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A cooling curve of naphthalene from liquid to solid.

A cooling curve is a line graph that represents the change of phase of matter, typically from either a gas to a solid or a liquid to a solid. Time is used in the x-axis while temperature is used for the y-axis. They are often used in chemistry and physics, and can apply (misleadingly) to matter behaviour during heating, as well as during cooling.

Cooling curve graphs are useful as they clearly show up a phenomenon of phase changes of matter. To explain this, we shall use water as our example. First we start with a sealed container of steam, temperature of which is, say, 150°C. As we measure the temperature of the rapidly cooling gas, we see that it falls at a rate that is typical of the specific heat capacity of steam; until when it reaches 100°C, when something odd happens - the temperature stagnates. After an amount of liquid water has collected the temperature continues to drop, again proportional to the specific heat capacity of water (different to that of steam), until again the temperature stagnates, this time at 0°C. Later, the temperature again resumes falling.

The explanation is that different phases of matter are associated with different energy levels. Steam at 100°C is the same temperature, but contains much more thermal energy than liquid water at 100°C. The same goes for water and ice at 0°C. This is because molecules of water are much more free to move around as a gas than as a liquid, and that freedom of movement means there is much more kinetic energy associated with each molecule, and that energy is transferred as the substance shifts phase - explaining why energy seemingly disappears when boiling a kettle.

Graph The kinetic energy decreases and the potential energy stays the same, except at the plateaus(phase change) the kinetic energy stays the same and the potential energy decreases.