Topic 3 of 4
Identify the gas, not just the bubbles
A named gas needs a specific test and observation.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
Identify the gas, not just the bubbles
A named gas needs a specific test and observation.
| Gas | Test | Positive observation |
|---|---|---|
| Hydrogen, H2 | Apply a lighted splint to a small collected sample | A pop |
| Oxygen, O2 | Insert a glowing splint | Splint relights |
| Carbon dioxide, CO2 | Bubble through limewater | White precipitate; prolonged excess CO2 can dissolve it |
| Ammonia, NH3 | Hold damp red litmus in the gas | Turns blue |
| Chlorine, Cl2 | Expose damp litmus paper to the gas | Bleaches; damp blue litmus may first redden |
| Sulfur dioxide, SO2 | Use acidified potassium manganate(VII) solution | Purple solution becomes colourless |
Litmus must be damp so the gas can dissolve and show its acidic or alkaline effect. Do not identify a gas by smell. Write "effervescence" for bubbles, then describe the confirmatory test. Carbon dioxide extinguishing a flame is less diagnostic than limewater; several gases fail to support combustion.
The sulfur dioxide test is required theory, but candidates are not required to carry out tests involving sulfur dioxide gas in the practical examination. Its colour change reflects reducing behaviour; other reducing substances can also decolourise acidified manganate(VII), so use the experimental context.