Give the characteristic tests for the following gases

(a) CO₂

(b) SO₂

(c) O₂

(d) H₂

(a) When carbon dioxide is made to run through lime water, the gas turns the solution milky. This is unique only to carbon dioxide.
The reaction produces Calcium carbonate which is a white precipitate, thus giving rise to the milky appearance. Lime water is nothing but a solution of Calcium hydroxide.
CO₂ + H₂O → H₂CO₃

H₂CO₃ + Ca(OH)₂→ CaCO₃ + 2H₂O

(b) The unique identifying feature of sulphur dioxide is its odour.It smells like burning of sulphur. Also, Sulphur dioxide turns moist litmus paper from blue to red.

When SO₂ is passed through acidified potassium permanganate solution (pink/purple in colour) , the solution turns green due to the formation of Chromium sulphate. It is a redox reaction where Potassium dichromate acts as an oxidising agent and sulphur dioxide acts as a reducing agent.
Cr₂O₇^2- (aq) + 2H+ (aq) + 3SO₂ (g) → 2Cr³⁺ (aq) + H₂O (l) + 3SO₄^2- (aq)

(c) When any fire element - a matchstick or a candle - is brought into an oxygen atmosphere, it starts burning more brightly.
Oxygen supports combustion.

(d) When a fire element - a matchstick or a candle - is brought into a Hydrogen atmosphere, it burns with a pop sound. This is unique to Hydrogen.