Cooling curves

Changes of state can be investigated by measuring the temperature as a substance changes state. There are two possibilities:

  • heat a substance and measure its temperature, for example, melt ice or boil water
  • allow a substance to cool and measure its temperature

It is often easier to allow a substance to cool down. Salol is a solid used in these investigations. To investigate its cooling curve:

  1. Put some salol and a thermometer into a boiling tube.
  2. Put the boiling tube in a hot water bath. Allow the salol to melt and reach the temperature of the hot water.
  3. Take the boiling tube out of the hot water.
  4. Measure and record the temperature of the salol every minute for about 20 minutes, stirring briefly to evenly mix the hot and cold parts.
  5. Plot a graph of temperature against time.

The diagram shows a cooling curve for salol. Notice that the temperature stays the same during the state change (freezing), and this is the melting point (or freezing point) of the salol.

A cooling curve for salolA cooling curve for salol

The temperature stays the same during a state change:

  • during melting and evaporating, internal energy increases as the motion of particles increases and bonds are broken
  • during condensing and freezing, internal energy decreases as the motion of particles decreases and new bonds are formed