Tests for the Common Gases
Four gases, four quick bench tests — this is the most reliably examined recall in C8, and the marks go to the exact method and the exact positive result. Learn each as a pair: what you do and what you see.
Hydrogenlit splint → squeaky pop
Oxygenglowing splint relights
Carbon dioxidelimewater turns milky
Chlorinedamp litmus bleached white
The four gas tests at a glance — each is a method paired with a single, unmistakable observation.
✅ The four tests — method and positive result
- Hydrogen (H2) — hold a lighted (burning) splint at the open end of the tube. Positive result: the gas burns rapidly with a squeaky “pop”.
- Oxygen (O2) — place a glowing splint inside the tube. Positive result: the splint relights.
- Carbon dioxide (CO2) — bubble the gas through limewater (calcium hydroxide solution). Positive result: the limewater turns milky / cloudy.
- Chlorine (Cl2) — hold damp litmus paper in the gas. Positive result: the litmus is bleached white.
💡 Two memory hooks
- A ligHted splint tests for Hydrogen; a glOwing splint tests for Oxygen.
- Chlorine bleaches — think of chlorine bleach. The colour is removed, leaving the paper white.
⚠️ Property vs test — where marks are lost
- CO2 putting out a lit splint is not the test. It is a property, but other gases (e.g. nitrogen) do the same, so it isn’t definitive. Quote the limewater test.
- “Chlorine smells like a swimming pool” is not a test. It’s a characteristic, not an identification — and chlorine is toxic, so use damp litmus in a fume cupboard.
- State the result precisely: oxygen relights the splint (not just “glows brighter”); limewater goes milky/cloudy (not “white”); litmus is bleached/white (not “clear”).
🧪 Exam-style questions
Q1
[1 mark]
What is the test for chlorine gas? Tick (✓) one box.
Chlorine bleaches damp litmus paper white. Relighting a glowing splint is oxygen; a squeaky pop is hydrogen; milky limewater is carbon dioxide.
Q2
[1 mark]
A gas relights a glowing splint. Which gas is it? Tick (✓) one box.
Oxygen relights a glowing splint — remember glOwing → Oxygen.
Q3
[2 marks]
A student says “carbon dioxide is the gas that puts out a lighted splint”. Why is this not a good test for carbon dioxide? Tick (✓) one box.
Putting out a splint is a property of CO2, but it’s shared by other unreactive gases (e.g. nitrogen), so it can’t identify CO2. The definitive test is limewater turning milky.
Q4
[1 mark]
Which test correctly identifies hydrogen? Tick (✓) one box.
Hydrogen burns rapidly at a lighted splint with a squeaky pop — ligHted → Hydrogen.