A heating curve (or cooling curve) shows how the temperature of a substance changes over time as it is steadily heated (or cooled).
Heating curve features
The graph has five distinct regions for a substance heated from solid to gas:
- Solid heating: temperature rises steadily. Energy increases KE of particles.
- Melting (flat section): temperature stays constant at the melting point. Energy breaks bonds between particles (latent heat of fusion). Solid and liquid coexist.
- Liquid heating: temperature rises again. Energy increases KE of particles.
- Boiling (flat section): temperature stays constant at the boiling point. Energy breaks bonds completely (latent heat of vaporisation). Liquid and gas coexist.
- Gas heating: temperature rises. Energy increases KE of particles.
Why temperature stays constant during state changes
All the energy supplied goes into breaking intermolecular bonds (overcoming attractive forces between particles). It does not increase the kinetic energy of the particles, so the temperature does not rise.
Cooling curves
The reverse process. Temperature drops, with flat sections at the boiling point (condensing) and melting point (freezing). During these flat sections, energy is released as bonds form.
Reading the graph
- The flat sections identify the melting and boiling points.
- The gradient of the sloping sections is related to the specific heat capacity. A steeper gradient means a lower heat capacity (the substance heats up faster).
- The length of the flat sections is related to the latent heat. A longer flat section means a higher latent heat.
Common errors and how to correct them
- Saying temperature decreases during boiling. Temperature stays constant during any change of state.
- Thinking the particles stop moving at a flat section. They continue moving; the energy goes into breaking bonds.
How to apply this in an exam
Identify each section of the graph. Explain what is happening to the particles and the energy in each section. Name the specific heat capacity or latent heat calculation that applies to each region.
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