What Is the Temperature of Ice Water: The Science of Melting Ice

Pure ice water is 0 °C (32 °F) when ice and liquid water coexist in thermal equilibrium at standard atmospheric pressure. If all the ice has melted, the liquid can warm above 0 °C. If the water contains dissolved substances, its freezing point may be lower.

“Ice water” means a mixture of solid ice and liquid water. When both phases are present and have reached equilibrium, they share the same temperature. The reading is not an average of the ice temperature and the water temperature.

Why ice water stays near 0 °C

When heat enters an ice-water mixture, it first helps melt the ice. This energy is called the latent heat of fusion. While ice remains and the mixture stays at equilibrium, the temperature stays close to 0 °C instead of rising with the added heat.

Once the last ice melts, further heat warms the liquid. The length of the melting period depends on how much ice there is and how quickly heat enters the mixture.

A thermometer reading can differ from the equilibrium temperature if the mixture has not been stirred or the thermometer is poorly placed. For a representative reading, stir the mixture and keep the thermometer in the liquid without letting it touch the container or rest against a piece of ice.

One example

Suppose a glass contains ice and water, with some ice still visible. If the mixture is pure and has reached equilibrium at ordinary atmospheric pressure, its temperature is about 0 °C (32 °F). Adding warmth to the glass melts some of the ice. The temperature remains near 0 °C while ice is still present. After all the ice melts, the liquid temperature can rise.

This example assumes the mixture is well mixed and the thermometer measures the liquid accurately. A reading taken near the glass wall or in an unmixed area may differ.

What can change the reading?

The 0 °C answer applies to pure water at standard atmospheric pressure. Several conditions can affect a real measurement:

• Dissolved substances: Salt and other solutes lower water’s freezing point. A salty ice-water mixture can therefore remain liquid below 0 °C.
• Pressure: The melting point of ice changes with pressure. At ordinary pressures, the change is small.
• Uneven mixing: Water in different parts of a container may have slightly different temperatures while heat is entering or the mixture is settling. Stirring helps reduce these local differences.
• Thermometer placement: A thermometer touching the container or ice may not show the temperature of the surrounding liquid.

For a simple check of a thermometer, use a well-mixed bath of crushed ice and water, with enough water to fill the spaces between the ice. Keep some ice present during the measurement. Under suitable conditions, the reading should be close to 0 °C.

Key Takeaways

• Pure ice water at standard atmospheric pressure is about 0 °C (32 °F) while ice and liquid water coexist in equilibrium.
• Heat melts the ice before it raises the temperature of the mixture.
• Once the ice has melted, the liquid can warm above 0 °C.
• Dissolved substances, pressure, uneven mixing, and thermometer placement can affect a real reading.
• Stir an ice-water mixture and keep the thermometer away from the container and ice for a more representative measurement.