Animated Solution for Chemistry - Environmental Chemistry: A water sample has ppm level concentration of the following metals: $\text{Fe}=0.2$; $\text{Mn}=5.0$; $\text{Cu}=3.0$; $\text{Zn}=5.0$. The metal that makes the water sample unsuitable for drinking is

Decoding Water Quality Standards

Imagine you are handed a glass of crystal-clear water. It looks perfectly fine, but at the microscopic level, it might be hiding a toxic secret. In Environmental Chemistry, we learn that just because water is transparent doesn't mean it is safe to drink. The safety of drinking water is strictly governed by the concentration of various trace metals, measured in parts per million (ppm).

In this problem, we are given a water sample containing four metals: Iron ($\text{Fe}$), Copper ($\text{Cu}$), Zinc ($\text{Zn}$), and Manganese ($\text{Mn}$). Our mission is to act as environmental chemists and identify which of these metals has crossed the red line, making the water unfit for human consumption.

Iron, Copper, and Zinc

To solve this, we must rely on our memory of the standard maximum permissible limits set by environmental guidelines (like those from the WHO or the European Environment Commission, which are standard in the NCERT curriculum).

Let's evaluate the metals one by one:

1. Iron ($\text{Fe}$): The sample contains $0.2\text{ ppm}$ of Iron. The maximum allowed limit for Iron in drinking water is exactly $0.2\text{ ppm}$. Since $0.2\le 0.2$, Iron is right at the boundary but still considered safe.

2. Copper ($\text{Cu}$): The sample has $3.0\text{ ppm}$ of Copper. The standard limit for Copper is also $3.0\text{ ppm}$. Again, it is perfectly within the acceptable range.

3. Zinc ($\text{Zn}$): We have $5.0\text{ ppm}$ of Zinc. The permissible limit for Zinc is $5.0\text{ ppm}$. Zinc gets a green light as well.

Manganese

Now, we turn our attention to Manganese ($\text{Mn}$). The given concentration in the sample is $5.0\text{ ppm}$. At first glance, $5.0\text{ ppm}$ might not seem like a huge number, especially since Zinc was allowed up to $5.0\text{ ppm}$.

However, here is the catch: different metals have vastly different toxicological profiles. The human body can tolerate Zinc much better than Manganese. The maximum allowed limit for Manganese in drinking water is a mere $0.05\text{ ppm}$.

Let's compare the values:

Our water sample contains 100 times the safe limit of Manganese! Consuming water with such high levels of Manganese can lead to severe neurological issues.

Because the concentration of Manganese drastically exceeds the recommended safety limit, it is the sole reason this water sample is completely unsuitable for drinking.

This question is a brilliant reminder of why rote memorization of certain standard values in Environmental Chemistry is non-negotiable for JEE. You cannot derive these limits in the exam hall; you simply have to know them. Keep these numbers at your fingertips, and you'll secure these easy marks in seconds!