Piggybacking off of a recent question asking whether drinking cold water means drinking the most water.
In: Physics
Nothing obligates substances to be “smaller” when they’re a solid vs. when they’re a liquid, per se. What makes a solid a solid is that the intermolecular forces are strong enough to force the molecules to stay together and stop them from floating around freely.
Water, for various quantum physics reasons, is shaped kinda weirdly; it’s angled, but not at a right angle, more like 1/6 of a hexagon. The oxygen has a sort-of-negative charge and the hydrogens have a sort-of-positive charge. Since positive and negative attract, two water molecules can generate intermolecular force by lining up the positive end of one with the negative end of another.
You can maximize these intermolecular forces by “lining up” a whole bunch of water molecules in a regular hexagonal pattern. The geometry of it guarantees that the positive parts are surrounded by negative parts and the negative parts are surrounded by positive parts. Then everything is tugging on everything else, so the molecules can’t move freely, which is what a solid is.
This hexagon structure isn’t that space efficient, since it leaves the middle of the hexagons empty. That doesn’t make it not a solid, since space efficiency isn’t what defines a solid per se. But it *is* different from most other substances which have a more space efficient packing, because of having a less awkward shape to have to work around. Long, stick-shaped molecules like the ones that make up solid fats and waxes, for example, can just lay side-by-side, which takes up comparatively less space.
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