Write two examples of double salts

Two common double salts are Mohr’s salt (FeSO₄·(NH₄)₂SO₄·6H₂O) and Alum (KAl(SO₄)₂·12H₂O).

Double salts are crystalline salts that contain two different simple salts combined in a fixed stoichiometric ratio and water of crystallization. They form one crystal lattice but, when dissolved in water, they dissociate into their simple ions and do not retain the complex structure. For example, alum gives K⁺, Al³⁺, and SO₄²⁻ ions in solution.

This distinguishes double salts from complex salts, which continue to behave as single complex ions in solution. Double salts are used in water purification, dyeing, and analytical chemistry. Mohr’s salt is useful in redox titrations because it is more stable than FeSO₄ alone.

Other examples

• Carnallite: KMgCl₃·6H₂O
• Potash alum: KAl(SO₄)₂·12H₂O
• Tutton’s salts: M₂M′(SO₄)₂·6H₂O family