Read the passage carefully and answer the Questions.
Werner in 1898, propounded his theory of coordination compounds. In coordination compounds metals show two types of linkages (valences)-primary and secondary. The primary valences are normally ionisable and are satisfied by negative ions. The secondary valences are non-ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal. The ions or group bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to the different coordination numbers. Both double salts as well as complexes are formed by the combination of two or more stable compounds in stoichiometric ratio. However, they differ in the fact that double salts such as carnallite, Mohr's salt, potash alum etc. dissociate into simple ions completely when dissolved in water. But complex ions do not dissociate completely into simple ions.
Which of the following is a double salt?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(KCl. MgCl_2.6H_2O\)
Carnallite is a double salt of potassium chloride and magnesium chloride. In water it gives only simple ions:
\( \displaystyle \ce{KCl.MgCl2.6H2O} \xrightarrow{\text{water}} \ce{K^+(aq) + Mg^2+(aq) + 3Cl^-(aq)} \)
So \( \ce{KCl.MgCl2.6H2O} \) is the double salt.
Why the other options are incorrect
Option (2) \([CrCl_2(H_2O)_4]Cl.2H_2O\) — Incorrect. This is a coordination compound: in water it gives the complex cation \( \ce{[CrCl2(H2O)4]^+} \) and one \( \ce{Cl^-} \); the two chlorides inside the bracket stay bound to chromium.
Option (3) \([FeCl(H_2O)_5]Cl_2.H_2O\) — Incorrect. This is also a coordination compound; the complex ion \( \ce{[FeCl(H2O)5]^2+} \) stays intact in solution.
Option (4) \([Ag(NH_3)_2][Ag(CN)_2]\) — Incorrect. Both ions are complex ions, the diamminesilver(I) cation and the dicyanidoargentate(I) anion, so the compound gives no simple \( \ce{Ag^+} \) ions.
NCERT (2026-27 edition):
Difference between a Double Salt and a Complex
