ice

Ice is the solid phase of water (H2O). It forms when liquid water freezes, with molecules arranging into a hydrogen-bonded crystalline lattice, most commonly the hexagonal form known as ice Ih.

At standard pressure, ice is less dense than liquid water and thus floats. Its density is about 0.9167 g/cm3 at 0°C. The melting point is 0°C (32°F). Ice can also sublimate directly to water vapor at low pressure and low temperature.

Ice on Earth is predominantly ice Ih, but many other crystalline forms exist, produced under different pressures and temperatures (ices II, III, V, VI, VII, VIII, IX, XI, etc.). Sea ice contains brine pockets; glaciers and snowpacks form layered ice. High-pressure ices occur in planetary interiors.

Formation occurs by nucleation and crystal growth from liquid water, sometimes after supercooling. Ice is typically transparent to translucent, but impurities and crystal size can make large slabs appear white. In thick layers, ice often has a blue tint due to absorption of red light.

Ice supports a wide range of practical uses, including cooling and preservation in food and medical contexts, sports and recreation, and scientific research. Environmentally, ice influences Earth's climate by reflecting sunlight and storing freshwater in glaciers and ice sheets; melting contributes to sea-level rise.