Why Ice Floats

Drop an ice cube into a glass of water and it floats. You have seen this so many times that it takes effort to notice how strange it is. Almost nothing else does this. Pour molten wax and the solid wax sinks. Freeze almost any metal and the solid sinks in its own melt. The general rule of matter is simple and almost universal: cool a thing down, its particles slow and pack closer, the solid is denser than the liquid, and the solid sinks.

Water breaks the rule. Solid water is about 9% less dense than the liquid it came from — roughly 0.92 grams per cubic centimeter against 1.00 — so ice rides on top. The company water keeps in this is a short list of oddballs: silicon, gallium, bismuth, antimony. Not one of them is something you’d find in your kitchen. Among the substances that fill an ordinary life — metals, oils, rock, wax, glass — water is essentially alone in floating on itself.

It would be easy to file this under “neat trick.” It isn’t a trick. It’s the reason there are fish.

Why most things sink

Start with the normal case, because the anomaly only makes sense against it.

Heat is motion. In a warm liquid the molecules are jostling, sliding past each other, keeping a loose average distance set by how hard they’re vibrating. Cool the liquid and you take energy out of that motion. The molecules slow, stop shoving each other apart so insistently, and settle closer together. Keep going and they lock into a solid — a crystal, usually — where each particle sits in a fixed spot in a tightly packed grid. Packed tighter means more mass in the same volume. The solid is denser. It sinks.

This is the expected story, and for water it’s even true — for a while. Cool water from warm and it contracts and densifies just like anything else, right down to about 4°C. And then, four degrees above freezing, it stops behaving and starts to do something that no ordinary liquid does. It begins to expand as it cools further. By the time it freezes it has swelled by nearly a tenth of its volume.

To see why, you have to look at the shape of a single molecule.

The bent molecule and the bond

A water molecule is one oxygen atom with two hydrogens stuck to it, and the thing that matters is that they are not in a straight line. The molecule is bent, the two hydrogens splayed off the oxygen at an angle of about 104.5 degrees. The oxygen hogs the shared electrons, so it carries a slight negative charge; the two hydrogens, left a little electron-poor, carry slight positive charges. Each molecule is a tiny lopsided magnet — negative at the oxygen end, positive at the hydrogen ends.

Opposite charges attract, so the hydrogen of one molecule reaches out to the oxygen of its neighbor. That weak, fleeting attraction is the hydrogen bond, and it is the hinge the whole story turns on. It’s much weaker than the bonds holding a single molecule together, weak enough that in liquid water these bonds are forming and breaking trillions of times a second, a churning network constantly rewiring itself.

In the liquid, that churn is the point. The bonds are there, but they are temporary, and molecules can slip into the gaps, packing reasonably close on average. As you cool the liquid toward 4°C, ordinary contraction wins: the molecules slow, the jostling shrinks, everything draws inward, denser.

Below 4°C the hydrogen bond takes over, and it has a demand. Each water molecule can make four hydrogen bonds — two through its hydrogens, two through lone pairs on its oxygen — and the bent geometry points those four bonds outward toward the corners of a tetrahedron. To satisfy all four at once, in their fixed directions, a molecule cannot let its neighbors crowd in close. It has to hold them at arm’s length, in the specific directions the bonds want to go.

When water freezes, every molecule gets its way at once. The molecules lock into an open, six-sided lattice — hexagonal rings with each oxygen tetrahedrally joined to four others, and conspicuous empty space in the middle of every ring. The crystal is a scaffold held rigid by billions of little outstretched arms, and a scaffold is mostly air. That open framework is why ice takes up more room than the liquid that made it. The molecules in ice are not packed tighter than in water. They are spaced further apart, pinned there by the geometry of the bond.

So the famous 4°C is not arbitrary. It’s the exact temperature where the two effects cross — where ordinary thermal contraction, pulling everything inward, finally loses to the hydrogen-bond lattice pushing everything out. Above 4°C, cooling makes water denser. Below 4°C, cooling makes it less dense. Water is at its heaviest at 4°C, a few degrees before it ever turns to ice.

