The method and the results are on the combustion lab. Do the analysis yourself first, then check the answers below.
Quick answer: Combustion is a fast reaction of a fuel with oxygen that releases heat and light. These answers go with the grade 8 science lab on combustion: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
Model answers follow the sample data in the table. Your own readings will differ, but the mist and the milky limewater should always appear for the candle gas and not for the control.
The mist is liquid water that condensed from water vapour made by the flame. It shows the wax contains hydrogen.
Question 3 The limewater result
Carbon dioxide is present. The white solid is calcium carbonate, which also shows the wax contains carbon.
Question 4 The control test
Ordinary air has a tiny amount of CO₂, so the control shows that the milkiness in the candle trials is caused by the extra CO₂ from burning. It makes the test fair.
Question 5 Equation for limewater
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Left: Ca 1, C 1, O 4, H 2. Right: Ca 1, C 1, O 4, H 2. Balanced.
Breath contains a higher concentration of CO₂ and may be blown in more directly; the burnt gas in the beaker is diluted with air. Breath has more CO₂ per volume.
Question 8 Improving the lab
Use the same volume of limewater and the same burn time each time, and repeat more times. Also keep the beaker above the flame at a fixed height. Control variables and repeat.
