Melting Point Versus Freezing Point

Melting point and freezing point aren't always the same

Melting point and freezing point aren't always the same, particularly for pure substances.
Melting point and freezing point aren't always the same, particularly for pure substances. Atomic Imagery / Getty Images

You might be thinking the melting point and freezing point of a substance occur at the same temperature. Sometimes they do, but sometimes they don't. The melting point of a solid is the temperature at which the vapor pressure of the liquid phase and the solid phase are equal and at equilibrium. If you increase the temperature, the solid will melt. If you decrease the temperature of a liquid past the same temperature, it may or may not freeze!

This is supercooling and it occurs with many substances, including water. Unless there is a nucleus for crystallization, you can cool water well below its melting point and it won't turn to ice (freeze). You can demonstrate this effect by cooling very pure water in a freezer in a smooth container to as low as −42 °C. Then if you disturb the water (shake it, pour it, or touch it), it will turn to ice as you watch. The freezing point of water and other liquids may be the same temperature as the melting point. It won't be higher, but it could easily be lower.

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Helmenstine, Anne Marie, Ph.D. "Melting Point Versus Freezing Point." ThoughtCo, Oct. 15, 2017, thoughtco.com/melting-point-versus-freezing-point-3976093. Helmenstine, Anne Marie, Ph.D. (2017, October 15). Melting Point Versus Freezing Point. Retrieved from https://www.thoughtco.com/melting-point-versus-freezing-point-3976093 Helmenstine, Anne Marie, Ph.D. "Melting Point Versus Freezing Point." ThoughtCo. https://www.thoughtco.com/melting-point-versus-freezing-point-3976093 (accessed January 21, 2018).