Assertion (A): Compounds which have the same composition but differ in the number of water molecules present as ligands and as molecules of hydration are called hydrate isomers.
Reason (R): Hydrate isomers such as [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2·H2O give different ions in aqueous solution.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Hydrate isomerism is solvate isomerism with water as the solvent.
Assertion (A): Compounds which have the same composition but differ in the number of water molecules present as ligands and as molecules of hydration are called hydrate isomers. — True. This is the definition: the formula is the same, only the place of the water (coordination sphere or lattice) differs.
Reason (R): Hydrate isomers such as [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2·H2O give different ions in aqueous solution. — True. The bracketed entity does not dissociate, so only the ions outside the bracket are set free:
\( \ce{[Cr(H2O)6]Cl3 -> [Cr(H2O)6]^3+ + 3Cl^-} \): the complex cation \( \ce{[Cr(H2O)6]^3+} \) and three chloride ions.
\( \ce{[Cr(H2O)5Cl]Cl2.H2O -> [Cr(H2O)5Cl]^2+ + 2Cl^- + H2O} \): the complex cation \( \ce{[Cr(H2O)5Cl]^2+} \) and two chloride ions.
Why Reason (R) does not explain Assertion (A)?
Reason (R) states only a consequence of where the water sits: the ions the two compounds give in solution. It does not say why such compounds are called hydrate isomers.
Assertion (A) is a definition: hydrate isomers are named by where the water sits, as a ligand or in the lattice.
NCERT (2026-27 edition):
5.4.6 Solvate Isomerism – “This form of isomerism is known as ‘hydrate isomerism’ in case where water is involved as a solvent. This is similar to ionisation isomerism. Solvate isomers differ by whether or not a solvent molecule is directly bonded to the metal ion or merely present as free solvent molecules in the crystal lattice. An example is provided by the aqua complex [Cr(H2O)6]Cl3 (violet) and its solvate isomer [Cr(H2O)5Cl]Cl2.H2O (grey-green).”
5.1 Werner’s Theory of Coordination Compounds – “… the atoms within the square brackets form a single entity which does not dissociate under the reaction conditions.”