What is the freezing point of water?
Pure water freezes at 0°C (32°F) at normal air pressure. Salt water freezes at lower temperatures, which is why salt is spread on icy roads.
As water cools, its molecules slow down. At 0°C, hydrogen bonds lock them into the open crystal pattern of ice. The Fahrenheit scale puts this point at 32 degrees, while the Kelvin scale used by scientists puts it at about 273.15 K.
Dissolved substances lower the freezing point, an effect called freezing-point depression. Seawater, with about 3.5 percent salt, freezes at about −2°C (28°F). Road salt works the same way: it dissolves into a thin layer of water on ice, forming a brine that stays liquid at lower temperatures.
Water that does not freeze at zero
Very pure water with nothing for ice crystals to form on can be supercooled well below 0°C and stay liquid. Tiny cloud droplets often remain liquid at −20°C or colder. When a supercooled droplet touches a cold surface, such as an airplane wing, it can freeze instantly.
Did You Know?
Some fish in Antarctic waters produce antifreeze proteins in their blood. These proteins stop small ice crystals from growing, so the fish can live in seawater that is below the normal freezing point of their body fluids.