This deep dive covers Individual Gas Tests in Detail within Gas Tests for GCSE Chemistry. Revise Gas Tests in Chemical Analysis for GCSE Chemistry with 20 exam-style questions and 15 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 4 of 13 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 4 of 13
Practice
20 questions
Recall
15 flashcards
🔥 Individual Gas Tests in Detail
1. Hydrogen (H₂) — Burning Splint Test
Method: Collect gas in test tube. Apply a lit wooden splint to the mouth of the test tube.
Positive result: A "squeaky pop" sound is heard.
Equation: 2H₂(g) + O₂(g) → 2H₂O(l)
Safety: Hydrogen is highly flammable — only use small volumes; point tube away from people.
2. Oxygen (O₂) — Glowing Splint Test
Method: Light a splint, blow it out so it is just glowing (not burning). Insert the glowing splint into the gas.
Positive result: The glowing splint relights (bursts back into flame).
Why glowing, not burning? A burning splint already has enough oxygen from the air. The glowing splint is just below the ignition threshold — extra oxygen pushes it back into combustion.
3. Carbon Dioxide (CO₂) — Limewater Test
Method: Bubble the gas through limewater (calcium hydroxide solution, Ca(OH)₂).
Positive result: Limewater turns milky/cloudy.
Equation: CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)
The white precipitate of calcium carbonate (CaCO₃) makes the clear solution appear milky.
4. Chlorine (Cl₂) — Damp Litmus Paper
Method: Hold damp blue litmus paper in the gas.
Positive result: Damp blue litmus paper is bleached white.
Equation: Cl₂(g) + H₂O(l) → HClO(aq) + HCl(aq)
The hypochlorous acid (HClO) formed destroys the colour in the litmus dye. The paper must be damp — dry litmus will not bleach.
Safety: Chlorine is toxic and corrosive — always use a fume cupboard.
5. Ammonia (NH₃) — Red Litmus or HCl Rod
Method 1: Hold damp red litmus paper in the gas → turns blue (ammonia is alkaline).
Method 2: Hold a glass rod dipped in concentrated HCl near the gas → produces white fumes of ammonium chloride.
Equation (Method 1): NH₃(g) + H₂O(l) → NH₄⁺(aq) + OH⁻(aq)
Equation (Method 2): NH₃(g) + HCl(g) → NH₄Cl(s)