The Gas Detective
Match each gas to the test that unmasks it
What this lesson covers
Why it matters
Ten unlabelled gas jars sit on the bench. Most are colourless, several are odourless — yet a chemist can name almost every one armed with just a splint, a strip of litmus and a drop of limewater. Under the right test, every gas betrays itself.
Predict first
A colourless, odourless gas fills a test tube. You bring a burning splint to its mouth and hear a sharp 'pop'. Which gas is it?
The 'pop' is a tiny explosion of hydrogen burning with air: 2H₂ + O₂ → 2H₂O. Oxygen never pops — it silently relights a glowing splint. Keep those two splint tests apart!
- Hydrogen — correct
- Oxygen
- Carbon dioxide
- Chlorine
What you do
Six gases, six confessions. Match each gas to the one observation that unmasks it beyond doubt — colour and smell give hints, but splints and test papers give proof.
Check yourself
Dilute acid on sodium sulphite gives a gas smelling of burning sulphur. Which result confirms it is SO₂ and not CO₂?
Limewater milkiness is shared by CO₂ and SO₂. Only SO₂ reduces acidified K₂Cr₂O₇, turning it from orange to green — the book's distinguishing test.
Steamy vapours from a heated crystal condense into colourless droplets on the cool part of the test tube. Which test proves the droplets are water?
Water regenerates the hydrate: CuSO₄ (white) + 5H₂O → CuSO₄·5H₂O (blue). Blue cobalt chloride paper turning pink works too.
Heating lead nitrate gives a reddish-brown pungent gas plus a gas that relights a glowing splint. Which pair has been released?
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. The reddish-brown colour marks NO₂; the relit glowing splint marks O₂. Pop (burning splint, H₂) and rekindle (glowing splint, O₂) are different tests.
- It turns limewater milky
- It turns acidified potassium dichromate paper from orange to green — correct
- It gives a pop with a burning splint
- They turn white anhydrous copper sulphate blue — correct
- They turn blue copper sulphate crystals white
- They turn limewater milky
- Nitrogen dioxide and oxygen — correct
- Nitrogen dioxide and hydrogen
- Chlorine and oxygen