`HgS darr+Na_(2)S hArrNa_(2)[HgS_(2)]`

Options

A. For precipitate formation reaction
B. For precipitate dissolution reaction
C. For precipitate exchange reaction
D. For no reaction

Answer

To solve the given reaction \( \text{HgS} + \text{Na}_2\text{S} \rightarrow \text{Na}_2[\text{HgS}_2] \), we will analyze the nature of the reaction step by step.

### Step 1: Identify the Reactants and Products
- The reactants are mercury sulfide (HgS) and sodium sulfide (Na₂S).
- The product formed is sodium mercuric sulfide (Na₂[HgS₂]).

### Step 2: Determine the Type of Reaction
- We need to classify the reaction. The options given are:
1. Precipitate formation reaction
2. Precipitate dissolution reaction
3. Precipitate exchange reaction
4. No reaction

### Step 3: Analyze the Formation of the Product
- In this reaction, HgS (which is a precipitate) reacts with Na₂S to form Na₂[HgS₂].
- The product Na₂[HgS₂] is a complex compound that can lead to the precipitation of mercury sulfide (HgS) again under certain conditions.

### Step 4: Understand the Behavior of the Compounds
- HgS is known as cinnabar and is not soluble in water.
- The formation of Na₂[HgS₂] indicates a reaction where the concentration of free sodium sulfide decreases, which can lead to the precipitation of HgS from the solution.

### Step 5: Conclude the Type of Reaction
- Given that the reaction involves the dissolution of a precipitate (HgS) and the formation of a new compound (Na₂[HgS₂]), it can be classified as a **precipitate dissolution reaction**.
- Therefore, the correct option is **Option 2: Precipitate dissolution reaction**.

### Final Answer
The reaction \( \text{HgS} + \text{Na}_2\text{S} \rightarrow \text{Na}_2[\text{HgS}_2] \) is classified as a **precipitate dissolution reaction**.

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