Why Ice Floats, and Why Almost Nothing Else Does

Density is the answer, but not the interesting half

Anything floats in a fluid less dense than itself, and ice at 0 °C has a density of about 0.917 g/cm³ while the water beneath it is very nearly 1.000. That settles the question of what happens and explains nothing about why it should.

The real question is why freezing makes water less dense at all, because it makes almost everything else denser. Melt candle wax and drop a lump of solid wax into it: the solid sinks. The same is true of molten metals, of most plastics, and of nearly every substance a school laboratory keeps. Cooling normally slows particles down, lets them pack closer, and increases density.

Water refuses. The reason is a particular kind of attraction between its molecules, and the whole answer lives there.

What hydrogen bonds do as the temperature falls to zero

A water molecule is bent, not straight, and the oxygen atom pulls the shared electrons towards itself. That leaves the oxygen end slightly negative and the two hydrogen ends slightly positive, so each molecule attracts its neighbours by those charges. These attractions are hydrogen bonds, and they are much stronger than the forces between most small molecules.

In liquid water the hydrogen bonds form, break and re-form constantly. Molecules slide past one another, and at any instant some are packed close together in gaps that happen to be available. The arrangement is disordered, and disorder here means efficiency: on average the molecules sit closer than a fixed arrangement would allow.

On freezing, that freedom ends. Each molecule locks to four neighbours in a fixed tetrahedral arrangement, and the whole assembly becomes a hexagonal lattice with open channels running through it. The bonds now hold the molecules apart at a set distance rather than letting them fall together. Same molecules, same mass, about nine per cent more volume — and therefore lower density.

How much of an iceberg is above the water

The density figures do more than answer yes or no; they say precisely how much of a floating object sits below the surface. A floating body displaces its own mass of water, so the fraction submerged is the ratio of the two densities.

For ice in fresh water that is 0.917 ÷ 1.000, so about 92 per cent is under the surface and roughly 8 per cent shows. In seawater, denser at around 1.025 g/cm³, the ratio is 0.917 ÷ 1.025 — about 89 per cent submerged and 11 per cent above. The tip of the iceberg is not a figure of speech invented for effect; it is a straightforward consequence of two densities, and it is why an iceberg is so much wider below the waterline than a ship's watch can see.

Why lakes freeze from the top down, and what that means for the fish

There is a second oddity in the same substance, and together the two decide how a lake behaves in winter. Fresh water is at its densest not at its freezing point but at 4 °C. Cool water below that and it starts to expand again, before it has frozen at all.

So as autumn cools a lake, the surface water chills, becomes denser, and sinks, pushing warmer water up to be cooled in turn. This circulation continues until the whole lake is at about 4 °C. After that, further cooling makes the surface water less dense than what is below it, so it stays on top, chills to 0 °C and freezes there. The ice then sits on the surface and acts as an insulating lid over the water beneath.

Reverse the density relationship and the consequences are severe. Ice would sink as it formed, fresh ice would freeze at the surface and sink in turn, and deep lakes in cold climates would fill with ice from the bottom upwards and never fully thaw in summer, because the sunlight would not reach it. Fish, insect larvae and plants overwinter in liquid water underneath a lid of ice, and they do so because of one anomaly in one substance.

The same nine per cent that splits pipes and shatters rock

Expansion on freezing is not always convenient. Water trapped in a closed pipe has nowhere to go when it freezes, and the pressure it generates is enough to split copper. This is why pipes in unheated spaces are lagged and why an outdoor tap is drained before winter — the burst usually appears on the thaw, when the ice plug melts and the water escapes through the crack it made.

The same process shapes landscapes. Water seeps into cracks in rock, freezes overnight, expands, widens the crack, thaws and seeps deeper. Repeat that for a few thousand cycles and the rock breaks apart. Geographers call it freeze-thaw weathering, and it is responsible for the scree slopes below mountain crags. It is the identical nine per cent, working on stone instead of plumbing.