Simple visual demonstrations allow me to introduce a science topic successfully and provide a good context for pupils to understand what they are learning. I have used this approach to introduce a variety of Year 7 topics, including combustion, and a lesson where pupils were required to design, build and test a simple fire extinguisher for putting out a lighted splint.
You need a two-litre plastic fizzy drink bottle, a sticky label and pen, a 600ml beaker, a carbon dioxide generator, a candle and matches. The carbon dioxide generator can be made using marble chips with 50mls of 1molar hydrochloric acid in a conical flask, and a delivery tube. Alternatively, you could use carbon dioxide from a compressed-gas cylinder.
Before the lesson, fill up the bottle with carbon dioxide and replace the lid. Label the bottle "magic air". Ask pupils to sit round the front and line up the bottle, beaker and candle. Light the candle. Explain that you are going to use "magic air" to extinguish the candle, but as the gas is invisible you will have to pour it very carefully to avoid spillages. Try to build up their anticipation - eg, by turning off the lights.
First, "pour" the air out of the empty beaker above the candle to show that normal air has no effect. Next, remove the lid from the "magic air" bottle and pour the carbon dioxide slowly into the beaker. Then carefully pour the contents of the beaker, now containing carbon dioxide, over the candle and the flame will be extinguished quickly.
Richard Waller is head of KS3 science, Comberton Village College, Cambridge