Burning Issues

Learners use a candle to investigate the products of combustion. When a glass rod is held over a lit candle, the candle flame deposits carbon on the rod. Then the candle and a drop of pH indicator are covered by a glass jar. Water vapor condenses on the sides of the jar, and carbon dioxide gas reacts with the water in the indicator to form carbonic acid. The candle goes out, and the indicator changes colors to show the presence of an acid. The combustion reaction of a candle is similar to the burning of wood or gas. Combustion reactions are responsible for some types of environmental pollution, including smog. Use this activity to introduce learners to combustion as well as greenhouse gases and global warming. For safety reasons, this activity works best as a demonstration for younger audiences.