Animated Solution for Chemistry - Environmental Chemistry: The depositions of $X$ and $Y$ on ground surfaces is referred to as wet and dry depositions, respectively. $X$ and $Y$ are

Wet and Dry Deposition

Have you ever wondered what happens to all the smoke, gases, and particulate matter that factories and vehicles pump into the atmosphere? They don't just stay up there forever. Eventually, what goes up must come down. This process of atmospheric pollutants settling back onto the Earth's surface is known as deposition.

In the realm of Environmental Chemistry, we broadly classify this settling process into two distinct categories: Wet Deposition and Dry Deposition. Understanding the difference between the two is crucial for tracking how pollution affects our soil, water bodies, and ecosystems.

Dry Deposition

Imagine a calm, dry day. Even without a drop of rain, pollutants are constantly making their way to the ground. This is dry deposition. It occurs when pollutants settle as gases or microscopic solid particles directly onto surfaces like leaves, buildings, and soil.

Acidic gases are the primary culprits here. Oxides of sulphur (like ${\text{SO}}_2$) and oxides of nitrogen (like ${\text{NO}}_x$) are heavy enough to gradually drift downwards. Because they settle without the aid of moisture, they are the classic examples of dry deposition. When these gases rest on surfaces, they can later react with dew or rain to form acids right on the spot, causing localized damage.

Wet Deposition

Now, imagine a heavy downpour. As raindrops fall through the polluted air, they act like a giant atmospheric washing machine. They dissolve soluble gases and capture suspended particles, dragging them down to the Earth. This is wet deposition.

When acidic gases like ${\text{SO}}_2$ dissolve in rain, they form acid rain (containing ${\text{H}}_2{\text{SO}}_4$). But it's not just acids. Ammonia gas (${\text{NH}}_3$) in the atmosphere often reacts with these acids to form ammonium salts (like ammonium sulphate or ammonium nitrate). These salts are highly soluble in water. Therefore, when it rains, these ammonium salts are washed down in the droplets, making them a textbook example of wet deposition.

Let's bring it all back to our specific problem. The question states that the depositions of $X$ and $Y$ are referred to as wet and dry depositions, respectively.

This means: - $X$ = Wet Deposition: This must be a substance that is washed down by rain. As we discussed, ammonium salts fit this perfectly. - $Y$ = Dry Deposition: This must be a gas or particle that settles directly. ${\text{SO}}_2$ is a prime example of an acidic gas that undergoes dry deposition.

Therefore, the correct match is $X=\text{Ammonium salts}$ and $Y={\text{SO}}_2$. It is a straightforward factual concept, but one that beautifully illustrates the dynamic relationship between our atmosphere and the ground we walk on.