Ice Should Sink

Take a block of wax and drop it into molten wax. It sinks. Take a lump of metal and drop it into the same metal, melted. It sinks. This is what solids do: cooling packs the particles closer, so the solid weighs more for its size than the liquid it came from. Now do it with water. The ice floats — and that is the wrong way round. Almost everything else sinks in its own liquid, and the reason water does not comes down to the shape of one molecule. Its two hydrogens do not come off opposite each other; they come off the same side, about a hundred and four degrees apart. The molecule is bent, and lopsided, and slightly sticky at both ends — but only in particular directions. Freeze it, and each molecule ends up holding exactly four neighbours. It cannot hold four at those angles and stay close to all of them. To give every bond the direction it wants, it has to hold its neighbours at arm's length. The crowd stops being a crowd and becomes a cage, with nothing in the middle of it. Same water. About nine percent more room. Nine percent does not sound like much until it has nowhere to go: a sealed bottle in a freezer does not bulge, it splits, and water freezing in a crack takes mountains apart one winter at a time. Water is not even the only one — silicon, germanium, gallium, bismuth and antimony all do it. And there is one more thing. Water is densest at about four degrees, not at zero, so a cooling lake turns over and over until all of it reaches four — and then stops. The coldest water is now the lightest, so it stays at the top, and that is where it freezes. The ice forms a lid instead of a filling. The water underneath stays liquid all winter. None of it is for anything. It is geometry — two bonds, a hundred and four degrees apart — and everything after that is consequence. 00:00 The wrong way round 00:40 Why solids sink 01:34 What water does instead 02:30 The cage 04:03 Nine percent 04:48 Water is not special 05:19 The lake 06:24 The turn Subtitles: English