Choose the correct statement from the following:

$KAl(SO_4)_2.12H_2O$, is a double salt and dissociate into simple ions completely when dissolved in water

The correct answer is Option (4) → $KAl(SO_4)_2.12H_2O$, is a double salt and dissociate into simple ions completely when dissolved in water

Option 1

K₄[Fe(CN)₆] is a double salt formed by the combination of Fe²⁺ and CN⁻ ions in stoichiometric ratio.

This statement is incorrect.
K₄[Fe(CN)₆] is a coordination compound, not a double salt. In this compound, Fe²⁺ forms a complex ion [Fe(CN)₆]⁴⁻ with six CN⁻ ligands through coordinate bonds. Therefore it is a complex salt, not a double salt.

Option 2

K₄[Fe(CN)₆] is a double salt and dissociates into simple Fe²⁺ and CN⁻ ions completely when dissolved in water.

This statement is incorrect.
K₄[Fe(CN)₆] dissociates in water as:

K₄[Fe(CN)₆] → 4K⁺ + [Fe(CN)₆]⁴⁻

The complex ion remains intact and does not break into Fe²⁺ and CN⁻ ions. This behavior is typical of coordination compounds, not double salts.

Option 3

KAl(SO₄)₂·12H₂O is a coordination complex in which SO₄²⁻ ligand is coordinated to Al.

This statement is incorrect.
KAl(SO₄)₂·12H₂O (potash alum) is not a coordination complex. It is a double salt formed from K₂SO₄ and Al₂(SO₄)₃. In this compound, sulfate ions are not acting as ligands bonded directly to Al³⁺ through coordinate bonds.

Option 4

KAl(SO₄)₂·12H₂O is a double salt and dissociates into simple ions completely when dissolved in water.

This statement is correct.
Double salts dissociate completely into their constituent ions in aqueous solution.

KAl(SO₄)₂·12H₂O → K⁺ + Al³⁺ + 2SO₄²⁻ + 12H₂O

Thus the ions behave independently in solution, which is the characteristic property of double salts.