Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or 32 degrees Fahrenheit. 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.
|Density (ρ)||0.9167–0.9168 g/cm3|
|Refractive index (n)||1.309|
|Young's modulus (E)||3400 to 37,500 kg-force/cm3|
|Tensile strength (σt)||5 to 18 kg-force/cm2|
|Compressive strength (σc)||24 to 60 kg-force/cm2|
|Poisson's ratio (ν)||0.36±0.13|
|Thermal conductivity (k)||0.0053(1 + 0.0015 θ) cal/(cm s K), θ = temperature in °C|
|Linear thermal expansion coefficient (α)||5.5×10−5|
|Specific heat capacity (c)||0.5057 − 0.001863 θ cal/(g K), θ = absolute value of temperature in °C|
|Dielectric constant (εr)||~3.15|
|The properties of ice vary substantially with temperature, purity and other factors.|
In the Solar System, ice is abundant and occurs naturally from as close to the Sun as Mercury to as far away as the Oort cloud objects. Beyond the Solar System, it occurs as interstellar ice. It is abundant on Earth's surface – particularly in the polar regions and above the snow line – and, 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 and aggregates from snow as glaciers and ice sheets.
Ice exhibits at least eighteen phases (packing geometries), depending on temperature and pressure. When water is cooled rapidly (quenching), up to three types of amorphous ice can form depending on its history of pressure and temperature. When cooled slowly, correlated proton tunneling occurs below −253.15 °C (20 K, −423.67 °F) giving rise to macroscopic quantum phenomena. Virtually all ice on Earth's surface and in its atmosphere is of a hexagonal crystalline structure denoted as ice Ih (spoken as "ice one h") with minute traces of cubic ice, denoted as ice Ic and, more recently found, Ice VII inclusions in diamonds. The most common phase transition to ice Ih occurs when liquid water is cooled below 0 °C (273.15 K, 32 °F) 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.