(a) Give the reason in each case why it is necessary to: (i) moisten a piece of litmus paper used in testing for the acidity or alkalinity of a gas. (i) aci...

(a)(i) A gas shows its acidic or alkaline character only when dissolved in water; moistening the litmus paper provides the water that dissolves the gas so it can ionize and change the colour of the paper.

(ii) The test solution is acidified with dilute hydrochloric acid so that any trioxocarbonate(IV) or trioxosulphate(IV) ions present (which would also give a white precipitate with BaCl₂) are removed. Only sulphate then gives a white precipitate of BaSO₄, which is insoluble in the dilute acid, confirming SO₄^2-.

(b)(i) Two gases that must not be prepared in the open laboratory: chlorine and hydrogen sulphide (both are poisonous).

(ii) Precaution: prepare them inside a fume cupboard (fume chamber) so the toxic gas is drawn away.

(iii) I. Wash bottle - holds distilled water for rinsing apparatus and washing precipitates. II. Tripod stand - supports a beaker or flask (with wire gauze) over the flame during heating.

(c)(i) When Pb(NO₃)₂ is heated, it crackles (decrepitates), gives off brown fumes of nitrogen(IV) oxide, and leaves a residue that is brown when hot and yellow (PbO) when cold.

(ii) When concentrated HCl is added to MnO₂ and warmed, a greenish-yellow gas (chlorine) with a choking, pungent smell is evolved:
\[MnO_2 + 4HCl \rightarrow MnCl_2 + Cl_2 + 2H_2O\]