What Celsius Temperature Does Water Freeze? The Simple Answer
Water freezes at 0 °C (32 °F) at standard atmospheric pressure. This is the freezing point of pure water, where liquid water and ice can coexist in equilibrium.
The exact freezing point depends on conditions. Dissolved substances such as salt lower it, while pressure can shift it. For most everyday situations, though, 0 °C is the useful answer.
Why water freezes at 0 °C
As water loses heat, it can change from liquid to solid. At standard atmospheric pressure, this change occurs at 0 °C. Freezing and melting happen at the same temperature under the same pressure: ice melts at 0 °C, and liquid water freezes at 0 °C.
The USGS Water Science School describes freezing point as one of water’s physical properties. The NIST Chemistry WebBook lists water’s thermophysical data, including its melting point.
What can change the freezing point?
Dissolved substances: Salt lowers water’s freezing point. This is why seawater and water mixed with road salt can remain liquid below 0 °C. The more dissolved material, the lower the freezing point can be, though the exact temperature depends on the mixture.
Pressure: Water’s freezing point shifts when pressure changes. Since ice is less dense than liquid water, increasing pressure lowers the freezing point. For ordinary weather and household conditions, this shift is usually small.
Very pure, still water can sometimes remain liquid below 0 °C. This is called supercooling. Disturbing the water or introducing a small ice crystal can prompt it to freeze.
Key Takeaways
- Pure water freezes at 0 °C (32 °F) at standard atmospheric pressure.
- Freezing and melting occur at the same temperature under the same pressure.
- Salt and other dissolved substances lower the freezing point.
- Pressure changes can shift the freezing point; the effect is small in most everyday settings.
- Pure, still water can sometimes supercool below 0 °C before freezing.
