Question

A solution when dilute with \(\mathrm{H}_{2} \mathrm{O}\) and boiled, it 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) \(\mathrm{Al}(\mathrm{OH})_{3}\) (b) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (c) \(\mathrm{Mg}(\mathrm{OH})_{2}\) (d) \(\mathrm{Zn}(\mathrm{OH})_{2}\)

Short answer

The precipitate is \( \mathrm{Al} (\mathrm{OH})_{3} \).

Step-by-step solution

Understanding the problem

We need to identify the precipitate that dissolves in the presence of \(NH_{4}OH\) and \(NH_{4}Cl\). The key is to understand which hydroxide behaves this way in chemistry.

Consider the behavior of aluminum hydroxide

First, consider \(Al(OH)_3\). Aluminum hydroxide is known to form a white gelatinous precipitate that will dissolve in excess \(NH_{4}OH\), forming a complex ion. Hence, it matches the description.

Consider other hydroxides

Now, examine \(Ca(OH)_2\), \(Mg(OH)_2\), and \(Zn(OH)_2\). \(Ca(OH)_2\) and \(Mg(OH)_2\) generally do not dissolve in \(NH_{4}OH\)/ \(NH_{4}Cl\). On the other hand, \(Zn(OH)_2\), like \(Al(OH)_3\), can dissolve in the presence of \(NH_{4}OH\) due to complex formation.

Elimination and Conclusion

The information provided indicates both dissolution and the formation of a white gelatinous precipitate. This characteristic is a strong indicator of \(Al(OH)_3\), which is known to dissolve in \(NH_{4}OH\)/\(NH_{4}Cl\). \(Zn(OH)_2\) also dissolves but is less common as a gelatinous precipitate under these conditions.

Key concepts

Precipitation Reactions

Precipitation reactions play a crucial role in inorganic chemistry, helping to identify substances through the formation of a solid out of a liquid solution. A precipitation reaction occurs when two aqueous solutions combine to form an insoluble solid, known as a precipitate.
This process is characterized by specific chemical conditions where the ions in the solution form a less soluble compound that falls out of the solution.

• In the given exercise, the formation of a white precipitate upon boiling suggests the presence of metal hydroxides like Aluminum Hydroxide \(Al(OH)_3\) or Zinc Hydroxide \(Zn(OH)_2\).
• The unique behavior of this precipitate when treated with different reagents provides further clues for identification.

Understanding and predicting precipitation reactions is foundational for analyzing reactions in solutions, especially in laboratory settings.

Complex Ions Formation

Complex ion formation is a key concept in understanding why certain precipitates dissolve in specific conditions. A complex ion is formed when a central metal ion binds to one or more ligands.
In this process, the metal ion and ligand interactions enhance the metal's solubility in solution.

• For instance, in the presence of \(NH_4OH\) and \(NH_4Cl\), Aluminum Hydroxide \(Al(OH)_3\) forms a soluble complex ion, \[Al(OH)_4^-\], which causes the precipitate to dissolve.
• Similarly, Zinc Hydroxide \(Zn(OH)_2\) interacts with these ions to form complexes like \[Zn(NH_3)_4^{2+}\].

The formation of such complex ions is crucial in processes where solubility can be manipulated by altering the chemical environment.

Hydroxide Solubility

Understanding hydroxide solubility is critical for predicting the behavior of metal hydroxides in solution. Not all hydroxides are soluble in water, which is why they often form precipitates. The solubility of a metal hydroxide depends on various factors, such as pH and presence of complexing agents.

• In the problem at hand, \(Al(OH)_3\) and \(Zn(OH)_2\) display solubility in complex ion form, while \(Ca(OH)_2\) and \(Mg(OH)_2\) remain largely insoluble in similar conditions.
• Hydroxides of metals like aluminum and zinc have a distinct ability to dissolve in excess \(NH_4OH\), unlike calcium or magnesium hydroxides, which are more stable as precipitates under these conditions.

This selective solubility helps in distinguishing metals based on their chemical behavior.

Aluminium Hydroxide

Aluminium Hydroxide \(Al(OH)_3\) is a prime example of a substance that undergoes unique chemical changes in specific conditions. As an amphoteric compound, it reacts both with acids and bases.
This dual nature allows \(Al(OH)_3\) to dissolve in excess \(NH_4OH\), forming a soluble complex ion. Here’s what happens:

• The gelatinous white precipitate of \(Al(OH)_3\) becomes a soluble species \[Al(OH)_4^-\] in the presence of an alkaline solution like \(NH_4OH\).
• Its reactivity with \(NH_4Cl\) aids in maintaining the reaction's alkaline environment, promoting further solubility.

Aluminium Hydroxide is notable for its amphoteric properties, making it an interesting subject in inorganic chemistry.

Zinc Hydroxide

Zinc Hydroxide \(Zn(OH)_2\) shares some properties with Aluminum Hydroxide but also carries unique characteristics. It is slightly soluble in water and pressures favoring a shift towards a complex ion form.

• When \(NH_4OH\) is added, \(Zn(OH)_2\) can dissolve by forming complex ions such as \[Zn(NH_3)_4^{2+}\].
• The dissolution under these conditions distinguishes it from many other hydroxides but it is less common to form a gelatinous precipitate compared to \(Al(OH)_3\).

Zinc Hydroxide demonstrates the nuances in solubility trends and the influence of chemical environments on solid-liquid interactions.