A Levels Chemistry (9701)•9701/12/M/J/21
Explanation
Nitrate runoff causes eutrophication
Steps:
- Uncontrolled nitrate fertilisers wash into streams via runoff.
- Excess nitrates promote algal blooms in water bodies.
- Algae die and decompose, consuming oxygen.
- This leads to hypoxic conditions, harming aquatic life.
Why B is correct:
- Low oxygen levels result from eutrophication, where nitrate enrichment fuels microbial decomposition that depletes dissolved oxygen (DO) in water, as per the process of aerobic respiration: organic matter + O₂ → CO₂ + H₂O.
Why the others are wrong:
- A: Acid rain stems from sulfur and nitrogen oxides in air pollution, not directly from nitrate fertilisers.
- C: Ozone depletion involves CFCs breaking down stratospheric ozone, unrelated to nitrates.
- D: The greenhouse effect is driven by gases like CO₂ and methane trapping heat, not nitrates.
Final answer: B
Topic: Nitrogen and sulfur
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