Individual Gas Tests in Detail

🔥 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)