Problem 7 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Water filled in two glasses \(\mathrm{A}\) and \(\mathrm{B}\) have \(\mathrm{BOD}\) values of 10 and 20 , respectively. The correct statement regarding, them, is: (a) \(\mathrm{B}\) is more polluted than \(\mathrm{A}\). (b) \(\mathrm{A}\) is suitable for drinking, whereas \(\mathrm{B}\) is not. (c) Both A and B are suitable for drinking. (d) \(\mathrm{A}\) is more polluted than \(\mathrm{B}\).

Step-by-Step Solution

Answer

The correct statement is (a) B is more polluted than A.

Step 1: Understand BOD

Biochemical Oxygen Demand (BOD) is a measure of the amount of oxygen that microorganisms will consume while decomposing organic matter in water. Higher BOD indicates more pollution because it means more organic material is present to decompose.

Step 2: Compare BOD Values

Glass A has a BOD value of 10, while Glass B has a BOD value of 20. Since BOD values are higher in more polluted waters, Glass B, with the higher BOD value, is more likely to be polluted than Glass A.

Step 3: Evaluate Drinking Suitability based on BOD

Typically, water with a low BOD value (less than 4) is suitable for drinking. Both Glass A and Glass B have BOD values higher than 4, indicating that neither is likely suitable for drinking in terms of BOD.

Step 4: Determine the Correct Statement

From the given options, (a) 'B is more polluted than A' is correct because Glass B has a higher BOD value than Glass A, indicating more pollution. Option (b) is incorrect because neither A nor B have suitable BOD levels for drinking.

Key Concepts

Water Pollution Assessment

Water pollution assessment is an essential process to understand the quality of water resources. One of the most important metrics in this process is Biochemical Oxygen Demand (BOD). BOD measures the amount of oxygen consumed by microorganisms as they decompose organic matter in water.
This makes it a useful indicator of pollution levels. If the BOD value is high, it suggests a significant presence of organic waste, leading to greater oxygen consumption by microorganisms. High BOD values point to high pollution, which affects aquatic life and can indicate the presence of harmful substances.
To properly assess water pollution, BOD is just one part of the equation. Other factors such as chemical pollutants, heavy metals, and physical attributes like temperature and pH must also be considered. However, BOD offers a valuable snapshot, as it correlates well with the biological activity in the water, providing insight into how polluted the water may be.

Suitability of Drinking Water

Determining the suitability of water for drinking purposes hinges on various key indicators, one of which is the BOD level. Typically, drinking water should have a BOD value less than 4. This low level ensures that the water is free from high concentrations of organic matter that microorganisms could decompose, which might otherwise compromise water quality.
It's essential for water to meet strict quality guidelines to be considered "safe" for drinking. Parameters such as clarity, chemical content, and pathogen presence all play a role. High BOD values suggest that the water is more polluted, making it unsuitable for consumption without proper treatment.
Assessing the BOD level alongside these other factors can help identify any potential health risks. For example, water with a BOD value above 4 typically requires further purification processes to deem it fit for human consumption.

Microorganism Activity in Water

Microorganisms play a critical role in natural water bodies, aiding the breakdown of organic materials. The level of microorganism activity is directly related to the BOD value. A higher BOD indicates intense microbial activity, as microorganisms require more oxygen to decompose the organic matter present.
This relationship becomes particularly significant for understanding pollution and water quality. When organic matter such as leaves, sewage, or industrial waste enters a water body, microorganisms feast on this material, consuming greater amounts of oxygen.
It's important to maintain a balance in microorganism activity. While they are necessary for breaking down pollutants, excessive microbial activity can deplete oxygen levels in water, harming aquatic life. Thus, monitoring BOD as a measure of microorganism activity helps manage and protect water ecosystems.