Read the passage carefully and answer the Questions.

Werner in 1898, propounded his theory of coordination compounds. In coordination compounds metals show two types of linkages (valences)-primary and secondary. The primary valences are normally ionisable and are satisfied by negative ions. The secondary valences are non-ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal. The ions or group bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to the different coordination numbers. Both double salts as well as complexes are formed by the combination of two or more stable compounds in stoichiometric ratio. However, they differ in the fact that double salts such as carnallite, Mohr's salt, potash alum etc. dissociate into simple ions completely when dissolved in water. But complex ions do not dissociate completely into simple ions.

Which of the following is a double salt?

$KCl. MgCl_2.6H_2O$

The correct answer is Option (1) → $KCl. MgCl_2.6H_2O$

A double salt is a crystalline salt made by the combination of two simple salts in a stoichiometric ratio, and it dissociates completely into its constituent ions in water.
Examples: Mohr’s salt, potash alum, carnallite, etc.

Let’s examine each option:

Option A: KCl·MgCl₂·6H₂O

This is Carnallite, a classic example of a double salt.
It dissociates completely into K⁺, Mg²⁺, and Cl⁻ ions in solution.
✅ Double salt

Option B: [CrCl₂(H₂O)₄]Cl·2H₂O

This contains a complex ion $[CrCl_2(H_2O)_4]^+$.
❌ It is a coordination compound, not a double salt.

Option C: [FeCl(H₂O)₅]Cl₂·H₂O

Contains a complex ion $[FeCl(H_2O)_5]^{2+}$.
❌ Also a coordination compound.

Option D: [Ag(NH₃)₂][Ag(CN)₂]

Contains coordination complexes.
❌ Not a double salt.