Calomel (Hg2Cl2) on reaction with ammonium hydroxide gives
(a) HgO
(b) Hg2O
(c) NH2—Hg—HgCl
(d) HgNH

Q: Why does the precipitate turn black when ammonia is added?

The black color is due to the formation of finely divided elemental mercury ($\text{Hg}$) during the disproportionation reaction of the Mercury(I) ions.

Q: What is the oxidation state of Hg in Calomel?

In Calomel ($\text{Hg}_2\text{Cl}_2$), mercury is in the $+1$ oxidation state. It uniquely exists as a diatomic cation, $\text{Hg}_2^{2+}$.

Q: Is $\text{HgNH}_2\text{Cl}$ soluble in water?

No, $\text{HgNH}_2\text{Cl}$ (mercury(II) amido chloride) is an insoluble white precipitate, often referred to as 'infusible white precipitate'.

Q: What exactly is a disproportionation reaction?

It is a specific type of redox reaction where a single substance is simultaneously oxidized and reduced. Here, $\text{Hg}_2^{2+}$ ($+1$) goes to $\text{Hg}^{2+}$ ($+2$, oxidation) and $\text{Hg}^0$ ($0$, reduction).

Q: Can we use NaOH instead of NH4OH to get the same result?

No. Using a strong base like NaOH would yield a different product, specifically black Mercury(I) oxide ($\text{Hg}_2\text{O}$), which further decomposes to $\text{HgO}$ and $\text{Hg}$. Ammonia is required to form the specific amido complex.

Question

Calomel ($	ext{Hg}_2	ext{Cl}_2$) on reaction with ammonium hydroxide gives

Options:

(A) $	ext{HgO}$

(B) $	ext{Hg}_2	ext{O}$

(C) $	ext{NH}_2	ext{—Hg—HgCl}$

(D) $	ext{HgNH}_2	ext{Cl}$

Sigma Percentile · Accepted Answer

### The Fascinating Chemistry of Calomel

Qualitative analysis is full of colorful and dramatic chemical transformations, and the reaction of Calomel with ammonia is one of the most classic examples. Calomel, chemically known as Mercury(I) chloride or $	ext{Hg}_2	ext{Cl}_2$, is a white, insoluble solid. When it encounters a solution of ammonium hydroxide ($	ext{NH}_4	ext{OH}$), a fascinating sequence of events unfolds.

### The Disproportionation Mechanism

The core of this reaction lies in the behavior of the Mercury(I) ion, $	ext{Hg}_2^{2+}$. In this diatomic ion, mercury is in the $+1$ oxidation state. However, this state is relatively unstable in the presence of certain ligands like ammonia.

When ammonia is added, the $	ext{Hg}_2^{2+}$ ion undergoes a **disproportionation reaction**. This means it acts as both an oxidizing and a reducing agent simultaneously. It splits into two different oxidation states:
1. It oxidizes to Mercury(II), $	ext{Hg}^{2+}$ (oxidation state $+2$).
2. It reduces to elemental Mercury, $	ext{Hg}^0$ (oxidation state $0$).

### The Visual Transformation

As the disproportionation occurs, the newly formed $	ext{Hg}^{2+}$ ions immediately react with the ammonia and chloride ions present in the solution. This forms **Mercury(II) amido chloride**, $	ext{Hg(NH}_2	ext{)Cl}$, which is an infusible **white precipitate**.

Simultaneously, the elemental mercury ($	ext{Hg}^0$) precipitates out as a very finely divided **black powder**.

When you mix a white precipitate with a black powder, what do you get? The entire mixture turns a distinct **grayish-black** color. This dramatic color change from pure white calomel to a dark gray/black mixture is the confirmatory test for the presence of Mercury(I) ions in qualitative salt analysis.

### The Master Equation

The complete balanced chemical equation for this beautiful transformation is:

$$	ext{Hg}_2	ext{Cl}_2 + 2	ext{NH}_4	ext{OH} \longrightarrow 	ext{Hg(NH}_2	ext{)Cl} \downarrow + 	ext{Hg} \downarrow + 	ext{NH}_4	ext{Cl} + 2	ext{H}_2	ext{O}$$

Looking at our options, the compound $	ext{HgNH}_2	ext{Cl}$ perfectly matches the white precipitate formed in this reaction, making it the correct answer.

Animated Solution for Chemistry - d and f-Block Elements: Calomel ( $Hg_{2} Cl_{2}$ ) on reaction with ammonium hydroxide gives

Select Answer:

Visualized Solution

\text{The Reaction Setup}

\text{Disproportionation of } \text{Hg}_2^{2+}

\text{Formation of Precipitates}

\text{Final Conclusion}

The Sigma Insight: d-block Elements

The Fascinating Chemistry of Calomel

The Disproportionation Mechanism

The Visual Transformation

The Master Equation

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Animated Solution for Chemistry - d and f-Block Elements: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Calomel (Hg2​Cl2​) on reaction with ammonium hydroxide gives