Students should be able to interpret heating and cooling graphs that…

The idea

Heating and cooling graphs show how temperature changes with time as energy is transferred. A sloping section shows a temperature change in a single state, where the gradient depends on the mass, specific heat capacity and rate of energy transfer. A flat section, or plateau, shows a change of state at a constant temperature; here energy is used to break or form intermolecular forces rather than to change the average kinetic energy of the particles. On a heating graph, flat sections occur at the melting point and boiling point. On a cooling graph, flat sections occur at the condensing point and freezing point. The length of a plateau depends on the mass and the specific latent heat of the substance.

For example: A pure ice sample heated steadily shows a plateau at 0 °C while melting and another at 100 °C while boiling.

What earns the marks

• Identify sloping sections as periods where the temperature of a single state changes and no change of state occurs.
• Identify flat sections as changes of state where energy is transferred but the temperature remains constant.
• Label the melting point and boiling point on a heating graph, and the condensing point and freezing point on a cooling graph.
• Relate the length of a plateau to the mass of substance and its specific latent heat.
• Explain that during a change of state, energy changes the potential energy of the particles rather than their average kinetic energy.

Common mistakes and how to put them right

• Believing that a flat section means no energy is being transferred.

  Put it right by stating that energy is still supplied or removed but is used to change the arrangement of particles, not the temperature.

• Labelling the first plateau on a heating graph as boiling rather than melting.

  Put it right by checking the order of state changes as temperature increases.

• Assuming the gradient of a sloping section depends only on the rate of heating.

  Put it right by noting that gradient also depends on the mass and specific heat capacity of the substance.

Exam tips

• Annotate each part of the graph with the state or state change before answering questions about it.
• Use the terms melting, boiling, condensing and freezing precisely, matching each to the correct plateau on a heating or cooling graph.
• When comparing plateaus, refer to both the mass of the sample and the relevant specific latent heat to justify differences in length.