Problem 135 - A Complete Resource Book in Chemistry for JEE Main

Question

A solution, when diluted with water 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 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: Recognize the context and properties

The problem involves a solution that, when boiled and diluted in water, produces a white precipitate. The precipitate behaves differently when exposed to \(\mathrm{NH}_{4} \mathrm{Cl} / \mathrm{NH}_{4} \mathrm{OH}\) by decreasing or dissolving partially.

Step 2: Understand the behavior with NH4Cl and NH4OH

When excess \( \mathrm{NH}_{4} \mathrm{Cl} / \mathrm{NH}_{4} \mathrm{OH} \) is added to the precipitate, the volume of precipitate decreases, indicating partial dissolution. This suggests that the precipitate may form a soluble complex with ammonia and chloride ions.

Step 3: Identify compounds with the specific reaction

Among the given options, \\mathrm{Zn(OH)}_2\ can dissolve in excess ammonia to form \(\mathrm{Zn(NH}_3\mathrm{)}_4^{2+} \) complex. \\mathrm{Al(OH)}_3\ forms a gelatinous precipitate and is not known to dissolve under these conditions. **So, the compound dissolving here in the given condition is \\mathrm{Zn(OH)}_2\**.

Key Concepts

Precipitation Reactions

Precipitation reactions occur when two soluble substances in aqueous solution combine to form an insoluble compound, known as a precipitate. This is a classic type of chemical reaction often seen in inorganic chemistry.
For example, when a solution containing zinc ions ( Zn^2+ ) is mixed with a base like hydroxide ions ( OH^- ), a white solid zinc hydroxide ( Zn(OH)_2 ) precipitates out. This precipitate is insoluble in water and can be observed as a solid forming.
The occurrence of a precipitate helps in identifying either the presence of specific ions or serves as a purification step in chemical reactions. Factors like temperature, concentration of reactants, and the completeness of mixing can influence the formation and characteristics of precipitation.

Complex Ions and Solubility

Complex ions involve a central metal cation bonded to one or more molecules or ions through coordinate covalent bonds. These can drastically change the solubility characteristics of certain compounds.
For instance, zinc hydroxide ( Zn(OH)_2 ) which is poorly soluble in water can exhibit increased solubility when transformed into a complex ion. When excess ammonia ( NH_3 ) is added, zinc hydroxide forms a soluble complex ion, Zn(NH_3)_4^{2+} .

• This process involves the ligand (in this case, ammonia) displacing the hydroxide ions.
• The formed complex ion is much more soluble in water compared to the original precipitate.

Complex ion formation is an important consideration in qualitative analysis of various ions, often employed to enhance solubility and aid in the separation and identification of ions.

Chemical Identification

Chemical identification involves determining the presence of certain ions or compounds in a solution. This is often done by observing the physical changes such as color change, precipitate formation, or reaction with specific reagents.
In the given scenario, the identification centers on discerning which of the listed compounds behaves as described when interacting with NH_4Cl / NH_4OH . By noting the reduction in the volume of the precipitate upon addition of NH_4OH , we can deduce that a complex ion is forming, thereby helping to identify Zn(OH)_2 as the compound that dissolves.

• This process utilizes our understanding of both precipitation and complex ion reactions.
• Careful observation and processing of the reactions can provide significant clues leading to accurate chemical identification.

Hence, chemical identification is a critical skill in both academic and practical chemistry applications. It hinges on recognizing key reaction signatures and understanding the underlying chemistry principles.