Q1. [4]
Acid-base indicators can be used to distinguish between an acid and a base. Universal indicator, a mixture of several indicators, shows different colours at different concentrations of acids and bases, thereby indicating their pH on the pH scale of 0 – 14. The pH of a solution is measured by pH paper, which is a paper impregnated with a universal indicator.
Read the case carefully and answer the questions that follow.
- (a) Solution P is a strong acid while solution Q is a strong base. On the pH scale, where would you place the solutions P and Q ? [1]
- (b) A solution has a pH of 7. Name a compound you would use to (i) increase its pH, and (ii) decrease its pH. [1]
- (c) (i) When the pH of a solution is decreased from 4 to 2, what effect does it produce on its hydronium ion concentration ? State the colour change shown by the pH paper. [2]
Previously asked in CBSE board exam
2025 31/2/1 Q37
Generated by claude-sonnet-4-6 · 2026-06-15 06:43 · grounding stimulus
Model Answer
(a) Solution P (strong acid) would be placed near pH 0–3 (towards the left/acidic end), and Solution Q (strong base) would be placed near pH 11–14 (towards the right/basic end) of the pH scale.
(b)
(i) To increase the pH (make it more basic): NaOH (sodium hydroxide)
(ii) To decrease the pH (make it more acidic): HCl (hydrochloric acid)
(c)(i) When pH decreases from 4 to 2, the hydronium ion (H₃O⁺) concentration increases 100 times (increases by a factor of 100).
Colour change on pH paper: from orange (pH 4) to red (pH 2).
Source: Chapter 2 – Acids, Bases and Salts; pH Scale section
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Explanation
- (a) Strong acids have very low pH (0–3); strong bases have very high pH (11–14). Place them at opposite ends of the scale.
- (b) Any valid strong base/acid is acceptable. NaOH and HCl are the most common NCERT examples.
- (c) pH is a logarithmic scale — a drop of 2 units means H⁺ concentration increases 100-fold (10² = 100). Examiners expect "100 times" specifically. Colour change from orange → red is read directly from the pH scale figure: pH 4 ≈ orange, pH 2 ≈ red.
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