(a) What is the pH scale? What does a pH value of less than 7, equal to 7 and greater than 7 indicate about a solution?
(b) Give two examples each of substances with pH less than 7 and pH greater than 7 from everyday life.
(c) How does pH affect the survival of aquatic life in rivers?
Generated by claude-sonnet-4-6 · 2026-06-17 12:38 · grounding rag
Model Answer
(a) The pH scale is a scale from 0 to 14 used to measure the hydrogen ion concentration in a solution, indicating its acidic or basic nature.
- pH less than 7 → acidic solution (higher H⁺ concentration)
- pH equal to 7 → neutral solution
- pH greater than 7 → basic (alkaline) solution (higher OH⁻ concentration)
(b)
- pH less than 7 (acidic): Lemon juice, tomato juice
- pH greater than 7 (basic): 1M NaOH solution, baking soda solution
(c) Living organisms can survive only within a narrow range of pH. When acid rain (pH less than 5.6) flows into rivers, it lowers the pH of the river water. This makes survival of aquatic life difficult, as most aquatic organisms require a near-neutral pH to carry out their life processes.
Source: Chapter 2, Section 2.3 and 2.3.1
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Explanation
- (a) Examiners expect the definition of pH scale and all three cases (acidic/neutral/basic) — missing any one loses a mark.
- (b) Give only two examples per category; lemon juice, tomato juice, vinegar, HCl are all valid for acidic; NaOH, milk of magnesia, baking soda for basic.
- (c) The key chain is: acid rain → lowers river pH → aquatic life cannot survive. Mention the acid rain threshold (pH < 5.6) for full marks. Keep it brief — this sub-part is worth ~1 mark.