The density of water is greater than that of ice at the freezing temperature. As the temperature of water is raised, the density rises to a maximum value near 4 °C, and falls after that.

The change in density is attributed to the collapse of the open structure found in ice. Ice involves the formation of an extended hydrogen bonding system which requires appropriate positioning of water molecules and, therefore, has considerable open space. When heated, this structure collapses, and the density rises. The structure continues to collapse as the temperature is increased. In general, when condensed matter is heated, its density decreases -- the matter expands. The density found for water is thought to arise from a combination of collapsing structure and expansion due to thermal motions.

	The Galileo thermometer is constructed on the basis of decreasing liquid density with increasing temperature. The glass spheres do not change density with temperature as much as does the surrounding liquid.
A cup, initially filled to the brim with liquid water, is frozen. Note the increased volume of the ice (solid water).