Generally speaking, objects expand when heated and contract when cooled, that is, for the same object, "the density of solids should be greater than that of liquids", while water is the opposite. This reason involves chemistry. After water forms ice, its mass remains unchanged, because the density of ice becomes smaller, so its volume becomes larger. Water has a high density, but ice has a low density. After freezing, the intermolecular gap becomes larger, but the total mass remains unchanged, so the volume will become larger. The density of water is 1 and the density of ice is 0.9.

Because water molecules are highly polar molecules, they can be combined into associated molecules by hydrogen bonding (multiple water molecules are combined together). Liquid water contains not only simple water molecules (H2O), but also associated molecules. The density of a substance is determined by the average spacing of molecules in the substance. When water is not frozen at 0℃, most water molecules exist as (H2O)3 associated molecules. When the temperature rises to 3.98℃( 10 1kPa), most water molecules exist in the form of bimolecular associated water molecules (in the process of water temperature rising from 0℃ to 4℃, the effect of water density increase caused by hydrogen bond breaking of associated water molecules is greater than that of water density decrease caused by accelerated molecular thermal motion, so in this process, the water density increases with the increase of temperature. )