Question

A solution when diluted with water 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) \(\mathrm{Al}(\mathrm{OH})_{3}\) (b) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (c) \(\mathrm{Mg}(\mathrm{OH})_{2}\) (d) \(\mathrm{Zn}(\mathrm{OH})_{2}\)

Short answer

(d) \( \mathrm{Zn(OH)_2} \) is the precipitate dissolving in ammonium compounds.

Step-by-step solution

Analyze the Initial Reaction

When the solution is diluted with water and boiled, a white precipitate forms. This indicates that the metal hydroxide precipitates from the solution under these conditions.

Reaction with Ammonium Compounds

The addition of excess \( \mathrm{NH}_{4} \mathrm{Cl} \) and \( \mathrm{NH}_{4} \mathrm{OH} \) reduces the precipitate volume, suggesting the formation of a complex. We need to identify which component forms a soluble complex with these reagents.

Determine the Soluble Complex

Among the hydroxides listed, \( \mathrm{Zn} (\mathrm{OH})_2 \) forms a soluble complex when reacted with \( \mathrm{NH}_{4} \mathrm{OH} \), specifically \( \mathrm{[Zn(NH_3)_4]^{2+}} \), which reduces the precipitate, distinguishing it from the other options.

Identify the Remaining Precipitate

After the addition of \( \mathrm{NH}_{4} \mathrm{OH} \) and \( \mathrm{NH}_{4} \mathrm{Cl} \), the solution leaves behind a white gelatinous precipitate. The precipitate that does not dissolve in \( \mathrm{NH}_{4} \mathrm{OH} \) is likely an insoluble hydroxide.

Conclusion: Identify the Dissolving Precipitate

The precipitate that dissolves in \( \mathrm{NH}_{4} \mathrm{OH} \) due to complexation is \( \mathrm{Zn(OH)_2} \). Thus, \( \mathrm{Zn} (\mathrm{OH})_2 \) is the hydroxide that's solubilized by \( \mathrm{NH}_{4} \mathrm{OH} / \mathrm{NH}_{4} \mathrm{Cl} \) as a complex, confirming it is the correct answer.

Key concepts

Solubility of Hydroxides

In chemistry, solubility is a measure of how well a substance dissolves in a solvent. When it comes to hydroxides, their solubility varies significantly. Hydroxides like

• Aluminium hydroxide (\( \text{Al(OH)}_3 \)
• Calcium hydroxide (\( \text{Ca(OH)}_2 \)
• Magnesium hydroxide (\( \text{Mg(OH)}_2 \)
• Zinc hydroxide (\( \text{Zn(OH)}_2 \))

exhibit different solubility behaviors in water. Generally, metal hydroxides tend to be more insoluble in water compared to other compounds.
In the case of zinc hydroxide, it has a unique ability to interact with various substances to form complex ions, which affects its solubility. The solubility increases when it reacts in the presence of certain ligands, such as ammonia forming a complex ion. The formation of such ionic complexes can lead to dissolution of otherwise insoluble compounds. Understanding the solubility of hydroxides is crucial for predicting their behavior in chemical reactions.

Complex Formation

Complex formation is a significant concept in chemistry where a compound forms complex ions by binding with other ligands. In this response, we discuss zinc hydroxide, which forms a complex with ammonia.
When zinc hydroxide (\( \text{Zn(OH)}_2 \)) is treated with ammonium compounds, it is solubilized by forming a tetra ammine zinc complex:

• The chemical equation for this interaction can be represented as follows: \[\text{Zn(OH)}_2 + 4\text{NH}_3 + \text{H}_2\text{O} \rightarrow \text{[Zn(NH}_3\text{)}_4\text{]}^{2+} + 2\text{OH}^- \]

This reaction indicates that the zinc hydroxide is dissolved because of the formation of the soluble \( \text{[Zn(NH}_3\text{)}_4\text{]^{2+}} \) complex ion.
This principle relies on the abilities of ammonia to act as a ligand, effectively surrounding the zinc ion and creating a stable complex that keeps zinc in solution. This dissolution of hydroxide into a complex form is a convenient method to change the solubility behavior of a compound. Understanding these interactions is crucial in recognizing how such transformations occur in various chemical processes.

Precipitation Reactions

Precipitation reactions are fundamental in chemistry, characterized by the formation of a solid from a solution. In this process, two soluble ions combine to produce an insoluble compound: the precipitate. Initially, when the solution is diluted and boiled in this exercise, a white precipitate forms indicating the arising of an insoluble phase likely due to a specific reaction condition, such as heat.
Here, when zinc sulfate solution is boiled and diluted, zinc hydroxide precipitates due to the low solubility of zinc hydroxide in water.
In general, a white precipitate signals the formation of a hydroxide like \( \text{Zn(OH)}_2 \).

• The residual precipitate after interaction with ammonium chloride and \( \text{NH}_4\text{OH} \) is often a metal hydroxide that remains insoluble while other forms dissolve.

The interaction and transition from precipitate to dissolved complex form is typical of the interplay in precipitation reactions. Understanding precipitation reactions is crucial when predicting the formation or dissolution of solids in chemical solutions.