Heating and cooling curves

This lesson covers: 

  1. What are heating and cooling curves?
  2. The energy changes that occur during heating and cooling.

What are heating and cooling curves?

Diagram showing heating and cooling curves with temperature changes during state changes from solid to liquid to gas.

Heating and cooling curves show how temperature changes when a substance is heated up or cooled down.

The curves are flat when the substance changes state, like melting, boiling, freezing or condensing.

Heating curves

Graph showing temperature changes over time during heating, with phases for solid, melting, liquid, boiling, and gas.
  1. When a solid is heated, the particles gain energy and vibrate more. The temperature rises as heat is added.
  2. At the melting point, energy goes into breaking bonds between particles, turning the solid to liquid.
  3. The temperature stays the same during melting, giving a flat line on the graph.
  4. The same happens when a liquid is heated to boiling point. Energy is used to break the bonds completely, forming a gas.
  5. The temperature stays steady during boiling, giving another flat line.

Cooling curves

Graph showing the cooling curve from gas to liquid to solid, with flat lines at the condensation and freezing points.
  1. When a gas is cooled, particles slow down and lose energy. The temperature drops as heat is given off.
  2. At the condensation point, particles come together and bond turning the gas into a liquid. The energy released stops the temperature dropping further.
  3. The temperature stays the same during condensation giving a flat line on the graph.
  4. The same happens a when liquid is cooled to freezing point. Particles arrange into solid structure, releasing heat energy.
  5. This stops temperature dropping as liquid becomes solid, giving another flat line.