JEE Advanced 2025
LEVELJEE Main
Visualized Solution
The Sigma Insight: Group 15 Elements
Imagine you are in a chemistry lab, carefully heating two white crystalline powders in separate test tubes. One is ammonium nitrite (), and the other is ammonium nitrate ().
This is a classic scenario that tests your understanding of thermal decomposition. The key to unlocking this problem lies in the nature of the anions attached to the ammonium ion ().
Both nitrite () and nitrate () are strong oxidizing agents. When heated, they don't just break apart; an internal redox reaction occurs!
The "Maximum Water" Trick
Here is a brilliant shortcut to predict the products of such reactions. We call it the Maximum Water Rule.
When an ammonium salt contains an oxygen-rich oxidizing anion, the hydrogen atoms from the ammonium ion will pair up with the oxygen atoms from the anion. They will form as many water () molecules as mathematically possible.
Whatever atoms are left behind will combine to form the remaining gaseous product. Let's apply this elegant logic to our two salts.
Analyzing Ammonium Nitrite
Let's look at the first reaction. We are heating at a moderate temperature of .
Applying our trick, we have 4 hydrogen atoms and 2 oxygen atoms. These will perfectly combine to yield exactly 2 molecules of water ().
Now, what is left in the formula? We have exactly two nitrogen atoms remaining. These will bond together to form nitrogen gas ().
Therefore, the complete balanced equation is:
This means our first unknown compound, X, is .
Analyzing Ammonium Nitrate
Now, let's shift our focus to the second salt. We are heating at a much higher temperature of .
We apply the exact same logic. The 4 hydrogen atoms will again consume 2 oxygen atoms to form 2 molecules of water ().
However, this time, the nitrate ion had 3 oxygen atoms to begin with. After forming water, we are left with two nitrogen atoms and one extra oxygen atom.
These remaining atoms combine to form nitrous oxide (), commonly known as laughing gas.
The balanced equation for this process is:
Thus, our second unknown compound, Y, is .
The Final Verdict
By systematically applying the Maximum Water Rule, we have successfully identified both gases. Compound X is nitrogen gas (), and compound Y is nitrous oxide ().
Comparing this with our given options, we can confidently conclude that Option (A) is the correct answer.
Always remember this fundamental difference: oxidizing anions yield nitrogen gases, while non-oxidizing anions (like chloride) yield ammonia gas!
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