eli5: Why is water the only substance that explands when it freezes?
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Crystal structures are usually very dense, but in some cases the liquid form can assume a denser state than it’s crystal form. Not only does water become less dense when it freezes, this also happens with bismuth, gallium and some other substances.
It is not the *only* substance to do this. It is, however, one of a rare few. When a substance solidifies, its molecules settle from a chaotic mess into a more organized structure. Think of it like magnets snapping together.
The shape of this structure depends on the ‘shape’ of our ‘magnets’; the way they want to arrange themselves. In water, this shape is less tightly packed than the chaotic soup of the liquid form, so water expands as it freezes.
Imagine that you have a box of matches and you start glueing them by their ends. Something simillar happens to the water: if I’m not misremembering, when water mollecules freeze they create hexagons, which are bigger in comparison
This has to do with water having a tetrahedral shape and it’s very good at sharing its hydrogen atoms. The ways to arrange waters to maximize hydrogen sharing have an average lower density than liquid water. Make the assumption water will maximize hydrogen sharing/bonding in ice to be true
It’s not the only one, but it’s one of the few. Water is made of oxygen and hydrogen atoms, this type of molecules interact with each other through hydrogen bonds, which are way stronger than the Van der Waals forces, which are usually the only ones that appear in most compounds. When temperature is lowered, atoms slow down* so they tend to get closer to each other. In the case of water, this deceleration makes it easier for Hydrogen bonds to keep molecules in place and eventually create more open structures.
Note: Hydrogen bonds appear in many more compounds, but they are especially strong in water
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