Problem 122
Question
A solution, when diluted with \(\mathrm{H}_{2} \mathrm{O}\) 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
Verified Answer
The precipitate that dissolves in \\(\mathrm{NH}_{4} \mathrm{OH} / \mathrm{NH}_{4} \mathrm{Cl}\\) is \\(\mathrm{Zn} \mathrm{(OH)}_{2}\\).
1Step 1: Understand the Precipitation Reaction
A white precipitate forms when a solution is boiled after adding water, signaling the formation of a metal hydroxide or similar compound. Metal hydroxides often precipitate under such conditions.
2Step 2: Analyze the Behavior with \\(\mathrm{NH}_{4} \mathrm{Cl} / \mathrm{NH}_{4} \mathrm{OH}\\)
Upon adding \(\mathrm{NH}_{4} \mathrm{Cl} / \mathrm{NH}_{4} \mathrm{OH}\), one part of the precipitate dissolves, leaving a white gelatinous precipitate. This behavior suggests that the precipitate may be an amphoteric hydroxide, as amphoteric substances can dissolve in weak base solutions like \(\mathrm{NH}_{4} \mathrm{OH}\).
Key Concepts
Amphoteric HydroxidesMetal HydroxidesAmmonium Hydroxide Reaction
Amphoteric Hydroxides
In chemical reactions, some substances can act as either acids or bases, depending on the environment. These are known as amphoteric substances. A classic example of an amphoteric hydroxide is zinc hydroxide, represented by \( \mathrm{Zn} (\mathrm{OH})_2 \). Amphoteric hydroxides dissolve in a weakly acidic or basic medium, making them versatile in neutralizing reactions.
For instance, in the presence of \( \mathrm{NH}_4 \mathrm{OH} \), an amphoteric hydroxide like zinc hydroxide can dissolve, forming a complex ion that transitions the solid into solution form. This characteristic is critical in separating amphoteric hydroxides from others that form precipitates under similar conditions.
Understanding the behavior of amphoteric hydroxides helps differentiate them based on their reactivity with different compounds.
For instance, in the presence of \( \mathrm{NH}_4 \mathrm{OH} \), an amphoteric hydroxide like zinc hydroxide can dissolve, forming a complex ion that transitions the solid into solution form. This characteristic is critical in separating amphoteric hydroxides from others that form precipitates under similar conditions.
Understanding the behavior of amphoteric hydroxides helps differentiate them based on their reactivity with different compounds.
Metal Hydroxides
Metal hydroxides are formed when metal ions react with hydroxide ions in solution. They usually form as precipitates—solid particles that separate from the solution upon reaction. The solubility of metal hydroxides in water can vary. Some are insoluble and readily precipitate out of solution when formed, like aluminum hydroxide, \( \mathrm{Al} (\mathrm{OH})_3 \), which forms a gelatinous white precipitate.
Upon heating or within specific alkaline conditions, some metal hydroxides may dissolve or alter their forms. Differentiating between metal hydroxides can involve their reaction behavior with other compounds, like acids that can break down or dissolve specific hydroxide forms. Recognizing these behaviors is crucial in predicting the outcomes in chemical experiments and industrial processes.
Upon heating or within specific alkaline conditions, some metal hydroxides may dissolve or alter their forms. Differentiating between metal hydroxides can involve their reaction behavior with other compounds, like acids that can break down or dissolve specific hydroxide forms. Recognizing these behaviors is crucial in predicting the outcomes in chemical experiments and industrial processes.
Ammonium Hydroxide Reaction
Ammonium hydroxide, \( \mathrm{NH}_4 \mathrm{OH} \), is a solution of ammonia in water, functioning as a weak base. When added to solutions containing certain metal hydroxides, it can induce specific reactions. For example, ammonium hydroxide can cause some metal hydroxides' precipitates to dissolve, as seen in reactions involving amphoteric hydroxides.
In the context of the exercise, adding \( \mathrm{NH}_4 \mathrm{OH} \) to a solution with precipitated substances allows us to determine the solubility and potential amphoteric nature of the hydroxides present. This reaction is used in chemical analyses to narrow down potential compounds by observing which ones dissolve upon treatment with ammonium hydroxide.
In the context of the exercise, adding \( \mathrm{NH}_4 \mathrm{OH} \) to a solution with precipitated substances allows us to determine the solubility and potential amphoteric nature of the hydroxides present. This reaction is used in chemical analyses to narrow down potential compounds by observing which ones dissolve upon treatment with ammonium hydroxide.
- Proton transfer plays a significant role in these reactions, altering the charge and solubility of compounds.
- Reaction with ammonium chloride enhances this by increasing the medium’s ionic strength, manipulating the equilibrium state of the compound in solution.
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