When a hydrocarbon burns, two substances are produced; water and carbon dioxide. This practical allows students to literally see that these are produced as the candle burns.
The candle should be lit and placed on a heatproof mat. An upturned empty and dry gas jar should be placed over the candle. Ensure there is no gap under the jar where gas can get through.
After a few seconds the candle will go out because it will have used up all of the oxygen in the jar. A lid is then placed onto the gas jar. To test for water present, students can rub a piece of cobalt chloride paper around the inside glass of the jar. If water is present the paper will turn from a vibrant blue to a pink colour.
Adding a little limewater to the jar and swilling it around will detect any carbon dioxide present. The limewater will turn cloudy if it comes into contact with carbon dioxide.
Equipment required (per set):
- Gas jar and lid
- Candle in tray
- Dry blue cobalt chloride paper
- Heatproof mat