What Is the Temperature of Ice Water? The Exact Answer

The temperature of ice water that is actively melting—a mixture of ice and liquid water at standard atmospheric pressure—is exactly 0 degrees Celsius (32 degrees Fahrenheit). This is the freezing point of water, and it remains constant as long as both solid ice and liquid water are present together.

Why the Temperature Stays Fixed

This fixed temperature is a result of a phase change. When ice melts, it absorbs heat energy from its surroundings to break the crystalline bonds of the solid, but this energy does not raise the temperature of the mixture. Instead, it drives the transformation from solid to liquid. As noted by the National Institute of Standards and Technology, the melting point of pure water ice at one atmosphere of pressure is a primary fixed point on the Celsius scale. The Royal Society of Chemistry explains that as long as any ice remains, the system will stay at 0 °C because any added heat goes into melting more ice, not warming the water.

A practical example: if you take a glass of ice water and leave it on a table at room temperature, the drink will remain at 0 °C until all the ice has melted. Only after the last ice cube disappears will the water temperature begin to rise toward the ambient air temperature.

How Pressure Changes the Melting Point

The 0 °C value holds only at standard atmospheric pressure (1 atmosphere, or about 101.325 kPa). If you change the pressure, the melting point shifts. Increasing pressure lowers the melting point of ice slightly. This is why ice skates work: the blade’s narrow edge exerts high pressure on the ice, causing a thin layer to melt and provide lubrication. The effect is small—roughly a drop of 0.0074 °C for every additional atmosphere of pressure.

Conversely, reducing pressure raises the melting point. At high altitudes, where atmospheric pressure is lower, the freezing point of pure water rises only slightly—by about 0.01 °C for a significant drop in pressure. These pressure dependencies are documented by the National Institute of Standards and Technology as part of the thermodynamic properties of water.

Key Takeaways

• Ice water at standard pressure is exactly 0 °C (32 °F) while both ice and liquid water coexist.
• The temperature stays constant because added heat energy is used for melting, not for raising temperature.
• Changing pressure or adding impurities shifts the melting point away from 0 °C.
• An ice-water bath is a reliable reference point for calibrating thermometers.