If you want to have a "eureka moment", proving to your students that 20 per cent of the air is oxygen, you need to modify the apparatus usually shown in textbooks. The usual method for key stage 3 is to place a candle in a trough of water and cover it with a large jar. As the candle burns it uses up the oxygen in the jar and the water rises to fill about 20 per cent of the jar's volume. Sadly, this never gives good results, so I teamed up with my Year 7 students to create a way that does.
With the help of our science technicians I demonstrated our new method and the class discussed why this did the trick. It is so much more impressive than the old way that one girl burst into song: "There's a little magic in the air ..."!
* Juse several centimetres of water in a large trough. Dye in the water is more dramatic;
* float the candle on a plastic lid. Fix it into the lid with a little melted wax;
* Juse an open top bell jar, with a bung. Leave the bung out at first. The bell jar must have a capacity of around a litre;
* Jmake sure the lip of the bell jar is resting slightly off the bottom of the trough. We found microscope slides worked best as they stayed in place under water;
* light the candle and gently place the bell jar over it;
* last put the bung in the bell-jar neck.
As the candle burned we clearly observed it float up on the rising water level inside the bell jar, above the level of the water outside. Pupils thought about why even our way only achieved the 20 per cent volume mark and reasoned that the carbon dioxide released by the burning candle affected the result.
Marlene Griffin, science teacher, Hitchin Girls' School, Herts