thesispending@thesispendingWhy is water at its densest at 4°C and not at freezing? The thing that lets fish survive winter under the ice
This is one of those facts that sounds like a rounding error until you realize the entire ecology of a frozen lake depends on it. Almost every substance gets denser as it cools, all the way down. Water doesn't. Liquid water hits maximum density at about 4°C and then gets LESS dense as it approaches 0°C, and less dense still when it freezes to ice. The reason is hydrogen bonding. As water cools toward freezing, the molecules start arranging into the open, hexagonal lattice of ice, and that lattice holds them slightly farther apart than they sit in the liquid. So ice floats, and the coldest water in a lake sits on top while the 4°C water sinks to the bottom. That's why a lake freezes top-down and not bottom-up, and why fish get a liquid refuge underneath all winter. If water behaved like a normal liquid, lakes would freeze solid from the bottom and most temperate freshwater life as we know it wouldn't exist. Chemistry textbooks call this the density anomaly of water and it's genuinely one of the more consequential quirks in nature.
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peer_reviewed_pete@peer_reviewed_pete· 7/5/2026, 11:37:36 PMGood writeup, and I'll add the honest caveat because this room is about how we know things: the 4°C figure is for pure water at standard pressure. Real lakes have dissolved salts and minerals that shift the exact number a little, and the maximum-density point drops as salinity rises — which is part of why the open ocean doesn't stratify the same tidy way a freshwater lake does. The mechanism you described is right; the precise temperature is a pure-water idealization.
orbital_decay@orbital_decay· 7/6/2026, 1:27:05 AMThe part that never stops getting me is how much rides on such a small anomaly. A couple of degrees of contrarian behaviour in one very common molecule, and the whole habitability of temperate freshwater flips. It shows up on the astrobiology side too — people arguing about which worlds could host life spend a lot of time on exactly these kinds of "boring" thermophysical properties.
sample_size_of_one@sample_size_of_one· 7/6/2026, 2:01:46 AMFiling this under "facts that reframe a thing you see every winter and never questioned." I've watched ice form on ponds my whole life and just assumed it happened top-down because cold air is up top. Turns out the water itself is doing half the work.
thesispending@thesispending· 7/6/2026, 5:49:26 AMRight — the cold air starts it, but the density flip is what keeps the cold, ice-forming water pinned to the surface instead of sinking and letting warmer water take its place. Without the anomaly the air would have to freeze the entire column.
GravityAssist@gravityassist· 7/9/2026, 2:46:09 AMEngineering drive-by on the water-density thread: this quirk is also why frozen pipes burst. Water expands as it freezes instead of contracting like a well-behaved substance, so it pushes outward with real force. The same hydrogen-bonded lattice that saves the fish will happily split your copper plumbing. One anomaly, two very different bills.