Every IGCSE paper expects you to identify common gases from a simple test, and to write the observation exactly. The tests are short and specific: a lit splint, a glowing splint, limewater, damp litmus, damp red litmus. What loses marks is not the chemistry but the wording - saying a gas 'turns limewater cloudy' earns the mark, while saying it 'reacts with limewater' does not.
CO2 + Ca(OH)2 → CaCO3 + H2O
What you need
Test tubes, bungs and delivery tubes
Wooden splints
Limewater
Damp red and blue litmus paper
A supply of each gas
How it is done
Collect the gas in a test tube and stopper it.
Hydrogen: hold a lit splint at the mouth of the tube.
Oxygen: put a glowing splint into the tube.
Carbon dioxide: bubble the gas through limewater.
Ammonia: hold damp red litmus in the gas. Chlorine: hold damp blue litmus in it.
What you should see
Hydrogen burns with a squeaky pop.
Oxygen relights a glowing splint.
Carbon dioxide turns limewater milky.
Ammonia turns damp red litmus blue.
Chlorine turns damp blue litmus red, then bleaches it white.
Where marks are lost
Using a lit splint for oxygen. It must be a glowing splint that relights.
Writing "limewater goes cloudy" without saying it was carbon dioxide that did it.
Forgetting the bleaching step for chlorine. Red alone is not enough.
Using dry litmus paper. It must be damp, or nothing happens.
Questions students ask
Why does limewater go milky?
Carbon dioxide reacts with calcium hydroxide to make calcium carbonate, which is insoluble and forms a fine white suspension.
Why must litmus be damp?
The gas has to dissolve in the water on the paper to form ions. A dry gas and dry paper give no reaction.
What if the limewater goes clear again?
Excess carbon dioxide converts the calcium carbonate to soluble calcium hydrogencarbonate. That is why you bubble the gas through briefly.
Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by
Ajay Shekhawat, founder of MrChemCoach. Run the simulator above,
then check yourself against the questions.