That crossover, sitting just above freezing, is the part that runs the living world.

The lid on the pond

Watch a freshwater lake go into winter.

The surface chills first, exposed to the cold air. As that surface water cools, it gets denser and sinks, and warmer water rises to take its place and be chilled in turn. The lake turns itself over, cooling from the top but mixing all the way down — until the whole body reaches 4°C. Now the rule flips. Water cooled below 4°C is no longer denser; it’s lighter. So the coldest water stops sinking. It stays on top, floating on the 4°C water beneath it, and it’s the surface that finally freezes.

The ice forms as a lid, and because ice floats, the lid stays at the top, where it shelters everything below it. Until it formed, the lake was shedding heat fast: the surface chilled, sank, and carried the warmth up to be lost to the air in a constant overturning churn. The solid lid stops that churn dead — liquid can’t circulate up through a sheet of ice — so heat can now leave only by conducting slowly up through the cap, and any snow that settles on top, mostly trapped air and an excellent insulator, slows even that to a crawl. Beneath the cap the water sits at a steady few degrees, densest and warmest at the very bottom, and the fish, the frogs in the mud, the whole submerged economy of the lake, wait out the winter in liquid water a hand’s breadth below solid ice.

Now run the counterfactual, the world where water obeyed the normal rule and ice sank. The surface would freeze and the ice would drop to the bottom, out of reach of the summer sun. Each winter would stack more ice on the floor, shielded from any thaw. Lakes and ponds would freeze solid from the bottom up and mostly stay that way, ice accumulating year over year. There would be no liquid refuge, because the refuge is created entirely by the lid floating instead of sinking. Through any prolonged cold — an ice age, a hard continental winter — life in fresh water would have nowhere to be. The thing that makes a frozen lake survivable is precisely that the ice is on the wrong side, on top, where a normal solid would never be.

The open ocean is a more complicated case — salt changes the math, and the deep sea is kept cold by global currents rather than by this simple stratification. But the headline survives the complication: because ice floats on seawater too, a freezing polar ocean grows a crust on its surface and insulates the dark water below, instead of freezing into a solid block from the seabed up. The pole gets a skin, not a tomb.

One angle, everywhere

It doesn’t stop at fish.

The floating crust of sea ice and the great ice caps sit on top of the ocean and the land, white and reflective, bouncing sunlight back to space and helping set the temperature of the whole planet — a climate regulator that exists because frozen water rises to where the sun can reach it. Under and on that floating ice, polar ecosystems run: algae in the sea ice, the animals that graze them, everything stacked above.

The same 9% expansion that saves the lake also breaks the world down to build it back. Water seeps into a crack in a rock, freezes, and expands with enough force to split stone. Repeat that through enough winters and mountains crumble into gravel, gravel into the mineral grit of soil. Freeze-thaw weathering — the patient shattering that turns bare rock into ground things can grow in — is the destructive face of the very same molecular geometry. And it is genuinely the same force, indifferent to outcome: the expansion that insulates the fish is the expansion that bursts your pipes in a cold snap and cracks the engine block of a car left without antifreeze. The bond does not know whether it is making soil or a plumbing bill. It just pushes outward by a tenth, every time, and the consequences sort themselves out.

Here is the part I keep turning over. None of this was arranged. There is no sense in which water was set up to keep lakes liquid or to grind rock into soil. All of it — the fish, the ice caps, the dirt under a forest — falls out of two brute physical facts that have nothing to do with life: that a water molecule is bent at about 104.5 degrees, and that the slightly-charged ends of those bent molecules reach for each other in four fixed directions. Get those two things, and ice must be an open lattice, and an open lattice must be less dense, and a less dense solid must float, and a floating solid must insulate from above, and a great deal of what lives does so in the shelter of that chain.

The living world is downstream of the angle of a molecule. Most of the time the foundations of things are hidden behind so many layers that you can’t see them at all. Once in a while you can see all the way down, and what’s holding everything up turns out to be 104.5 degrees and a weak attraction between a hydrogen and an oxygen — a piece of geometry that was never going anywhere and that, almost as an afterthought, made the rest possible.

— Cael