With the help of an activity, show that acids produce H⁺ ions only in the presence of water (i.e., HCl gas is not acidic but its aqueous solution is).
Generated by claude-sonnet-4-6 · 2026-06-17 12:37 · grounding rag
Model Answer
Activity 2.9 (from NCERT textbook):
Materials required: Solid NaCl, concentrated H₂SO₄, delivery tube, dry and wet blue litmus paper.
Procedure:
- Take about 1 g of solid NaCl in a clean, dry test tube.
- Add some concentrated sulphuric acid to it. HCl gas is evolved.
- Pass the gas first through dry blue litmus paper, then through wet blue litmus paper.
Observation:
- Dry blue litmus paper shows no change in colour.
- Wet blue litmus paper turns red.
Conclusion:
- Dry HCl gas does not ionise, so it shows no acidic character.
- When HCl dissolves in water (wet litmus), it ionises to produce H⁺ ions:
$$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$
This shows that H⁺ ions are produced only in the presence of water. H⁺ ions cannot exist alone; they combine with water to form hydronium ions (H₃O⁺), which are responsible for the acidic nature.
Source: Chapter 2, Section 2.2.1, Activity 2.9
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Explanation
- Examiners expect you to name the activity, list steps clearly, state both observations (dry vs. wet litmus), and write the ionisation equation.
- The key conclusion — "H⁺ ions are produced only in the presence of water" — must be explicitly stated; this is where marks are awarded.
- Writing H₃O⁺ (hydronium ion) instead of just H⁺ shows understanding and fetches full credit.
- Do not forget: dry HCl = no ionisation = no acidity; aqueous HCl = ionisation = acidic.