A double salt contains two or more different cations or anions in a definite stoichiometric ratio, forming a crystalline compound that dissociates into its constituent ions in solution. Unlike complex salts, double salts do not retain complex ions in solution.
Step 1: Define double salts
Double salts ionize completely in water into individual ions (e.g., ), unlike coordination compounds with complex ions (e.g., ).
Step 2: Analyze options
- (A) : Carnallite, a double salt containing , , and ions, with water of crystallization. Ionizes as .
- (B) }: Mohr’s salt, a double salt with , , and ions. Ionizes completely.
- (C) }: Potash alum, a double salt with , , and ions. Ionizes as individual ions.
Step 3: Conclusion
All are double salts, matching option (D).