Ice

Ice
An ice block, photographed at the Duluth Canal Park in Minnesota
Physical properties
Density (ρ)	0.9167[1]–0.9168[2] g/cm3
Refractive index (n)	1.309
Chemical properties
Chemical formula	H2O
Mechanical properties
Young's modulus (E)	3400 to 37,500 kg-force/cm3[2]
Tensile strength (σt)	5 to 18 kg-force/cm2[2]
Compressive strength (σc)	24 to 60 kg-force/cm2[2]
Poisson's ratio (ν)	0.36±0.13[2]
Thermal properties
Thermal conductivity (k)	0.0053(1 + 0.0015 θ) cal/(cm s K), θ = temperature in °C[2]
Linear thermal expansion coefficient (α)	5.5×10−5[2]
Specific heat capacity (c)	0.5057 − 0.001863 θ cal/(g K), θ = absolute value of temperature in °C[2]
Electrical properties
Dielectric constant (εr)	~95[3]
The properties of ice vary substantially with temperature, purity and other factors.

Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 °C, 32 °F, or 273.15 K. It occurs naturally on Earth, on other planets, in Oort cloud objects, and as interstellar ice. As a naturally occurring crystalline inorganic solid with an ordered structure, ice is considered to be a mineral. 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.

Virtually all of the ice on Earth is of a hexagonal crystalline structure denoted as ice I_h (spoken as "ice one h"). Depending on temperature and pressure, at least nineteen phases (packing geometries) can exist. The most common phase transition to ice I_h occurs when liquid water is cooled below 0 °C (273.15 K, 32 °F) at standard atmospheric pressure. When water is cooled rapidly (quenching), up to three types of amorphous ice can form. Interstellar ice is overwhelmingly low-density amorphous ice (LDA), which likely makes LDA ice the most abundant type in the universe. When cooled slowly, correlated proton tunneling occurs below −253.15 °C (20 K, −423.67 °F) giving rise to macroscopic quantum phenomena.

Ice is abundant on the Earth's surface, particularly in the polar regions and above the snow line, where it can aggregate from snow to form glaciers and ice sheets. As snowflakes and hail, ice is a common form of precipitation, and it may also be deposited directly by water vapor as frost. The transition from ice to water is melting and from ice directly to water vapor is sublimation. These processes plays a key role in Earth's water cycle and climate. In the recent decades, ice volume on Earth has been decreasing due to climate change. The largest declines have occurred in the Arctic and in the mountains located outside of the polar regions. The loss of grounded ice (as opposed to floating sea ice) is the primary contributor to sea level rise.

Humans have been using ice for various purposes for thousands of years. Some historic structures designed to hold ice to provide cooling are over 2,000 years old. Before the invention of refrigeration technology, the only way to safely store food without modifying it through preservatives was to use ice. Sufficiently solid surface ice makes waterways accessible to land transport during winter, and dedicated ice roads may be maintained. Ice also plays a major role in winter sports.