Inorganic Chemistry · 4 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 8% of your exam marks.
Ion tests, flame tests and gas tests all appear regularly; expect to recall observations.
| Gas | Test | Positive result |
|---|---|---|
| Ammonia, NH3 | Hold a piece of damp red litmus paper at the mouth of the tube | The paper turns blue |
| Carbon dioxide, CO2 | Bubble the gas through limewater (Ca(OH)2(aq)) | The limewater turns milky (cloudy white) |
| Chlorine, Cl2 | Hold a piece of damp blue litmus paper at the mouth of the tube | The paper turns red and is then bleached white |
| Hydrogen, H2 | Hold a lit splint at the mouth of the tube | A loud squeaky pop sound (hydrogen burns rapidly with the oxygen in the air) |
| Oxygen, O2 | Hold a glowing splint inside the mouth of the tube | The splint relights |
Testing for chlorine gas
What comes up: Describe the test and the positive result for chlorine.
Write (two marks): (1) Hold a piece of damp blue litmus paper at the mouth of the tube. (2) The paper first turns red, then is bleached white.
Watch out: Chlorine dissolves in the moisture on the paper to form an acidic solution, so damp blue litmus turns red before the chlorine bleaches it white. If you start with red litmus paper the red-to-red colour change is not visible, so only the bleaching (turning white) shows. The mark scheme credits only one mark if you use red litmus paper (the bleaching step only). To score both marks you must specify damp blue litmus paper and state it turns red then is bleached white, or simply state it is bleached/turns white.
Hydrogen vs oxygen — lighted splint or glowing splint?
What comes up: State the test and the positive result for hydrogen or oxygen.
Write: For hydrogen, hold a lighted (lit) splint at the mouth of the tube — you hear a squeaky pop. For oxygen, insert a glowing splint into the tube — the splint relights.
Watch out: The mark scheme explicitly rejects using a glowing splint to test for hydrogen. A lighted splint for the oxygen test will also not score full marks. Keep the two separate: lighted = hydrogen, glowing = oxygen.