Use this for questions related to the solid phase of Water ($\mathrm{H_2O}$).
Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or 32 °F. Ice can have different forms and properties, depending on the pressure, temperature and impurities. For example, ice can be clear, cloudy, or colored; it can be hard, soft, or brittle; it can have different crystal structures and shapes.
The freezing point of water is the temperature at which water changes from liquid to solid phase. The freezing point of water depends on the pressure above it, but not very much. At atmospheric pressure (1 atm or 101.3 kPa), the freezing point of water is 0°C or 273.15 K. If the pressure is increased, the freezing point of water decreases slightly, because water expands when it freezes and higher pressure makes it harder for water molecules to move apart. For example, at a pressure of 200 MPa (about 2000 times the atmospheric pressure), the freezing point of water is -0.83°C or 272.32 K. If the pressure is decreased, the freezing point of water increases slightly, until it reaches the triple point of water, where liquid, solid and gas phases can coexist. The triple point of water is at a pressure of 0.006 atm or 0.61 kPa and a temperature of 0.01°C or 273.16 K. Below this pressure, water cannot exist as a liquid, only as ice or vapor.
