Problem 76 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The precipitate that dissolves in \(\mathrm{NH}_{4} \mathrm{OH} / \mathrm{NH}_{4} \mathrm{Cl}\) is \(\mathrm{Zn(OH)}_2\).

Step 1: Analyze Precipitate Formation

When the solution is diluted with water and boiled, a white precipitate forms. This indicates the presence of a compound that is less soluble in hot water. Common hydroxides like \(\mathrm{Zn(OH)}_2\), \(\mathrm{Al(OH)}_3\), \(\mathrm{Mg(OH)}_2\), and \(\mathrm{Ca(OH)}_2\) can behave this way.

Step 2: Evaluate Precipitate Solubility in NH4Cl/NH4OH

Upon addition of excess \(\mathrm{NH}_{4} \mathrm{Cl}/\mathrm{NH}_{4} \mathrm{OH}\), the volume of the white precipitate decreases, suggesting that some of the precipitate dissolves in this mixture. This behavior matches the properties of amphoteric hydroxides, which dissolve in both acidic and basic environments. \(\mathrm{Zn(OH)}_2\) and \(\mathrm{Al(OH)}_3\) are amphoteric.

Step 3: Reason Out the Remaining Precipitate

The remaining precipitate is described as a white gelatinous substance. \(\mathrm{Al(OH)}_3\) is known for forming a gelatinous precipitate when the solution is alkaline. This step helps differentiate amphoteric hydroxides by considering the physical characteristics of the remaining precipitate.

Step 4: Identify the Dissolving Precipitate

Considering that \(\mathrm{Zn(OH)}_2\) dissolves in an excess of \(\mathrm{NH}_{4} \mathrm{Cl}/\mathrm{NH}_{4} \mathrm{OH}\) leaving behind a gelatinous \(\mathrm{Al(OH)}_3\), we identify that \(\mathrm{Zn(OH)}_2\) is responsible for the reduction in precipitate volume because it specifically dissolves in these conditions.

Key Concepts

Precipitate Formation

The process of precipitate formation is a fascinating aspect of chemistry, particularly noticeable when certain solutions undergo changes due to factors such as temperature or dilution. In the context of the exercise, when a solution is diluted with water and boiled, a white precipitate forms. This is a clear sign that some compound within the solution is less soluble in the new conditions, leading to the formation of a solid out of solution.
This is because solubility is often dependent on temperature, and changes can prompt some substances to exceed their solubility limit and precipitate out as solids.
In this scenario, common hydroxides like

• Zn(OH) ₂,
• Al(OH)₃,
• Mg(OH)₂,
• Ca(OH)₂

are known to form precipitates under specific conditions. Precipitation is a critical process used in various scientific applications, including purifying compounds or removing undesirable substances from solutions.

Hydroxide Solubility

Hydroxide solubility is crucial in understanding how compounds behave in solution, especially in the presence of specific reagents. Some hydroxides, like zinc and aluminum hydroxides, have unique solubility characteristics. Upon adding excess distinguishing factors that play into hydroxide solubility:

• temperature,
• pH level,
• presence of complexing agents.

The exercise specifically describes the addition of ammonium chloride/ammonium hydroxide ( NH ₄Cl/ NH ₄OH) leading to the dissolution of the precipitate. This behavior of dissolving in an ammoniacal medium indicates compound amphoterism, where it can interact with both acidic and basic solutions, becoming more soluble.
Thus, monitoring how a compound reacts under varying conditions helps in predicting the solubility and potential reactions in given chemical processes.

Amphoterism in Chemistry

Amphoterism is a fascinating chemical property referring to a compound's ability to react with both acids and bases. Amphoteric hydroxides, such as Zn(OH) ₂ and Al(OH) ₃, exemplify this property. In the exercise, amphoterism explains why, when excess NH ₄Cl/ NH ₄OH is added, some precipitate dissolves, while others remain, such as the gelatinous Al(OH) ₃.
Amphoteric compounds are incredibly valuable due to their versatility in chemical reactions. They

• can help in buffering solutions,
• participate in complex formation,
• play a role in metallurgy for leaching metals.

Comprehending the concept of amphoterism requires recognizing these dual character interactions, as they deeply influence how compounds like the hydroxides in the exercise behave in the presence of different chemical environments.