The freezing point of water is 0 degrees Celsius under standard atmospheric pressure.
At this temperature, liquid water and solid ice can coexist in equilibrium. As water loses heat below the freezing point, its molecules settle into the ordered crystal structure of ice. The Celsius scale was historically based on water’s phase changes, with 0 degrees marking its freezing point and 100 degrees marking its boiling point at sea-level pressure.
The exact freezing point can vary slightly with conditions. Dissolved substances such as salt lower water’s freezing point, which is why salt helps melt ice on roads. Pressure also affects the melting point of ice, although ordinary changes in weather pressure have only a tiny effect. Pure water can even remain liquid below 0 degrees Celsius if ice crystals do not begin forming; this is called supercooling.
Do not confuse Celsius with Fahrenheit: 0 degrees Celsius equals 32 degrees Fahrenheit. Water’s boiling point is normally about 100 degrees Celsius, but it falls at high elevations because atmospheric pressure is lower.