hydrogen bond") which causes the molecules to be very strongly attracted to eachother. This attraction is very hard to over come, and is only complete (that is, water only becomes a gas) at 373.15K, or boiling point. O₂ and N₂ for example do not have this dipole, which is one reason why they boil at much lower temperatures.

I'll try to explain a little more simply.

Oxygen attracts electrons more strongly than hydrogen. This means one side of the water molecule is slightly negative while the other side is slightly positive. The water molecules align so the negative end is facing the positive end of another molecule. The ends are attracted because opposites attract, just as north attracts south, positive attract negative. The attraction makes up a bond. These bonds are strong enough to stop water molecules flying away.

In oxygen and nitrogen gas the molecules are the same so there are no permenent charges on the molecule. The bonds formed are different and a lot weaker, so weak that they can't stop the molecules flying away.

First answer by Theozster. Last edit by TIM18. Contributor trust: 4 [recommend contributor]. Question popularity: 1 [recommend question]