Ice (from the Old English “īs”, in turn from the Proto-Germanic “*isaz”) is water frozen into a solid state. Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaque bluish-white color. In the Solar System, ice occurs naturally from as close to the Sun as Mercury to as far as the Oort cloud. Beyond the Solar System, it occurs as interstellar ice. It is abundant on Earth’s surfaceparticularly in the polar regions and above the snow lineand, as a common form of precipitation and deposition, plays a key role in Earth’s water cycle and climate. It falls as snowflakes and hail or occurs as frost, icicles or ice spikes. Ice molecules exhibit different phases (packing geometries) that depend on temperature and pressure. Virtually all the ice on Earth’s surface and in its atmosphere is of a hexagonal crystalline structure denoted as ice I (spoken as “ice one h”). The most common phase transition to ice I occurs when liquid water is cooled below (, ) at standard atmospheric pressure. It may also be deposited directly by water vapor, as happens in the formation of frost. The transition from ice to water is melting and from ice directly to water vapor is sublimation. Ice is used in a variety of ways, including cooling, winter sports and ice sculpture.
