Piggybacking off of a recent question asking whether drinking cold water means drinking the most water.
In: Physics
When water freezes its molecules form in a lattice arrangement, it just so happens that in this lattice the molecules of water are further apart than when it was liquid. In very basic terms, there’s more space between the molecules when water is frozen than when it is liquid
The way water bonds when it becomes a solid makes it less dense. Process called hydrogen bonding
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.
Water molecules are formed at an angle, like this:
H H
/
O
That oxygen atom is hungry for electrons, so it pulls the one electron from each of the hydrogen atoms closer to its nucleus than the hydrogen nuclei. This means that there is a slight “magnetic” difference: the hydrogen atoms are more positive (because they don’t have the balancing electron nearby) and the oxygen atom is slightly more negative because it’s hogging the electrons.
In liquid form this “magnetic” difference (i.e, polarity) is negligent but you see it in surface tension. When water is frozen into a solid, this polarity forces the molecules to line up neatly and more rigidly with other slightly positive oxygen atoms attracted to other hydrogen atoms, in a lattice formation.
It’s a relatively unique property for a small molecule and it’s fundamentally important to so much biochemistry.
edit: molecule spacing ain’t right
When water is liquid, it’s molecules are all nice and snug, relatively.
When most substances freeze, their molecules just get even closer together.
But because of specific properties of water molecules (their shape and polar nature), instead of getting closer, they arrange themselves into a sponge-like structure, with voids in-between. This is what makes ice less dense AND why it expands.
When water is liquid the water atoms are only loosely bonded together so they can smush together to completely fill the space they’re in as closely as they can pack together. When it’s frozen it crystallises. As a solid the water atoms form stronger bonds but only in certain ways (set number and angle of bonds). This makes fitting the water atoms together in its frozen state is like playing a game of tetris. It’s not the most efficient way of smooshing them together, they have to stay in place and follow new rules in how they connect to one another, so they can’t pack as closely as in liquid form where they can do what they want to fit in and fill a space.
Others have answered, but what’s interesting is that water does get more and more dense as it gets colder like other liquids, until it hits 4C.
I’ll just add that while water is definitely the most common everyday substance which bucks the trend of being more dense as a solid than as a liquid, it’s not unique. Gallium, germanium, and silicon are also less dense in solid form.
Also, there are other kinds of ice with different crystal structures, and some of them are more dense than liquid water. But at the temperatures and pressures found on Earth, hexagonal ice (aka ice I) is pretty much the only kind that can form.
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