Question

Ice is iess dense than water, which allows it to float on liquid water. Why does this happen?

Answer 1

Hydrogen bondings pushing molecules further away from each other, decreasing its density.

Explanation:

When liquid water turns solid, a crystalline structure is formed and held together by hydrogen bonding. These hydrogen bonds are very capable of decreasing the density in this structure due to their ability to increase the amount of space between each molecule. That is to say that the frozen water molecules don't pack as tightly into an area of the same volume of liquid water.

This is unique amongst liquid and frozen water. Interestingly, liquid water is incredibly hard to compress, where as frozen water can be compressed much easier by comparison. It's all due to their strange, out-of-the-norm density comparisons.

Water has a lot of special characteristics that makes it so unique, and this is just one of them. Here's more information on that.