Water samples with BOD values of 4 ppm and 18 ppm, respectively, are
(a) clean and clean
(b) highly

Q: Why does polluted water have a higher BOD?

Polluted water contains more organic waste. Bacteria need more oxygen to decompose this large amount of waste, hence the demand for oxygen (BOD) increases.

Q: What exactly does 'ppm' mean in this context?

ppm stands for parts per million. In water analysis, $1 \text{ ppm}$ is equivalent to $1 \text{ milligram}$ of oxygen dissolved in $1 \text{ liter}$ of water.

Q: Is BOD the only way to measure water pollution?

No, there is also COD (Chemical Oxygen Demand), which measures the oxygen required to oxidize both biodegradable and non-biodegradable organic matter chemically.

Q: Can BOD be zero?

In perfectly pure, distilled water with absolutely no organic matter, the BOD would be zero. However, in natural water bodies, it's rarely exactly zero.

Q: What happens if the BOD is between 5 ppm and 17 ppm?

Water with a BOD in this range is considered moderately polluted. It's not pristine, but it's not severely contaminated either.

Question

Water samples with BOD values of 4 ppm and 18 ppm, respectively, are

Options:

(A) clean and clean

(B) highly polluted and clean

(C) highly polluted and highly polluted

(D) clean and highly polluted

Sigma Percentile · Accepted Answer

Imagine a bustling microscopic city inside a drop of water. The citizens are aerobic bacteria, and their primary job is to clean up the city by eating organic waste. But just like us, these bacteria need oxygen to survive and do their heavy lifting. The more waste there is, the more bacteria show up to the feast, and the more oxygen they consume. This invisible hunger for oxygen is exactly what we measure when we talk about **Biochemical Oxygen Demand (BOD)**.

### Decoding the BOD Scale

Environmental chemists use BOD as a direct indicator of water pollution. It tells us how much dissolved oxygen (in parts per million, or ppm) is required by these microorganisms to break down the organic material present in a specific volume of water.

To make sense of these numbers, standard benchmarks have been established:

*   **Clean Water:** If the water is pristine, there is very little organic waste. The bacteria aren't working overtime, so the oxygen demand is low. Clean water typically has a BOD value of strictly less than $5 	ext{ ppm}$.
*   **Highly Polluted Water:** When water is heavily contaminated with sewage or industrial runoff, it becomes an all-you-can-eat buffet for bacteria. Their population explodes, and they rapidly deplete the dissolved oxygen. Water is considered highly polluted when its BOD reaches or exceeds $17 	ext{ ppm}$.

### Analyzing the Given Samples

Let's apply this knowledge to the problem at hand. We are given two distinct water samples with specific BOD values.

**Sample 1:** The first sample has a BOD of $4 	ext{ ppm}$. 
Since $4 	ext{ ppm} < 5 	ext{ ppm}$, this falls perfectly into the safe zone. The oxygen demand is minimal, indicating that the water is free from significant organic contamination. Therefore, Sample 1 is **clean**.

**Sample 2:** The second sample has a BOD of $18 	ext{ ppm}$. 
Since $18 	ext{ ppm} > 17 	ext{ ppm}$, this triggers a massive red flag. The bacteria in this sample are consuming oxygen at an alarming rate to break down a heavy load of organic waste. Therefore, Sample 2 is **highly polluted**.

### The Final Verdict

By simply comparing the given values against the standard environmental benchmarks, the conclusion is crystal clear. The first sample is clean, and the second sample is highly polluted. This perfectly matches option (d).

As a quick thought experiment: what happens if you encounter a water sample with a BOD of, say, $10 	ext{ ppm}$? It doesn't fit into either extreme. In environmental chemistry, water with a BOD between $5 	ext{ ppm}$ and $17 	ext{ ppm}$ is classified as *moderately polluted*. Always keep these threshold values at your fingertips!

Animated Solution for Chemistry - Environmental Chemistry: Water samples with BOD values of 4 ppm and 18 ppm, respectively, are

Select Answer:

Visualized Solution

$Understanding BOD$

$Standard BOD Values$

$Analyzing Sample 1$

$Analyzing Sample 2$

$Final Conclusion$

$The Way Forward$

The Sigma Insight: Water and Soil Pollution

Decoding the BOD Scale

Analyzing the Given Samples

The Final Verdict

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