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Ice Expands to be bigger than water

Why icebergs float, and why water pipes burst in frost

When water freezes it turns into ice. Ice is bigger than water and takes up more space.

As water freezes and turns to ice, it becomes larger but stays the same weight, so it is a lower density. It's lighter. That's why icebergs float on water.

Ice is bigger, and it expands on its previous size as water, and that is why water pipes can burst. That's a cause for claiming on your home insurance. The water freezes, turns to ice, bursts the pipes, and then melts again, and then the pressure of water in the pipes causes the water to gush through the holes made by the split.

The ice is not much bigger than the water, but inside a rigid container it is likely to break it. In the deep freezer fun page, there are things about this issue.

Density of water is 1000Kg/m3 whereas the density of ice is about 920Kg/m3, or to put it another way, a tonne of water stops taking up a cubic metre (1000 litres) and starts taking up 1087 litres.

Although water does this under almost all circumstances, most other substances don't do this. Most liquids get smaller when they freeze into solid. On a planet with liquid methane oceans, solid methanebergs would sink, not float. You can even see this usual relationship between solid and liquid with molten lead. Pieces of solid lead sink in it.

There's something unusual about H2O. It could be something to do with the crystalline structure of ice and the six-sided nature of snowflakes. This regular structure makes the ice take up more room.

We might wonder if crystalline carbon (diamond) is somehow bigger than "liquid carbon", but carbon is only a liquid at exotically high pressures and temperatures, so it wouldn't be fair to compare soot and diamond and somehow try to conclude something from the fact that the crystalline form is heavier. It would be like comparing ice with water vapour at ultra-low pressure.

The fact is that water and ice at everyday human temperatures have an upsidedown relationship in their comparative densities.

Other helpful things: Is Water Incompressible? ... and home insurance!