Testing for carbon dioxide in combustion

 We burned natural gas. The products of full combustion are carbon dioxide and water. To show that carbon dioxide was produced we used the fact that hot gases are less dense than the surrounding air and float upwards into the boiling tube. We had a bung ready on the table. When the tube had cooled we added limewater.

 Limewater is normally clear but when you shake the tube, the carbon dioxide reacts with the soluble calcium hydroxide to produce calcium carbonate which is insoluble. Hence the limewater turns cloudy.