Which of the following is a double salt?

Options

A. Carnalite
B. Potassium ferrocyanide
C. Potasium ferricyanide
D. Nessler's reagent

Explanation

To determine which of the given options is a double salt, we need to understand what a double salt is and analyze each option accordingly.

### Step-by-Step Solution:

1. **Definition of Double Salt**:
A double salt is formed by the combination of two different salt compounds that crystallize together. When dissolved in water, a double salt will completely dissociate into its constituent ions.

2. **Analyze the Options**:
- **Option 1: Carnalite (KCl·MgCl2·6H2O)**
- Carnalite is composed of potassium chloride (KCl) and magnesium chloride (MgCl2), along with water of crystallization. Since it is formed from two different salts, it qualifies as a double salt.

- **Option 2: Potassium Ferrocyanide (K4[Fe(CN)6])**
- This compound contains a central iron atom surrounded by cyanide ligands. It does not dissociate into simple ions in solution but rather remains as a complex ion. Therefore, it is classified as a complex salt, not a double salt.

- **Option 3: Potassium Ferricyanide (K3[Fe(CN)6])**
- Similar to potassium ferrocyanide, this compound has a central iron atom with cyanide ligands. It also does not dissociate into simple ions in solution, making it a complex salt.

- **Option 4: Nestler's Reagent (K2[HgI4])**
- Nestler's reagent contains mercury and iodide ions, forming a complex structure. Like the previous two, it does not dissociate into simple ions and is classified as a complex salt.

3. **Conclusion**:
From the analysis, the only compound that qualifies as a double salt is **Carnalite (KCl·MgCl2·6H2O)**.

### Final Answer:
The correct option is **Carnalite (KCl·MgCl2·6H2O)**, which is a double salt.

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