Freezing is the change that occurs when a liquid changes into a solid as the temperature decreases. Melting is the opposite change, from a solid to a liquid as the temperature increases. These are both examples of changes in the states of matter of substances.

Substances freeze at exactly the same temperature as they melt. As a consequence, the temperature at which—under a specified pressure—liquid and solid exist in equilibrium is defined as the melting or freezing point. When the pressure is one atmosphere, this temperature is known as the normal freezing (or melting) point. A change in pressure will change the temperature at which the change in the state of matter occurs. A decrease in pressure will decrease the temperature at which this occurs and an increase in pressure will increase the temperature required.

At a fundamental level freezing and melting...