(a) What is the pH scale? Explain how pH is related to the concentration of H⁺(aq) ions in a solution. How does the pH of a solution change when it is diluted?
(b) Explain why our body is sensitive to pH changes. Give two specific examples from everyday life where pH plays a critical role in living systems or health.
(c) A universal indicator turns orange (pH ≈ 4) in one solution and violet (pH ≈ 11) in another. Compare the concentrations of H⁺ and OH⁻ ions in these two solutions.
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
(a) The pH scale is a scale from 0 to 14 used to measure the concentration of H⁺(aq) ions in a solution. Higher the H⁺ ion concentration, lower the pH value. A neutral solution has pH = 7; values below 7 indicate acidic solutions, and above 7 indicate basic solutions. On dilution, the concentration of H⁺ (or OH⁻) ions per unit volume decreases, so the pH of an acid moves closer to 7 (increases), and that of a base also moves closer to 7 (decreases).
(b) Our body works within the pH range of 7.0 to 7.8; living organisms can survive only in a narrow pH range. Even a small change disturbs metabolic activities.
- Digestion: The stomach produces HCl for digestion. Excess acid (indigestion) causes pain; antacids (mild bases) neutralise it.
- Tooth decay: Bacteria degrade food, producing acids. When mouth pH falls below 5.5, tooth enamel (calcium hydroxyapatite) gets corroded, causing decay.
(c) Solution with pH ≈ 4 (orange) is acidic — high H⁺ concentration, low OH⁻ concentration. Solution with pH ≈ 11 (violet) is strongly alkaline — very low H⁺ concentration, high OH⁻ concentration. Thus, the first solution has much greater H⁺ concentration than the second, while the second has much greater OH⁻ concentration than the first.
Source: Chapter 2, Sections 2.3 and 2.3.1
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Explanation
- (a): Examiners want the definition, the inverse relationship (higher H⁺ → lower pH), the neutral/acid/base ranges, and the effect of dilution (pH shifts toward 7). Don't just memorise the range — explain the direction of shift on dilution.
- (b): Two specific examples with the mechanism (excess acid → antacid neutralises; pH < 5.5 → enamel corrodes) earn full marks. Just naming examples without explanation loses marks.
- (c): This is a comparison question — clearly state which solution has more H⁺ and which has more OH⁻. No calculation is needed; reasoning from pH values (below 7 = acidic, above 7 = basic) is sufficient.