Given below are two statements:One is labelled as Assertion A and the other is labelled as Reason R. Assertion A: Polluted water may have a value of BOD of the order of 17ppm.

The correct answer is C. Complete step-by-step solution with explanation available on PadhoLikhoJEE.

JEE Main 2022 Environmental Chemistry — Two Statements Labelled Assertion Other Labelled

Quick Summary

Concept Tested: Environmental Chemistry (BOD values & pollution levels)
Chapter: Environmental Chemistry
Difficulty: ⭐⭐⭐ (Medium)
Time to Solve: 1-2 Minutes
Key Formula: $$\text{BOD} = \frac{\text{Dissolved } O_2 \text{ consumed (mg)}}{\text{Volume of sample (L)}}$$
Answer: C
One-Liner Reason: Assertion A is correct because highly polluted water can indeed have BOD values up to 17 ppm, but Reason R is incorrect because BOD only measures biodegradable organic matter, not non-biodegradable ones (which are measured by COD).

The Question

Assertion (A): Polluted water may have a value of BOD of the order of 17 ppm.
Reason (R): BOD is a measure of oxygen required to oxidise both biodegradable and non-biodegradable organic material in water.

Which of the following is correct?

(A) Both A and R are correct and R is the correct explanation of A.

(B) Both A and R are correct but R is NOT the correct explanation of A.

(C) A is correct but R is not correct.

(D) A is not correct but R is correct.

Quick Answer

The correct option is (C) A is correct but R is not correct.

Reasoning: Assertion A is true because highly polluted water (like sewage) typically has a BOD value of 10 ppm or higher, often reaching 17 ppm. However, Reason R is false because BOD specifically measures the oxygen required for the biological breakdown of biodegradable organic matter. Non-biodegradable organic matter is measured by Chemical Oxygen Demand (COD).

Why Other Options Are Incorrect

Option A is incorrect because: While Assertion A is true, Reason R is false. The statement claims BOD measures both biodegradable and non-biodegradable matter, which is chemically inaccurate. Since R is not a correct explanation (and is factually wrong), Option A cannot be the answer.

Option B is incorrect because: This option requires both statements to be correct. However, Reason R is incorrect. BOD is a biological parameter that only accounts for biodegradable organics, not non-biodegradable ones. Therefore, the premise of Option B is invalid.

Option D is incorrect because: This option claims Assertion A is incorrect. However, water with BOD values of 17 ppm is a standard characteristic of highly polluted sewage water, confirming that Assertion A is factually correct.

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Understanding the Concept

Biological Oxygen Demand (BOD) is a crucial parameter used to determine the amount of organic pollution in a water body. It represents the amount of dissolved oxygen (DO) consumed by aerobic microorganisms (bacteria) as they decompose organic matter in the water over a specific period (usually 5 days at 20°C).

The unit of BOD is ppm (parts per million), which is equivalent to mg/L (milligrams per liter). A high BOD value indicates a high concentration of biodegradable organic pollutants, which depletes the oxygen available for aquatic life.

Standard BOD ranges help classify water quality:

Clean Water: BOD < 1 ppm
Moderately Polluted: BOD = 2–8 ppm
Highly Polluted (Sewage/Industrial Waste): BOD ≥ 10 ppm (often 17 ppm or more)

Detailed Step-by-Step Solution

Step 1: Evaluate Assertion A (Is the statement correct?)

First, we analyze the Assertion regarding the BOD value of 17 ppm. The mathematical definition of BOD is given by the formula:

$$\text{BOD} = \frac{\text{Dissolved } O_2 \text{ consumed (mg)}}{\text{Volume of sample (L)}}$$

When we say a water sample has a BOD of 17 ppm, it means that 17 mg of dissolved oxygen are consumed per liter of water by bacteria. In the context of environmental standards:

Values below 5 ppm generally indicate clean water.
Values between 5-10 ppm indicate moderate pollution.
Values above 10 ppm indicate severe pollution, such as raw sewage.

Therefore, a BOD value of 17 ppm is a realistic and correct representation of highly polluted water.

Conclusion for Step 1: Assertion A is Correct.

Step 2: Evaluate Reason R (Is the definition correct?)

Next, we examine the Reason R. It states that BOD is a measure of oxygen required to oxidise both biodegradable and non-biodegradable organic material.

This definition is chemically incorrect. BOD is a biological parameter. It specifically measures the oxygen demand of biodegradable organic matter (like sugars, proteins, and fats) which microorganisms can break down using their metabolic processes.

