ice
Water in a solid state. There are ten types of ice in nature, with different crystalline structures, depending on pressure and temperature conditions. The ice we know and that is most abundant on Earth is Ih, which crystallizes in the hexagonal system and is known as ordinary ice, although we refer to it simply as ice. It can form both through the freezing of liquid water and through the sublimation of water vapor present in the atmosphere. It presents an infinite variety of shapes and textures. Pure ice - with no air trapped inside - is transparent. The white color that most forms of ice usually display is a consequence of the air they contain. The whiteness of snow - made up of aggregates of ice crystals - is due to its fluffiness. Ice can also display an intense blue color, which occurs when it is particularly compact, though with trapped air; this circumstance causes the scattering of light striking it, in a manner similar to what happens in the sky (Rayleigh scattering). This striking bluish tint is observed, for example, in deep crevasses of glaciers and in some icebergs. Pure water freezes at 0°C at sea level under normal pressure conditions (P = 1013 hPa). The density of the resulting ice is 0.9168 g/cm3, a value lower than that of liquid water (1 g/cm3). This peculiarity is responsible for ice floating on water.