Question:
Which one of the following cations gives a chocolate brown precipitate upon addition of aqueous solution of \(\text{K}_4[\text{Fe}(\text{CN})_6]\)?
Which one of the following cations gives a chocolate brown precipitate upon addition of aqueous solution of \(\text{K}_4[\text{Fe}(\text{CN})_6]\)?
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Potassium ferrocyanide is a classic analytical reagent used to identify transition metal ions:
- $\text{Cu}^{2+}$ gives a chocolate brown precipitate.
- $\text{Fe}^{3+}$ gives a Prussian blue precipitate.
- $\text{Zn}^{2+}$ gives a white precipitate.
Remembering these distinct colors is highly beneficial for qualitative analysis questions.
- $\text{Cu}^{2+}$ gives a chocolate brown precipitate.
- $\text{Fe}^{3+}$ gives a Prussian blue precipitate.
- $\text{Zn}^{2+}$ gives a white precipitate.
Remembering these distinct colors is highly beneficial for qualitative analysis questions.
Updated On: May 28, 2026
- $\text{Fe}^{3+}$
- $\text{Cu}^{2+}$
- $\text{Zn}^{2+}$
- $\text{Ca}^{2+}$
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The Correct Option is B
Solution and Explanation
Step 1: Understanding the Question:
The question asks us to identify the metal cation that forms a characteristic chocolate brown precipitate when it reacts with an aqueous solution of potassium ferrocyanide, $\text{K}_4[\text{Fe}(\text{CN})_6]$.
Step 2: Detailed Explanation:
When copper(II) ions ($\text{Cu}^{2+}$) react with potassium ferrocyanide, a characteristic chocolate-brown precipitate of copper ferrocyanide is formed.
The chemical equation for this reaction is given by:
\[ 2\text{Cu}^{2+}(\text{aq}) + [\text{Fe}(\text{CN})_6]^{4-}(\text{aq}) \rightarrow \text{Cu}_2[\text{Fe}(\text{CN})_6]\downarrow \text{ (Chocolate brown precipitate)} \]
Let us look at the other cations for comparison:
- $\text{Fe}^{3+}$ reacts with ferrocyanide to form a deep blue precipitate (Prussian blue), $\text{Fe}_4[\text{Fe}(\text{CN})_6]_3$.
- $\text{Zn}^{2+}$ reacts to form a white or grayish-white precipitate of zinc ferrocyanide, $\text{Zn}_2[\text{Fe}(\text{CN})_6]$.
- $\text{Ca}^{2+}$ does not form a chocolate brown precipitate with this reagent.
Thus, $\text{Cu}^{2+}$ is the correct cation.
Step 3: Final Answer:
The correct option is (B).
The question asks us to identify the metal cation that forms a characteristic chocolate brown precipitate when it reacts with an aqueous solution of potassium ferrocyanide, $\text{K}_4[\text{Fe}(\text{CN})_6]$.
Step 2: Detailed Explanation:
When copper(II) ions ($\text{Cu}^{2+}$) react with potassium ferrocyanide, a characteristic chocolate-brown precipitate of copper ferrocyanide is formed.
The chemical equation for this reaction is given by:
\[ 2\text{Cu}^{2+}(\text{aq}) + [\text{Fe}(\text{CN})_6]^{4-}(\text{aq}) \rightarrow \text{Cu}_2[\text{Fe}(\text{CN})_6]\downarrow \text{ (Chocolate brown precipitate)} \]
Let us look at the other cations for comparison:
- $\text{Fe}^{3+}$ reacts with ferrocyanide to form a deep blue precipitate (Prussian blue), $\text{Fe}_4[\text{Fe}(\text{CN})_6]_3$.
- $\text{Zn}^{2+}$ reacts to form a white or grayish-white precipitate of zinc ferrocyanide, $\text{Zn}_2[\text{Fe}(\text{CN})_6]$.
- $\text{Ca}^{2+}$ does not form a chocolate brown precipitate with this reagent.
Thus, $\text{Cu}^{2+}$ is the correct cation.
Step 3: Final Answer:
The correct option is (B).
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