A student adds excess sodium hydroxide solution to a fixed volume of hydrochloric acid. (a) Write the ionic equation for the neutralisation reaction that occurs. (b) Explain, using the concept of ions, why the resulting solution is basic rather than neutral. (c) Predict the colour of universal indicator in the original HCl solution and in the final solution after excess NaOH has been added, giving reasons for each.
Generated by claude-sonnet-4-6 · 2026-06-26 01:04 · grounding rag
Model Answer
(a) Ionic equation for neutralisation:
$$\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$$
(b) Why the resulting solution is basic:
HCl provides H⁺(aq) ions and NaOH provides OH⁻(aq) ions. During neutralisation, H⁺ and OH⁻ combine to form water. Since NaOH is added in excess, all the H⁺ ions are used up, but surplus OH⁻ ions remain in solution. These extra OH⁻ ions make the solution basic (pH > 7).
(c) Universal indicator colours:
- Original HCl solution: The colour will be red, because HCl is a strong acid producing a high concentration of H⁺(aq) ions, giving a very low pH (around 1–2).
- Final solution after excess NaOH: The colour will be violet/purple, because excess OH⁻ ions remain, making the solution strongly alkaline with a high pH (above 10).
Source: Chapter 2, Section 2.2 and 2.2.1
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Explanation
- Examiners expect the net ionic equation in (a) — not the full molecular equation. Na⁺ and Cl⁻ are spectator ions and must be omitted.
- In (b), the key phrase is "excess OH⁻ ions remain" — link the ionic concept directly to basicity. Simply saying "more NaOH was added" without mentioning ions will lose marks.
- In (c), remember the universal indicator colour scale: red/orange = strongly/weakly acidic; green = neutral; blue/violet = alkaline. HCl (strong acid) → red; strongly alkaline solution → violet. Always give a reason linked to ions or pH.