Why does water expand when it freezes?

11 October 2009

Question

Why does water expand when it freezes?

Answer

The default is for things to shrink when they freeze. This is because, normally, if you make something hotter, it vibrates more. When it vibrates more, it tends to take up more space, so it tends to expand.

Ice is very unusual in that, as it gets colder, although the particles are vibrating less, it expands.

The reason for that is due to the strange shape of water. If you've ever seen a picture of a water molecule, it looks like a Mickey Mouse head with an oxygen atom where Mickey Mouse's face is, and then two hydrogen atoms where his ears are.

The oxygen atom is slightly negative, and the hydrogens are slightly positively charged, so water molecules tend to stick together forming what are called hydrogen bonds. And because of that shape of the molecule, the way water molecules tend to link together is actually a very open structure with big holes. That means, there's quite a lot of extra "empty" space.

When water freezes it releases energy because lots of extra strong bonds can be made. But it does take up more space. And so, ice expands when it freezes.

Comments

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