Colourless gases look identical, but each one reacts in a way that gives it away: a squeaky pop, a splint bursting into flame, limewater turning milky. Learn the six tests and the exact words the mark scheme expects.
Ammonia turns damp red litmus paper blue. Carbon dioxide turns limewater milky. Chlorine bleaches damp litmus paper. Hydrogen gives a squeaky pop with a lighted splint. Oxygen relights a glowing splint. Sulfur dioxide turns acidified aqueous potassium manganate(VII) from purple to colourless. The litmus must be damp, because a gas has to dissolve in water before it can act as an alkali, acid or bleach.
Learn the test and the result as a pair. The mark is usually for the observation, in the exact words.
| Gas | Test | Positive result |
|---|---|---|
| ammonia, NH3 | damp red litmus paper | turns blue |
| carbon dioxide, CO2 | bubble through limewater | limewater turns milky |
| chlorine, Cl2 | damp litmus paper | bleached (turns white) |
| hydrogen, H2 | lighted splint | squeaky pop |
| oxygen, O2 | glowing splint | relights |
| sulfur dioxide, SO2 | acidified aqueous potassium manganate(VII) | turns from purple to colourless |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which gas relights a glowing splint?
What is the test for carbon dioxide?
Which gas bleaches damp litmus paper?
Which gas turns acidified aqueous potassium manganate(VII) from purple to colourless?
Warm an ammonium salt with alkali in the simulator and hold damp red litmus at the mouth of the tube.
| Gas produced by | Gas | Test that confirms it |
|---|---|---|
| ammonium salt + aqueous NaOH, warmed | ammonia | damp red litmus turns blue |
| carbonate + dilute acid | carbon dioxide | limewater turns milky |
| reactive metal (Mg, Zn) + dilute acid | hydrogen | squeaky pop |
| hydrogen peroxide with a manganese(IV) oxide catalyst | oxygen | relights a glowing splint |
| electrolysis of concentrated aqueous sodium chloride (anode) | chlorine | bleaches damp litmus |
| sulfite + dilute acid; burning sulfur | sulfur dioxide | acidified KMnO4 purple to colourless |
A lighted splint is for hydrogen (squeaky pop); a glowing splint is for oxygen (relights). It is the limewater that turns milky, not the gas. Ammonia turns red litmus blue, so blue litmus would show no change. Chlorine bleaches litmus; "turns red" alone does not score.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
A gas turns damp red litmus paper blue. Which gas is it?
Zinc is added to dilute sulfuric acid. Which test identifies the gas produced?
Why must the litmus paper be damp when testing for ammonia?
A sample of a sulfite is warmed with dilute hydrochloric acid. Which observation identifies the gas?
This skill is now recorded on your profile. Keep the chain going.