(a) Name the substance oxidised and reduced in the following reaction: $\mathrm{ZnO + C\longrightarrow Zn + CO}$ (b) Balance the following chemical reaction: $\mathrm{Pb(NO_3)_2 + KI\longrightarrow PbI_2 + KNO_3}$ (c) Give one example each of electrolytic decomposition and decomposition by sunlight.

This is a three-part structured question drawing on Sections 1.2.5 (Oxidation and Reduction), 1.1.2 (Balancing Chemical Equations), and 1.2.2 (Decomposition Reaction) of the chapter.

(a) Oxidised and reduced substances: By the NCERT definition, "if a substance gains oxygen during a reaction, it is said to be oxidised. If a substance loses oxygen during a reaction, it is said to be reduced." In $\mathrm{ZnO + C \rightarrow Zn + CO}$, carbon starts with no oxygen and ends up bonded to one oxygen atom in CO, so carbon is oxidised; ZnO starts with one oxygen atom and ends as metallic Zn with no oxygen, so ZnO is reduced. This is precisely the book's own example, Eq. (1.31), where the text states outright: "carbon is oxidised to CO and ZnO is reduced to Zn." Since one reactant is oxidised while the other is simultaneously reduced, this is also an example of a redox (oxidation-reduction) reaction, as defined in the same section.

(b) Balancing $\mathrm{Pb(NO_3)_2 + KI \rightarrow PbI_2 + KNO_3}$: Using the hit-and-trial method described in Section 1.1.2 (box each formula, tabulate atom counts, and balance one element at a time without altering any chemical formula), lead and the nitrate group are already balanced (1 Pb, 2 N, 6 O on each side once balanced), but iodine (1 on LHS vs 2 on RHS) and potassium (1 on LHS vs 1 on RHS, but tied to iodine's coefficient) are not. Placing a coefficient of 2 before KI balances iodine and simultaneously raises K to 2 on the LHS, which is then balanced by placing a coefficient of 2 before $\mathrm{KNO_3}$ as well. This reproduces the reaction from Activity 1.2 in the chapter (lead nitrate solution + potassium iodide solution), which the book describes as forming an insoluble yellow precipitate of lead iodide ($\mathrm{PbI_2}$) — also a double displacement/precipitation reaction, consistent with Section 1.2.4.

(c) Electrolytic decomposition vs decomposition by sunlight: The text explains that "decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants," and calls these endothermic reactions. Activity 1.7 demonstrates electrolytic decomposition: passing electric current through acidified water (using carbon electrodes connected to a battery) splits it into hydrogen and oxygen gas, $2\mathrm{H_2O(l)} \xrightarrow{\text{Electricity}} 2\mathrm{H_2(g)} + \mathrm{O_2(g)}$, with hydrogen collected in twice the volume of oxygen, matching the 2:1 mole ratio. Activity 1.8 demonstrates decomposition by sunlight: white silver chloride, left in sunlight, decomposes into grey metallic silver and chlorine gas, $2\mathrm{AgCl(s)} \xrightarrow{\text{Sunlight}} 2\mathrm{Ag(s)} + \mathrm{Cl_2(g)}$, a reaction the text notes underlies black-and-white photography (the analogous silver bromide reaction, Eq. 1.23, behaves the same way).