Problem 15 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A solution when diluted with \(\mathrm{H}_{2} \mathrm{O}\) and boiled, gives a white precipitate. On addition of excess \(\mathrm{NH}_{4} \mathrm{Cl} / \mathrm{NH}_{4} \mathrm{OH}\), the volume of precipitate decreases leaving behind a white gelatinous precipitate. Identify the precipitate which disolves in \(\mathrm{NH}_{4} \mathrm{OH} / \mathrm{NH}_{4} \mathrm{Cl}\) (a) \(\mathrm{Al}(\mathrm{OH})_{3}\) (b) \(\mathrm{Zn}(\mathrm{OH})_{2}\) (c) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (d) \(\mathrm{Mg}(\mathrm{OH})_{2}\)

Step-by-Step Solution

Answer

The correct answer is (b) \( \text{Zn(OH)}_2 \).

Step 1: Understand the Problem

We need to identify which hydroxide dissolves when treated with an excess of ammonium chloride/ammonium hydroxide. We have 4 potential candidates: (a) \( \text{Al(OH)}_3 \), (b) \( \text{Zn(OH)}_2 \), (c) \( \text{Ca(OH)}_2 \), and (d) \( \text{Mg(OH)}_2 \).

Step 2: Analyze Solubility in \( \text{NH}_4 \text{Cl} \)/\( \text{NH}_4 \text{OH} \)

The precipitate which dissolves in \( \text{NH}_4 \text{Cl} \)/\( \text{NH}_4 \text{OH} \) is typically amphoteric. \( \text{Zn(OH)}_2 \) is known to dissolve in \( \text{NH}_4 \text{Cl} \) and \( \text{NH}_4 \text{OH} \) to form complex zinc ions such as \( [\text{Zn(NH}_3)_4]^{2+} \), indicating amphoteric behavior.

Step 3: Check Other Substances

\( \text{Al(OH)}_3 \) is amphoteric and forms \( [\text{Al(NH}_3)_6]^{3+} \), but typically the question implies a smaller change in precipitate with \( \text{Zn(OH)}_2 \). \( \text{Ca(OH)}_2 \) and \( \text{Mg(OH)}_2 \) are basic and do not form such complexes, so they do not dissolve in the solution.

Step 4: Conclude the Correct Answer

Since \( \text{Zn(OH)}_2 \) dissolves in excess \( \text{NH}_4 \text{Cl} \)/\( \text{NH}_4 \text{OH} \) while others do not, the precipitate that dissolves is \( \text{Zn(OH)}_2 \).

Key Concepts

Solubility of Hydroxides

Hydroxides, in general, have varying levels of solubility in water and other solutions. The solubility of a hydroxide can greatly affect its behavior in a chemical reaction. Some hydroxides, like those of alkaline earth metals, tend to have low solubility, meaning they do not dissolve easily in water. This is why substances like \( \text{Mg(OH)}_2 \) and \( \text{Ca(OH)}_2 \) appear as solid precipitates when they react. They remain undissolved due to their basic nature and inability to form complex ions with common reagents.
In contrast, hydroxides like \( \text{Zn(OH)}_2 \) and \( \text{Al(OH)}_3 \), which are amphoteric, can dissolve in certain conditions. Their solubility increases in the presence of excess ammonium ions from \( \text{NH}_4 \text{OH} \) or \( \text{NH}_4 \text{Cl} \), enabling them to form dissolved complex ions. This is a critical clue in identifying a substance, as in the case where \( \text{Zn(OH)}_2 \) dissolved indicates how amphoteric and solubility properties interact.

Amphoteric Behaviour

Amphoteric substances have the unique ability to react both as acids and bases. This dual characteristic is pivotal in many chemical reactions and helps in distinguishing compounds based on their reactions with different chemicals.
For instance, \( \text{Zn(OH)}_2 \) is an example of an amphoteric hydroxide. It can react with acids, like hydrochloric acid, by accepting protons. Meanwhile, it can also dissolve in basic solutions such as \( \text{NH}_4 \text{OH} \) forming a complex ion \( [\text{Zn(NH}_3)_4]^{2+} \). This behavior is due to its ability to both donate and accept electrons depending on the surrounding environment.
Similarly, \( \text{Al(OH)}_3 \) exhibits amphoteric properties by forming \( [\text{Al(NH}_3)_6]^{3+} \) in certain solutions, which allows it to dissolve, albeit to a different degree compared to \( \text{Zn(OH)}_2 \). Understanding amphoterism is essential to predict solubility and reactions of hydroxides in various scenarios.

Chemical Precipitation

Chemical precipitation involves forming a solid from a solution due to a chemical reaction. This is often observed when two solutions react, resulting in an insoluble compound that separates from the liquid.
In the exercise, a white precipitate forms initially due to the introduction of a hydroxide into the solution, where it is less soluble. However, upon addition of \( \text{NH}_4 \text{Cl} \) or \( \text{NH}_4 \text{OH} \), some precipitates like \( \text{Zn(OH)}_2 \) dissolve, demonstrating its amphoteric nature and forming a complex soluble ion.
The process of decrementing precipitate through specific introductions identifies that the undissolved portion may be more gelatinous, like what is observed with \( \text{Al(OH)}_3 \). By understanding what causes precipitation and dissolution, experiments can be tailored to identify unknown compounds or purify mixtures.