(a) A reaction in which oxidation and reduction occur simultaneously is called an oxidation-reduction, or simply, a redox reaction. Redox reactions involve the transfer of electrons between two chemical species. The reaction in which electron is gained is called a reduction reaction and the reaction in which electron is lost is called oxidation reaction. The compound that loses an electron is said to be oxidized, the one that gains an electron is said to be reduced.

(b)(i) Oxidation :Zn → Zn ²⁺

Reduction : Pb²⁺ Pb

(ii) Oxidation : Zn → Zn ²⁺

Reduction : Cu²⁺ Cu

(iii) Oxidation : Br^- → Br

Reduction : Cl₂ → 2Cl^-

(iv) Oxidation : Sn^₂+ → Sn⁴⁺

Reduction : 2Hg²⁺ → Hg_²

(v) Oxidation : Cu⁺ → Cu ²⁺

Reduction : Cu⁺ → Cu

(c)(i)  Potassium undergoes oxidation as it loses an electron and forms a cation.

(ii)  Chlorine undergoes reduction as it gains an electron and forms chloride anion.

(iii)  Potassium acts a reducing agent and gets oxidised.

(iv)  Chlorine acts an oxidizing agent and gets reduced.