Choose the correct statement from the following:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(KAl(SO_4)_2.12H_2O\), is a double salt and dissociates into simple ions completely when dissolved in water
A double salt dissociates completely into simple ions in water; a complex ion stays intact.
Potash alum is a double salt (\( \ce{K2SO4.Al2(SO4)3.24H2O} \)); in water it gives only simple ions:
\( \displaystyle \ce{KAl(SO4)2.12H2O -> K^+ + Al^3+ + 2SO4^2- + 12H2O} \)
Potassium ferrocyanide is a complex; it gives potassium ions and the complex ion, but no free \( \ce{Fe^2+} \) or \( \ce{CN^-} \):
\( \displaystyle \ce{K4[Fe(CN)6] -> 4K^+ + [Fe(CN)6]^4-} \)
So the correct statement is Option (4).
Why the other options are incorrect
Option (1) \(K_4[Fe(CN)_6]\) is a double salt formed by the combination of \(Fe^{2+}\) and \(CN^-\) ions in stoichiometric ratio — Incorrect. It is a coordination compound: six cyanide ions are bonded to \( \ce{Fe^2+} \) in the complex ion \( \ce{[Fe(CN)6]^4-} \).
Option (2) \(K_4[Fe(CN)_6]\) is a double salt and dissociates into simple \(Fe^{2+}\) and \(CN^-\) ions completely when dissolved in water — Incorrect. The complex ion stays intact in water, so the solution gives no test for \( \ce{Fe^2+} \) or \( \ce{CN^-} \).
Option (3) \(KAl(SO_4)_2.12H_2O\), is a coordination complex in which \({SO_4}^{2-}\) ligand is coordinated to Al — Incorrect. Alum is a double salt; the sulphate ions are free ions in its solution, not ligands bonded to aluminium.
NCERT (2026-27 edition):
Difference between a Double Salt and a Complex
