Read the following and answer the questions:
Rahul and Priya are performing an experiment in the school laboratory. They dissolve different substances — hydrochloric acid, sulphuric acid, glucose solution, and alcohol — in water and connect each solution to a simple electrical circuit containing a bulb. They observe that the bulb glows brightly for hydrochloric acid and sulphuric acid solutions, but does not glow at all for glucose and alcohol solutions. Their teacher then explains that when an acid is dissolved in water, something special happens at the ionic level that allows electricity to flow.
(i) Why does the bulb glow when HCl or H₂SO₄ solution is used but not with glucose or alcohol? (1 mark)
(ii) Write the equation showing what happens when HCl dissolves in water. What is the ion formed called? (1 mark)
(iii) NaOH solution conducts electricity but NaOH in its solid state does not. Explain why, and state what type of ion is responsible for the basic nature of NaOH solution. (1 mark)
(iv) Both glucose and alcohol contain hydrogen atoms. Why are they not classified as acids? (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
(i) HCl and H₂SO₄ dissolve in water to produce ions (H⁺ and respective anions), which carry electric current, making the bulb glow. Glucose and alcohol do not ionise in water; they produce no ions, so no current flows and the bulb does not glow.
(ii)
$$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$
The ion formed (H₃O⁺) is called the hydronium ion.
(iii) In solid state, NaOH ions are held in a fixed crystal lattice and cannot move, so it does not conduct electricity. When dissolved in water, it dissociates into free Na⁺ and OH⁻ ions that move freely. The hydroxide ion (OH⁻) is responsible for the basic nature of NaOH solution.
(iv) Glucose and alcohol do contain hydrogen, but they do not release H⁺ (hydrogen) ions when dissolved in water. Only compounds that produce H⁺(aq) ions in solution are classified as acids. Since glucose and alcohol produce no ions at all, they are not acids.
Source: Chapter 2, Section 2.2 and 2.2.1
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Explanation
- (i) The key concept is ionisation — examiners want the word "ions" linked to conductivity.
- (ii) Write the equation correctly and name H₃O⁺ as "hydronium ion" — both parts carry marks.
- (iii) Two points needed: why solid doesn't conduct (no free ions) + OH⁻ causes basic nature.
- (iv) The distinction is not the presence of hydrogen but whether H⁺(aq) ions are released in water — this is the core idea examiners test here.