Identification of Common Gases (C8.2)

Gas tests — the essential four

These tests are examined directly and must be memorised precisely, including the reagent and the expected observation.

Why these tests work

Hydrogen + lit splint → squeaky pop:
H₂ is flammable. Mixture of H₂ and O₂ in air ignites explosively, producing water vapour.
2H₂ + O₂ → 2H₂O (rapid exothermic reaction → "pop" sound)

Oxygen + glowing splint → relights:
O₂ is needed for combustion. A glowing splint (almost extinguished) still has hot carbon which reignites when exposed to O₂ rich gas.

CO₂ + limewater → milky:
CO₂ reacts with calcium hydroxide:
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
CaCO₃ is insoluble → white precipitate → turns solution milky. (Excess CO₂ dissolves the precipitate again: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂)

Chlorine + damp litmus → bleaches:
Cl₂ dissolves in water → HCl + HClO (hypochlorous acid).
HCl → H⁺ (turns litmus red/acid); HClO is a bleaching agent → destroys dye in litmus → white.

Additional notes

• Water (vapour): tested using anhydrous copper sulfate (white) — turns blue in presence of water.
  OR anhydrous cobalt(II) chloride (blue) — turns pink in presence of water.

• Ammonia (NH₃): holds damp red litmus → turns blue (ammonia is alkaline); characteristic pungent smell; forms white fumes with HCl (ammonium chloride).

Common exam errors

1. Saying oxygen relights a lit splint — oxygen relights a glowing splint (not already burning).
2. Saying CO₂ turns limewater blue — it turns it milky/cloudy (white precipitate).
3. Forgetting damp litmus for chlorine — dry litmus paper doesn't work because Cl₂ must dissolve in water to produce bleach.