Why Ice Floats When Almost Nothing Else Does

A water molecule is one oxygen atom with two hydrogen atoms attached at an angle of about 104 degrees, and the electrons are shared unevenly, leaving the oxygen slightly negative and the hydrogens slightly positive. Each molecule is therefore drawn to its neighbours: its hydrogens to their oxygens and its oxygen to their hydrogens. These attractions are hydrogen bonds, and they are the reason water is a liquid at room temperature when molecules of similar size are gases.

In liquid water the bonds form and break constantly and the molecules jostle into a fairly close arrangement. As the water cools towards freezing, the molecules slow down, and the hydrogen bonds get the chance to hold. Each molecule locks to four neighbours in a fixed pattern, and that pattern is a hexagonal lattice with a lot of empty space in it. The molecules in ice are, on average, further apart than the molecules in cold water.

The oddness starts before freezing. Water is densest at about 4 degrees Celsius, not at 0. Between 4 and 0 degrees the molecules are already beginning to arrange themselves into the open structure, so the water expands slightly as it cools further. When it finally freezes, the lattice snaps fully into place and the volume jumps by about nine percent.

That number is what you see in an ice cube: a little over nine tenths of it is below the surface and the rest stands above. Archimedes' principle says a floating object displaces its own weight of liquid, so an object nine percent less dense than water sits with about 91 percent of its volume submerged. The same arithmetic governs an iceberg, which is why about nine tenths of it is hidden.

Because water is densest at 4 degrees, a lake cooling in autumn behaves in a particular way. The surface water cools, becomes denser and sinks, and warmer water rises to replace it, until the whole lake is at 4 degrees. After that, water cooling further at the surface becomes lighter, not heavier, and stays on top. It freezes there, and the ice, floating, insulates the water beneath it.

The result is that a lake freezes from the top down and almost never freezes solid. Fish, insects and plants spend the winter in liquid water at 4 degrees under a lid of ice. If ice sank, lakes and shallow seas would freeze from the bottom up, thaw only partly each summer, and most of the aquatic life that evolved on Earth would have had nowhere to do it.

Water expanding as it freezes is destructive whenever it is trapped. A pipe left full in a cold house bursts not because ice pushes along it but because ice needs nine percent more room than the water did and iron or copper will not stretch that far. Water that seeps into a crack in rock, freezes, expands and thaws, over and over, splits the rock; this freeze-thaw weathering breaks down mountains and pits road surfaces every winter.

It also explains why frozen food loses texture. The ice crystals that form inside cells take up more room than the water did, puncture the cell walls, and let the contents leak out on thawing, which is why a defrosted strawberry is soft. Fast freezing forms smaller crystals and does less damage, which is the whole idea behind flash freezing.

Seawater complicates the picture in one way: dissolved salt lowers both the freezing point and the temperature of maximum density, so ocean water keeps getting denser right down to its freezing point of about minus 1.9 degrees. Sea ice still floats, because as it forms it rejects most of the salt, leaving nearly fresh ice on top of saltier, denser water. That expelled brine sinking away from forming sea ice is one of the engines of the deep ocean circulation.

Water is not entirely alone in expanding when it freezes. Silicon, germanium, gallium and bismuth do it too, for similar structural reasons, which is why gallium can crack the container it solidifies in. But water is the only common one, and the only one that matters to living things. The chain of consequences from one bent molecule:

• •Hydrogen bonds hold molecules in an open hexagonal lattice
• •Ice is nine percent less dense than water and floats
• •Water is densest at 4 degrees, so lakes freeze from the top
• •Life survives winter under the ice rather than inside it
• •Trapped water splits pipes, rocks and cells as it freezes

A common textbook explanation of ice skating says the weight of the skater on a thin blade raises the pressure enough to melt the ice, leaving a film of water to glide on. The idea has a real basis in water's oddness. Because ice is less dense than water, squeezing it favours the liquid, so pressure does lower the melting point, which is the opposite of what happens with most substances. But the effect is tiny: it takes roughly a hundred and thirty times normal atmospheric pressure to lower the melting point by a single degree.

Run the numbers for a skater and the drop is a fraction of a degree to a few degrees at most, yet people skate happily on ice at minus twenty. Pressure melting cannot be the main reason. Physicists now point to two other effects: the surface of ice carries a thin, disordered layer whose molecules are less tightly bound than those inside, so it behaves somewhat like a liquid even well below freezing, and friction from the moving blade generates heat that melts a little more. Exactly how these share the work is still an active research question, which is surprising for something so familiar.

The ice in your freezer is only one of many solid forms of water. Squeeze water hard enough, at the right temperatures, and its molecules lock into other crystal arrangements, and more than a dozen have been made in laboratories, numbered with Roman numerals. Ordinary ice is called ice Ih, the h standing for hexagonal. The high-pressure forms abandon its open lattice and pack the molecules more tightly, so they are denser than liquid water and would sink in it. The novelist Kurt Vonnegut invented a deadly ice-nine that stayed frozen at room temperature; the real ice IX exists only under high pressure and intense cold.

Those dense ices matter beyond the laboratory. Ganymede, Jupiter's largest moon, is thought to hold a deep salty ocean beneath its icy crust, and models suggest that below the ocean, where pressures are enormous, the water turns to high-pressure ice that sits on the bottom instead of floating, sandwiching the liquid between two kinds of ice. Similar layering is proposed for Titan and other large icy moons. Even the Earth carries a trace: researchers have identified ice VII in microscopic pockets inside diamonds that formed deep in the mantle, where the rigid diamond kept the trapped water squeezed into its dense form even after it reached the surface.

Because floating ice displaces exactly its own weight of water, melting it changes the level surprisingly little. Put ice cubes in a glass of water, mark the level and wait: when the cubes melt, the meltwater fills almost exactly the space their submerged parts were taking up, and the level stays put. The same holds for Arctic sea ice, which is already floating. When it melts, it does not by itself raise sea level, a point often lost when shrinking summer ice is discussed. Its loss matters for other reasons, chiefly because dark open water absorbs much of the sunlight that white ice would have reflected.

Two qualifications sharpen the picture. Floating ice is nearly fresh water sitting in salt water, which is denser, so when it melts the fresh water takes up slightly more room than the seawater it displaced; melting all the world's floating ice shelves would add only a few centimetres to the oceans. Ice resting on land is a different matter. The Greenland ice sheet holds enough water to raise global sea level by about seven metres, and the Antarctic ice sheet roughly fifty-eight, so ice flowing off the land and melting, together with seawater expanding as it warms, drives the rise in sea level, not melting sea ice.

Ice floats because water molecules lock into an open, hydrogen-bonded lattice as they freeze, making the solid about nine percent less dense than the liquid. That one exception to the usual rule keeps lakes liquid beneath their ice, hides most of every iceberg, bursts pipes and breaks rock.