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) \(\mathrm{Ca}(\mathrm{OH})_{2}\)

Short answer

The precipitate that dissolves in \(\mathrm{NH}_{4} \mathrm{OH} \ldots\ / \mathrm{NH}_{4} \mathrm{Cl}\) is \(\mathrm{Zn}( ext{OH})_{2}\).

Step-by-step solution

Analyze Initial Precipitate Formation

When the solution is diluted and boiled, a white precipitate forms. This is a typical behavior of hydroxides that have low solubility in water. Possible candidates could include Zn(OH)2, Al(OH)3, Mg(OH)2, and Ca(OH)2.

React with Ammonium Chloride and Ammonium Hydroxide

Adding excess \(\mathrm{NH}_{4} \mathrm{Cl}\)/\(\mathrm{NH}_{4} \mathrm{OH}\) causes the white precipitate to decrease in volume. Ammonium hydroxide can dissolve some metal hydroxides, specifically Zn(OH)2 due to complex formation, while others like Al(OH)3 become gelatinous.

Identify the Dissolved Precipitate

The problem requires identifying the precipitate that dissolves in ammonium hydroxide. Zn(OH)2 is known to form complex ions like \([\mathrm{Zn}( ext{NH}_3)_4]^{2+}\), which are soluble in ammonium hydroxide. This characteristic does not apply to Al(OH)3 or Mg(OH)2, which form gelatinous precipitates.

Key concepts

Hydroxide Solubility

Hydroxides are compounds that contain the hydroxide ion (OH⁻) and are known for varying degrees of solubility in water. When a solution containing metal ions is diluted with water and boiled, hydroxides like Zn(OH)₂, Al(OH)₃, Mg(OH)₂, and Ca(OH)₂ may precipitate if they have low solubility.

• Water Solubility: Some metal hydroxides are more soluble in water than others. Zinc hydroxide (Zn(OH)₂) and aluminum hydroxide (Al(OH)₃) have relatively low solubility, which leads to their formation as white precipitates when the solution conditions change.
• Precipitate Formation: When the solution is boiled, hydroxides typically form precipitates due to decreased solubility at higher temperatures.

Understanding which hydroxides are less soluble in water helps in identifying the precipitate, especially when matched with other chemical behaviors they exhibit when solutions are modified.

Complex Ion Formation

The concept of complex ion formation is pivotal in understanding how certain precipitates dissolve in the presence of specific chemicals. Complex ions are formed when metal ions bind with other molecules or ions, stabilizing the metal ion in solution. For Zn(OH)₂, the interaction with ammonium hydroxide is particularly noteworthy.

• Complexation with Ammonia: Zinc hydroxide can react with ammonia (NH₃) from ammonium hydroxide (NH₄OH) to form soluble complex ions such as \[ [\text{Zn}(\text{NH}_3)_4]^{2+} \]. This explains the dissolution of Zn(OH)₂ in ammonium hydroxide.
• Solubility Enhancement: The formation of these complexes increases the apparent solubility of the metal, allowing it to stay in solution instead of precipitating out.

The complexity of these aqueous reactions highlights the importance of complex ion stability in influencing the outcome of precipitation reactions.

Ammonium Hydroxide Reactions

Ammonium hydroxide (NH₄OH) is a weak base solution of ammonia in water and reacts differently with various hydroxides. It is notable for its ability to form complexes with certain metal ions, which significantly alter the behavior of precipitates in a solution.

• Behavior with Zinc Hydroxide: When added to solutions containing Zn(OH)₂, the ammonium hydroxide promotes the dissolution of this hydroxide by forming a complex ion \[ [\text{Zn}(\text{NH}_3)_4]^{2+} \]. This phenomenon distinguishes zinc hydroxide from other precipitates such as Al(OH)₃, which does not form similar complexes.
• Interaction with Other Hydroxides: Aluminum hydroxide (Al(OH)₃) remains largely insoluble and forms a gelatinous precipitate in the presence of ammonium hydroxide, as it does not possess the same complex-forming capabilities.

These reactions are crucial for identifying various chemical behaviors during precipitation reactions, as they help differentiate between similar precipitates based on their interaction with different substances.