Science (086) — AI-generated practice question

(a) A neutralisation reaction is when an acid reacts with a base to form salt and water, with the evolution of heat. Example: NaOH + HCl → NaCl + H₂O.

The ionic equation for all strong acid–strong base neutralisation reactions is:

$$\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)$$

(b) The blue-green solution indicates formation of copper sulphate, so metal oxide X is copper(II) oxide (CuO).

$$\text{CuO}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{CuSO}_4(aq) + \text{H}_2\text{O}(l)$$

CuO is a basic oxide because it reacts with an acid (H₂SO₄) to form salt and water only, showing no acidic character.

(c) Zinc reacts with sodium hydroxide as follows:

$$\text{Zn}(s) + 2\text{NaOH}(aq) \rightarrow \text{Na}_2\text{ZnO}_2(aq) + \text{H}_2(g)$$

Difference: When a metal reacts with an acid, hydrogen gas is displaced and a salt of the acid is formed. Here, zinc reacts with a base (NaOH) and still produces hydrogen gas, forming sodium zincate. This shows zinc is an amphoteric metal — it can react with both acids and bases, unlike most metals which react only with acids.

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• (a) Examiners expect the word "salt and water" and the net ionic equation $H^+ + OH^- \rightarrow H_2O$. Avoid writing full ionic equations with spectator ions.
• (b) "Blue-green" is the key clue for Cu²⁺ ions in solution → CuSO₄ → CuO was the reactant. Basic oxide reacts with acids (not bases) to give salt + water only.
• (c) The reaction of Zn with NaOH is a standard NCERT example showing amphoteric behaviour. The key contrast to emphasise: metals normally react with acids to give H₂, but Zn also reacts with a base to give H₂ — this is the difference examiners look for.