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 is \(\mathrm{Zn(OH)}_{2}\).

Step 1: Analyzing Initial Conditions

Upon dilution with water and boiling, the solution forms a white precipitate. This suggests the presence of metal hydroxides or sulfates that are insoluble in boiling water. The key is that the precipitate is white, which narrows our focus to possible candidates like zinc, aluminum, magnesium, and calcium hydroxides.

Step 2: Impact of NH4Cl/NH4OH Addition

When excess \(\mathrm{NH}_{4}\mathrm{Cl}/\mathrm{NH}_{4}\mathrm{OH}\) is added, the volume of the precipitate decreases. This suggests that some of the precipitate is dissolving. Ammonium salts, particularly \(\mathrm{NH}_{4}\mathrm{OH}\) formed in solution, can dissolve amphoteric hydroxides like \(\mathrm{Zn(OH)}_{2}\) or \(\mathrm{Al(OH)}_{3}\), but not regular hydroxides like \(\mathrm{Mg(OH)}_{2}\) or \(\mathrm{Ca(OH)}_{2}\).

Step 3: Identifying the Amphoteric Nature

\(\mathrm{Zn(OH)}_{2}\) and \(\mathrm{Al(OH)}_{3}\) exhibit amphoteric behavior; they can react with both acids and bases. The fact that adding \(\mathrm{NH}_{4}\mathrm{Cl}/\mathrm{NH}_{4}\mathrm{OH}\) decreases the volume of precipitate indicates the dissolution of one such amphoteric hydroxide.

Step 4: Final Identification Based on Reaction Trends

\(\mathrm{Zn(OH)}_{2}\) is known to dissolve in \(\mathrm{NH}_{4}\mathrm{Cl}/\mathrm{NH}_{4}\mathrm{OH}\) to form \([\mathrm{Zn(NH}_3)_4]^{2+}\), while \(\mathrm{Al(OH)}_{3}\) is less soluble in this mixture. This difference hints at \(\mathrm{Zn(OH)}_{2}\) as the compound that dissolves significantly under these conditions.

Key Concepts

Ammonium Hydroxide

Ammonium hydroxide is a solution of ammonia in water. Even though it is called hydroxide, it does not consist of ammonium ions and hydroxide ions. Instead, it forms by dissolving ammonia gas (\(\text{NH}_3\)) in water. This mixture gives a weakly basic solution because the ammonia reacts with water to yield ammonium ions (\(\text{NH}_4^+\)) and hydroxide ions (\(\text{OH}^-\)).

• Ammonium hydroxide is often used in cleaning agents and as a precursor in various chemical syntheses.
• In a chemical context, it can react with amphoteric substances, which possess characteristics of both acids and bases.

Understanding its interactions is key in determining the behavior of compounds such as zinc and aluminum hydroxides when \(\text{NH}_4\text{OH}\) is added. Ammonium hydroxide can facilitate the dissolution of amphoteric hydroxides due to its ability to donate \(\text{OH}^-\) ions.

Zinc Hydroxide

Zinc hydroxide, denoted as \(\text{Zn(OH)}_2\), is an amphoteric hydroxide. This means it can react both as an acid and as a base: an exceptional property. In the presence of bases like ammonium hydroxide, zinc hydroxide can dissolve, forming complex ions. When \(\text{NH}_4\text{Cl} / \text{NH}_4\text{OH}\) is added, \(\text{Zn(OH)}_2\) dissolves to form a soluble complex, \([\text{Zn(NH}_3)_4]^{2+}\).

• This dissolution process reduces the amount of solid \(\text{Zn(OH)}_2\), explaining why the volume of the precipitate diminishes.
• Moreover, the characteristic white precipitate of zinc hydroxide is initially formed when solutions containing zinc ions are boiled with water.

The ability of zinc hydroxide to dissolve in excess ammonia solution highlights its amphoteric nature, a crucial factor in identifying it as the precipitate when \(\text{NH}_4\text{Cl} / \text{NH}_4\text{OH}\) is present.

Aluminum Hydroxide

Aluminum hydroxide, \(\text{Al(OH)}_3\), shares a feature similar to zinc hydroxide in that it is amphoteric. It can react with acids, forming aluminum salts, and with bases to produce aluminate ions.

• This characteristic permits aluminum hydroxide to react in different chemical environments.
• However, compared to zinc hydroxide, \(\text{Al(OH)}_3\) has limited solubility in ammonium hydroxide solutions.

When ammonium hydroxide is added to solutions containing aluminum ions, some reaction occurs, partially dissolving the aluminum hydroxide. However, the resulting gelatinous precipitate remains more pronounced than the clear dissolution observed with \(\text{Zn(OH)}_2\). Aluminum hydroxide is less efficient at reacting with \(\text{NH}_4\text{Cl}/\text{NH}_4\text{OH}\), helping to differentiate it from zinc hydroxide in tests.