Revision Notes

2.3.2 Convection

What is Convection?

  • Convection is the main way that heat travels through liquids and gases
    (Convection cannot happen in solids)
  • When a liquid (or gas) is heated:
    • The molecules push each other apart, making the liquid/gas expand
    • This makes the hot liquid/gas less dense than the surroundings
    • The hot liquid/gas rises, and the cooler (surrounding) liquid/gas moves in to take its place
    • Eventually the hot liquid/gas cools, contracts and sinks back down again
    • The resulting motion is called a convection current

Convection, IGCSE & GCSE Physics revision notes

When a liquid or gas is heated, it becomes less dense and rises


Demonstrating Convection

  • A simple demonstration of convection in liquids involves taking a beaker of water and placing a few crystals of potassium permanganate in it, to one side, as shown in the diagram above
  • When the water is heated at that side, the potassium permanganate will dissolve in the heated water and rise along with the warmed water, revealing the convection current

Potassium convection, IGCSE & GCSE Physics revision notes

Diagram showing an experiment with potassium permanganate to demonstrate convection


Exam Tip

If a question on heat mentions liquids or gases the answer will probably be about convection.

Heat does not rise (only hot gases or liquids rise).


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Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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