Non-biodegradable organic matter (like certain plastics or synthetic chemicals) does not react with bacteria in the same way and therefore does not contribute to the BOD value. The measurement for non-biodegradable organic matter is COD (Chemical Oxygen Demand), which uses chemical oxidants like dichromate to break down matter chemically.

Conclusion for Step 2: Reason R is Incorrect.

Step 3: Determine the relationship between A and R

From Step 1, we established that Assertion A is true. From Step 2, we established that Reason R is false.

Since the Reason is not a correct explanation (and is factually wrong), we must select the option that reflects this specific relationship.

The correct choice is (C) A is correct but R is not correct.

Final Answer

Option (C) is the correct answer.

Essential Formulas for This Topic

1. BOD Calculation:
$$\text{BOD} = \frac{\text{Initial Dissolved } O_2 – \text{Final Dissolved } O_2}{\text{Volume of sample (L)}} \times 1000$$

2. Chemical Oxygen Demand (COD):
$$\text{COD} = \frac{\text{Dichromate consumed (mg)}}{\text{Volume of sample (L)}} \times \text{Equivalent factor}$$

Note: COD is always higher than BOD because it oxidizes all organic matter (including non-biodegradable), whereas BOD only oxidizes biodegradable matter.

Common Mistakes to Avoid

Mistake 1: Confusing BOD with COD

Wrong Thinking: Students often think BOD measures all organic pollution.

Correct Approach: Remember that BOD is biological (microbes), while COD is chemical (dichromate). Non-biodegradable matter contributes to COD but not BOD.

Mistake 2: Misinterpreting the “17 ppm” value

Wrong Thinking: Thinking 17 ppm is normal or low.

Correct Approach: Recognize that 17 ppm is extremely high. Standard clean water is < 1 ppm. 17 ppm is typical for untreated sewage. Always memorize the threshold of 10 ppm for "high pollution."

Mistake 3: Assuming the Reason explains the Assertion

Wrong Thinking: Selecting Option B (Both correct, no explanation) because A is true.

Correct Approach: If R is factually false, it cannot explain A. The relationship must be A is true, R is false (Option C).

Key Concept Summary

BOD measures the oxygen consumed by microorganisms to decompose biodegradable organic matter.
BOD is measured in ppm or mg/L.
Typical BOD ranges: Clean (< 1 ppm), Moderate (2–8 ppm), Polluted (≥ 10 ppm).
COD measures the oxygen required to oxidize all organic matter (biodegradable and non-biodegradable).

Golden Rule: High BOD means high biological oxygen demand, leading to low dissolved oxygen levels and fish kills.

Frequently Asked Questions

Q: Why is BOD usually measured after 5 days?

A: The 5-day incubation period (BOD5) is a standard test to estimate the oxygen depletion rate over a practical period relevant to aquatic life recovery.

Q: What is the BOD of drinking water?

A: Drinking water (potable water) should ideally have a BOD value of less than 1.5 ppm, though the standard often cited is < 1 ppm.

Q: Can BOD be negative?

A: Yes, if the water sample is supersaturated with dissolved oxygen (e.g., water aerated by a waterfall), the oxygen consumed by bacteria might be less than the initial oxygen, resulting in a negative BOD value.

Q: What is the relationship between BOD and DO?

A: BOD is the cause of the depletion of Dissolved Oxygen (DO). As BOD increases, DO levels in the water body decrease, eventually leading to hypoxic (low oxygen) or anoxic (no oxygen) conditions.

Prerequisites to Solve This Question

Understanding the definition of BOD (Biochemical Oxygen Demand).
Familiarity with the units of measurement (ppm vs mg/L).
Knowledge of the distinction between biological and chemical oxidation processes.
Understanding of standard water quality classifications based on BOD values.

After Solving This, You Can:

✔ Classify water samples as clean, moderately polluted, or highly polluted based on BOD values.
✔ Differentiate between BOD and COD in environmental chemistry contexts.
✔ Identify the limitations of biological testing for water pollution.

Study Tips for This Topic

For JEE Main/Advanced questions involving Environmental Chemistry, focus on memorizing the standard ranges for BOD and COD. Often, questions will ask you to identify the type of pollution (industrial vs. domestic) based on the value. Remember that domestic sewage typically has very high BOD (around 200-600 mg/L), while industrial effluents might have lower BOD but higher COD.

Difficulty Rating & Exam Frequency

Difficulty: ⭐⭐⭐ (Medium)

JEE Main Frequency: Occasional (appears in Environmental Chemistry sections).

Importance: High. Understanding BOD is fundamental for environmental science questions in competitive exams.