Skip to content
Grade 7 Beginner About 15 min Version 1 Awaiting review Premium

Convection currents in water

Convection is the way heat travels through liquids and gases. When water at the bottom of a pan is heated, it expands and becomes less dense, so it rises. Cooler, denser water from above sinks to take its place, is heated in turn, and rises too. This circulation is a convection current. It carries heat all through the water. A crystal of potassium permanganate dissolves into a purple dye that makes the current visible.

What you will learn

  • Describe convection as the movement of a heated liquid or gas.
  • Explain convection using density.
  • Give examples of convection in everyday life.

Before you start

  • Know that most substances expand when heated.

Materials

  • Water
  • A small crystal of potassium permanganate (or a drop of food colouring)

Equipment

  • Large beaker, tripod and gauze
  • Burner or spirit lamp

Procedure

  1. Put the dye crystal in the corner without heating it, wait 60 s and record.
  2. Light the burner on low under that corner and record the dye after 30 s and 60 s.
  3. Turn the burner to high and compare.

How this simulation is simplified

  • The water starts at 25 °C. The current flows at 2 mm/s for each unit of burner power, and one loop round the beaker is 400 mm.