Give reasons

(a) Hydrogen is collected by the downward displacement of water and not of air, even though it is lighter than air.

(b) A candle brought near the mouth of a jar containing hydrogen gas starts burning but is extinguished when pushed inside the jar.

(c) Apparatus for laboratory preparation of hydrogen should be airtight and away from a naked flame.

Write half-reactions for the following reaction :

A⁺ + B ⟶ A + B⁺ and name the following:

(a) oxidizing agent

(b) substance oxidized

(c) reducing agent.

State, giving reasons, whether the substances printed in bold letters have been oxidised or reduced.

(a) PbO + CO ⟶ Pb + CO₂

(b) Mg + 2HCl ⟶ MgCl₂ + H₂

(c) H₂S + Cl₂ ⟶ 2HCl + S

(d) Cl₂ + H₂S ⟶ 2HCl + S

Is it essential that oxidation and reduction must occur side by side in a chemical reaction? Explain.