Eli5: Materials expand when heated and contracts when cooled. So why does water expand when it freezes instead of contracting?
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“Materials expand when heated…” is a general rule that applies to a lot of things, but not all. The rule is really “Materials get more chaotic when heated, and more ordered when cooled.” It’s to do with something called entropy, which is really just a fancy term for the amount of disorder.
Imagine you have a big cardboard box of table tennis balls. When the box is still, they’re all nicely settled. But if you start to shake the box a bit, giving them energy, they begin to jiggle about. This makes them take up more space. If the box was almost full, then some will start to overflow the box. When you stop shaking it, they settle down and take up less space again. This is what happens when most things are heated and expand.
Water is a bit different. Water molecules are sort of L-shaped, and they get attracted to each other. This means they like to settle into a nice pattern. When we give them a bit of energy, we break the attraction between molecules and split the pattern up into a jumble. This actually takes up slightly *less* space than the structure. If we keep adding in more energy, then the water *will* begin to expand again, but it’s still denser than nicely formed ice.
Rubber is another example. Rubber is made up of long chain molecules. Imagine taking a chain and laying it out straight on a table so each end just reaches the edges of the table. Now start shaking the table. You’ll make some of the sections of chain get twisted and folded back on themselves. This effectively shortens the chain, meaning it doesn’t reach the full table. Just like this, rubber *also* shrinks when heated.
But, in all of these examples, adding in energy makes the thing less organised and more chaotic.
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