Q1. [2] medium thorough-understanding
Acid rain flows into a freshwater river. Describe the chemical change it causes in the river water and explain, with reference to pH, why a sustained change of this kind poses a threat to aquatic life.
Generated by claude-sonnet-4-6 · 2026-06-26 01:04 · grounding rag
Model Answer
Acid rain increases the concentration of H⁺(aq) ions in the river water, lowering its pH to below 7, making it acidic. Living organisms carry out their metabolic activities within an optimal pH range. A sustained drop in pH disrupts these processes, making survival impossible for most aquatic life.
Source: Chapter 2, Acids, Bases and Salts
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Explanation
- The key chemical change to mention is the increase in H⁺(aq) ions and the resulting fall in pH below 7.
- Link the threat directly to the textbook point: "Living beings carry out their metabolic activities within an optimal pH range."
- Don't just say "water becomes acidic" — always connect it to pH scale and biological consequence for full marks.
- For 2 marks: 1 mark for the chemical change (pH drops/acidic), 1 mark for the biological threat (optimal pH disrupted → aquatic life harmed).