Question: How does ice float on water?

Ben Smart answered on 10 Mar 2012:

The short answer is that ice is less dense than water. An object will float on anything that is more dense than it, and it will sink in anything that is less dense than it.

What I find interesting though is why ice is less dense than water. Normally a chemical in its solid state is more dense than when it’s in its liquid state.
When water forms ice, all the water molecules line up next to each other to form a specific pattern. The pattern water molecules form when they become ice is a pattern in which each water molecule is quite far away from each of its neighbours. In fact it’s further away from its neighbours than it would be if it was liquid water and not ice. And because the water molecules are further apart when they are ice, it means that ice is less dense.
Now I’ll be honest I had to go look up this next bit to check: it turns out water molecules form this specific pattern when they become ice because of the shape that water molecules are, and the fact that they are joined to one another by what is called ‘hydrogen bonding’. I could go into an explanation of all that, but I’ll leave it for now 😛 Ask another question about it if you want to know more!

One last thought: at the start of the answer I said that an object will float on anything that is more dense than it, and it will sink in anything that is less dense than it. Well this idea works for objects in air too! We are more dense than air, so we sink in air and stay on the ground. If we were less dense than air we would float away as we would be floating on air. A helium balloon (or an airship) is less dense than air, and that’s why it can fly, because it floats on air.