The fact that water expands upon freezing causes icebergs to float. The fact that water reaches a maximum density at about 4°C causes bodies of water to freeze on the top first. Then the further expansion as a part of the phase change keeps the ice floating with some 8% of its mass above the surface. The expansion during the phase change may be shown on a PvT surface, and contrasts with the contraction upon freezing of most substances.

The expansion upon freezing comes from the fact that water crystallizes into an open hexagonal form. This hexagonal lattice contains more space than the liquid state.

Water at ordinary temperatures contracts and increases in density as it is cooled, like most substances. But at about 4°C it reaches a maximum density and then decreases in density as it approaches the freezing point. This decrease in density is only about one part in 10,000 as it cools from 4°C to 0°C, but this is sufficient to cause the water near freezing to come to the top. The water further expands upon freezing, so that water freezes from the top down, and ice floats on water.