The following is not a water pollutant?

Analysis

The question asks us to identify which of the listed substances does not cause water pollution. A water pollutant is any substance that, when added to a water body, makes it harmful to living organisms or unsuitable for its intended use.

Concept Explanation

• Warm water affluent (Thermal pollution): When industries discharge hot water into rivers, it reduces the amount of dissolved oxygen available for aquatic life, causing thermal pollution.
• Inorganic fertilizers: These contain nitrates and phosphates. When washed into water bodies, they cause eutrophication (excessive growth of algae), which depletes oxygen and kills aquatic life.
• Oxygen gas ($O_2$): Oxygen is essential for the survival of aquatic organisms (fish, plants, etc.). It is a natural component of water and is necessary for the aerobic decomposition of organic matter. It is not a pollutant; rather, its depletion is a sign of pollution.
• Biodegradable waste: While these can be broken down by bacteria, if they enter water in large quantities, they increase the Biological Oxygen Demand (BOD), which leads to oxygen depletion and pollution.

Step-by-Step Solution

1. Evaluate A: Warm water alters the temperature of the ecosystem, which is a form of pollution.
2. Evaluate B: Inorganic fertilizers lead to nutrient enrichment (eutrophication), which is a major water pollutant.
3. Evaluate C: Oxygen gas is a life-sustaining gas. It is naturally present in water and is required for the health of the aquatic ecosystem. It does not cause harm or contamination.
4. Evaluate D: Large amounts of biodegradable waste lead to the depletion of dissolved oxygen as microorganisms consume it to break down the waste, thus acting as a pollutant.

Comparing these, oxygen gas is the only substance that is beneficial and natural to the aquatic environment in its standard state.

Conclusion

Exam Tip

When answering "which is not a pollutant" questions, look for substances that are naturally required for life (like Oxygen) versus substances that disrupt the chemical or physical balance of the environment (like heavy metals, nitrates, or heat).