Solid form of liquid water and water vapour. Below 32 °F (0 °C), liquid water forms a hard solid and water vapour forms frost on surfaces and snowflakes (see snow) in clouds. Unlike most liquids, water expands on freezing, so ice is less dense than liquid water and therefore floats. It consists of compact aggregates of many crystals (with hexagonal symmetry), although ice formed from the bulk liquid does not normally have crystal faces. Molecules in the crystal are held together by hydrogen bonds (see hydrogen bonding). With a very high dielectric constant, ice conducts electricity much better than most nonmetallic crystals. At very high pressures, at least five other crystal forms of ice occur.

This entry comes from Encyclopædia Britannica Concise.
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