Animated Solution for Chemistry - Environmental Chemistry: Water sample is called cleanest on the basis of which one of the BOD values given below

Understanding Dissolved Oxygen

Imagine a pristine, crystal-clear river flowing through a dense forest. Beneath the surface, a complex and delicate ecosystem thrives. Fish, insects, and microscopic organisms all depend on one invisible lifeline: Dissolved Oxygen (DO).

Just as we need oxygen in the air to breathe, aquatic life requires oxygen dissolved in the water to survive. When a water body is healthy, it has a high concentration of dissolved oxygen. This allows a diverse range of species to flourish.

However, when organic waste—such as sewage, agricultural runoff, or dead plant matter—enters the water, this delicate balance is disrupted. This is where the concept of Biochemical Oxygen Demand (BOD) comes into play.

What is BOD?

When organic waste enters a lake or river, it doesn't just sit there. It becomes an all-you-can-eat buffet for aerobic bacteria. These microscopic cleaners immediately get to work, breaking down the organic matter into simpler substances.

But this cleanup process comes at a cost. To decompose the waste, these bacteria consume the precious dissolved oxygen from the water. The more waste there is, the more bacteria multiply, and the more oxygen they consume.

Biochemical Oxygen Demand (BOD) is precisely the measure of this oxygen consumption. It is defined as the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period.

Therefore, BOD acts as a direct indicator of organic pollution. A high BOD means there is a massive amount of organic waste, leading to severe oxygen depletion. A low BOD means the water is relatively free of organic waste, leaving plenty of oxygen for fish and other aquatic life.

Grading Our Water

Environmental scientists and agencies have established specific thresholds to grade the quality of water based on its BOD value. These values are typically measured in parts per million (ppm) or milligrams per liter (mg/L).

For a water sample to be classified as clean water, its BOD value must be strictly less than $5\text{ ppm}$. At this level, the oxygen demand from bacteria is minimal. The water retains enough dissolved oxygen to support a healthy and vibrant aquatic ecosystem.

On the other end of the spectrum, if a water sample has a BOD value greater than $17\text{ ppm}$, it is classified as highly polluted water. In such conditions, the bacteria are consuming oxygen at an alarming rate. This rapid depletion can lead to hypoxia, creating "dead zones" where most aquatic life suffocates and dies.

These two critical thresholds—$<5\text{ ppm}$ for clean water and $>17\text{ ppm}$ for highly polluted water—are essential facts to memorize for competitive exams like JEE and NEET.

Now, let's apply this knowledge to the problem at hand. We are given four different water samples with varying BOD values, and we need to identify the cleanest one.

The given BOD values are $11\text{ ppm}$, $15\text{ ppm}$, $3\text{ ppm}$, and $21\text{ ppm}$. Let's evaluate each one against our standard thresholds.

First, consider the sample with a BOD of $21\text{ ppm}$. Since $21\text{ ppm}>17\text{ ppm}$, this water is highly polluted. It is severely contaminated with organic waste and would be toxic to most aquatic life.

Next, we have the samples with BOD values of $11\text{ ppm}$ and $15\text{ ppm}$. Both of these values fall between $5\text{ ppm}$ and $17\text{ ppm}$. This indicates that the water is moderately to heavily polluted. It is certainly not clean.

Finally, we look at the sample with a BOD of $3\text{ ppm}$. This is the only value that satisfies the condition for clean water, which is $\text{BOD}<5\text{ ppm}$. Because its oxygen demand is so low, it represents the purest and least contaminated sample among the choices.

You might be wondering how scientists actually determine this BOD value in a laboratory. The process is a fascinating application of chemical kinetics and biology.

To measure BOD, a water sample is first collected and its initial dissolved oxygen concentration is carefully measured. This gives us a baseline of how much oxygen is currently available in the water.

The sample is then sealed in an airtight bottle to prevent any additional oxygen from the atmosphere from dissolving into it. This bottle is placed in an incubator, typically maintained at a standard temperature of $20^{\circ }\text{C}$, and left in complete darkness for exactly five days.

The darkness is a crucial part of the setup. It prevents any photosynthetic algae present in the water from producing new oxygen, which would skew the results. We only want to measure the oxygen consumed by the bacteria.

After the five-day incubation period, the bottle is opened, and the final dissolved oxygen concentration is measured. The Biochemical Oxygen Demand is simply the difference between the initial and final dissolved oxygen levels.

This standard five-day test is universally known as ${\text{BOD}}_5$. It provides a reliable and standardized metric for environmental agencies worldwide to monitor water quality and enforce pollution control regulations. Understanding this experimental background gives you a much deeper appreciation for the simple numbers you see in your textbook!

By simply comparing the given values to the standard environmental thresholds, we can confidently conclude that the water sample with a BOD of $3\text{ ppm}$ is the cleanest. Therefore, the correct option is (c).

Questions based on Environmental Chemistry in the JEE often test your memory of these specific factual data points. While the math is non-existent, the conceptual clarity required is absolute.

Always remember the inverse relationship between BOD and water quality: Lower BOD means higher water quality, and higher BOD means lower water quality. Keep these standard values at your fingertips, and you will easily secure these crucial marks in your examination!