State how addition of –

(i) copper

(ii) NaCl

to hot conc. H₂SO₄ serves as a test for the latter.

(i) When Cu is added to hot conc. H₂SO₄, colourless suffocating (SO₂) gas is evolved. The gas turns orange coloured acidified potassium dichromate solution to green colour.

Cu + 2H₂SO₄ (conc.) $\xrightarrow{\Delta}$ CuSO₄ + SO₂ + 2H₂O

(ii) When NaCl is heated with conc. H₂SO₄, colourless pungent smelling HCl gas is evolved.

$\text{NaCl} + \text{H}$2\text{SO}4 [\text{conc.}] \xrightarrow{\lt 200 \degree\text{C}} \text{NaHSO}_\text{4} + \text{HCl}

Dense white fumes are produced when a rod dipped in NH₃ soln. is brought near the test tube containing HCl gas.