Model Question Paper — 1 — Question 13

(i) On addition of dil. sulphuric acid to sodium sulphide, hydrogen sulphide (H₂S) gas is evolved which can be identified by its rotten egg smell. Hydrogen sulphide gas turns moist lead acetate paper silvery black.

Na₂S + H₂SO₄ ⟶ Na₂SO₄ + H₂S ↑

On addition of dil. sulphuric acid to sodium sulphite, sulphur dioxide (SO₂) gas is evolved. Sulphur dioxide gas has a suffocating odour and turns lime water milky and potassium permanganate solution from pink to clear colourless.

Na₂SO₃ + H₂SO₄ (dil.) ⟶ Na₂SO₄ + H₂O + SO₂↑

(ii) HCl and chlorine gas

Chlorine (Cl₂) turns moist starch iodide paper blue black whereas HCl does not.

Cl₂ + 2KI ⟶ 2KCl + I₂

Starch + I₂ ⟶ Blue black colour