What Is the Water Freezing Point? Quick Answer & Facts

Water freezes at 32 degrees Fahrenheit (0 degrees Celsius) at standard atmospheric pressure. That is the direct answer. This value is a fundamental reference in science and daily life, used to calibrate thermometers and define weather forecasts.

Why the Freezing Point Matters

The freezing point of water is a fixed point in the International System of Units (SI), where 0 °C is defined as the temperature at which pure water transitions from liquid to solid at sea level. This standard is maintained by organizations such as the National Institute of Standards and Technology NIST. In practice, water does not always freeze at exactly 0 °C. Impurities like salt lower the freezing point, which is why roads are salted in winter. For example, seawater freezes at about -2 °C (28.4 °F) because of its dissolved salts. Pure water can also supercool, remaining liquid below 0 °C until disturbed.

The Royal Society of Chemistry explains that the freezing point is the same as the melting point for a pure substance Royal Society of Chemistry. This means ice melts at 0 °C just as water freezes at 0 °C, under the same pressure. Pressure changes the freezing point slightly. At higher altitudes, where air pressure is lower, water freezes at a temperature very close to 0 °C but not exactly the same. The difference is tiny, usually less than a hundredth of a degree, so it matters only in precise laboratory work.

The Effect of Pressure on Freezing

Pressure shifts the freezing point of water in a way that is unusual compared to most liquids. For most substances, increasing pressure raises the freezing point. Water does the opposite: higher pressure lowers its freezing point. This happens because ice is less dense than liquid water. When you squeeze water, it resists forming the more open crystal structure of ice, so it stays liquid at a slightly lower temperature.

A practical example is ice skating. The pressure from a skate blade lowers the freezing point of the ice directly beneath it, creating a thin film of liquid water that reduces friction. Once the blade passes, the water refreezes. The effect is small. To lower the freezing point by just 1 °C, you need about 130 atmospheres of pressure. That is roughly 1,900 pounds per square inch, far more than a skate blade applies. The thin liquid layer on ice is better explained by frictional heating than by pressure melting.

Key Takeaways

• The freezing point of pure water is 0 °C (32 °F) at standard atmospheric pressure.
• Impurities, especially salt, lower the freezing point, which is why salt is used for de-icing.
• Pure water can supercool, staying liquid below 0 °C until it is disturbed or seeded with an ice crystal.
• Higher pressure lowers the freezing point of water, unlike most other liquids.