What temperature does water freeze at?
Quick answer
Pure water freezes at 0°C, which is 32°F or 273.15 K, at normal sea-level pressure. Salt water freezes at a lower temperature: seawater freezes at about −2°C (28°F).
By the numbers
- 0 °C
- Freezing point of pure water
- 32 °F
- The same temperature in Fahrenheit
- 273.15 K
- The same temperature in kelvin
- −2 °C
- Approximate freezing point of seawater
Freezing and boiling in three scales
| Celsius | Fahrenheit | Kelvin | |
|---|---|---|---|
| Water freezes | 0 °C | 32 °F | 273.15 K |
| Water boils (sea level) | 100 °C | 212 °F | 373.15 K |
To convert Celsius to Fahrenheit, multiply by 9/5 and add 32. So 10°C is 50°F, and 25°C is 77°F.
Why salt water freezes at a lower temperature
Dissolved salt gets in the way of water molecules trying to line up into ice crystals, so the water has to get colder before it can freeze. This is why cities spread salt on icy roads: it lowers the freezing point and melts the ice, though it stops working well below about −10°C (14°F).
Can water stay liquid below 0°C?
Yes. Very pure, still water can be supercooled well below its freezing point, in extreme cases to around −40°C, because ice needs a starting point such as a speck of dust or a scratch to begin forming. Shake or tap a bottle of supercooled water and it can turn to slush in seconds.
Why does ice float?
Most substances get denser when they freeze. Water does the opposite: as it freezes, its molecules lock into an open crystal pattern that takes up about 9% more space. That makes ice lighter than liquid water, so lakes freeze from the top down, and the water beneath stays liquid for fish through the winter.
Cooking or measuring? Here’s how many ounces are in a cup, and why a cup of water weighs more than a cup of flour.
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