An effect of thermal pollution on water bodies is that the

Analyze the question: Thermal pollution is the rise in water temperature caused by human activity. A common source is industrial plants (power stations, refineries) discharging hot cooling water into rivers, lakes, or the sea. The question asks what effect this has on the water body.

Concept: solubility of gases and temperature

The solubility of gases in liquids decreases as temperature rises. This is why a warm soft drink goes flat faster than a cold one. Dissolved oxygen (DO) in water follows the same rule.

Step-by-step solution:

1. Hot effluent is discharged into a water body, so the water temperature rises.
2. Warmer water holds less dissolved oxygen, so the oxygen level falls.
3. Higher temperature also speeds up the metabolism of aquatic organisms, so they need more oxygen just when less is available.
4. The result is stress and death of fish and other aquatic life, along with disruption of the ecosystem.

Checking each option:

• A. Volume of water reduces: Some evaporation may occur, but this is not the main effect of thermal pollution.
• B. Volume of chemical waste increases: Thermal pollution is about heat, not chemical waste.
• C. Level of oxides of nitrogen increases: These are linked to air pollution (vehicle exhaust, combustion), not heated water.
• D. Level of oxygen reduces: Correct. Higher temperature lowers the solubility of oxygen in water.

**Answer: D.

Exam tip: Remember "Hot water, less oxygen." Whenever you see thermal pollution, think of reduced dissolved oxygen, which leads to fish kills. The same idea appears in questions on gas solubility and temperature.