Answer: 20 degrees C approximates temperate river temperature and supports active decomposer bacteria; 5 days oxidises about 70-80% of biodegradable organics without requiring an impractically long incubation.
- A 20 degrees C is the boiling point of water at high altitude; 5 days allows complete sterile chemical oxidation overall in most cases under typical conditions according to standard textbooks in general practice
- B 20 degrees C prevents all microbial growth; 5 days enables mainly chemical (not biological) oxygen demand as frequently described in most textbook accounts during normal conditions as generally observed
- C 20 degrees C approximates temperate river temperature and supports active decomposer bacteria; 5 days oxidises about 70-80% of biodegradable organics without requiring an impractically long incubation
- D 20 degrees C maximises dissolved oxygen solubility; 5 days ensures all heavy metals precipitate out of solution in typical laboratory settings under usual circumstances according to most researchers
Correct answer: C. 20 degrees C approximates temperate river temperature and supports active decomposer bacteria; 5 days oxidises about 70-80% of biodegradable organics without requiring an impractically long incubation
Explanation: BOD<sub>5</sub> at 20 degrees C is internationally standardised because 20 degrees C is typical of temperate rivers and supports active growth of mixed sewage bacteria. Five days allows ~70-80% of readily biodegradable organic matter to be consumed, providing a reproducible pollution index without requiring weeks of incubation.
Classical smog is a reducing mixture of SO₂ and fog, while photochemical smog is an oxidising mixture generated by UV-driven reactions of NOx and hydrocarbons.
Concept context
Understand how human activities alter the atmosphere, water, and soil. Covers air pollutants, smog types, ozone depletion, acid rain, greenhouse effect, water and soil pollution, and the principles of green chemistry.