Which one among the following is a double salt?

A double salt is a substance formed by the combination of two different salts that crystallize together in a fixed ratio but dissociate into their constituent ions when dissolved in water. Option 2, K2SO4·Al2(SO4)3·24H2O, commonly known as potash alum, is a classic example of a double salt consisting of potassium sulphate and aluminium sulphate [1]. Unlike complex salts like potassium ferrocyanide (K4[Fe(CN)6]), which retain their complex ion structure in solution [1], double salts lose their identity in aqueous form. Option 3, CuSO4·5H2O (copper sulphate pentahydrate), is a simple hydrated salt, and Option 4, NaCl (sodium chloride), is a simple salt [3]. Potash alum belongs to the 'alum' family of double sulphates, which are typically isomorphous and follow the general formula of M2SO4·M'2(SO4)3·24H2O.

Sources

1. [1] https://uou.ac.in/sites/default/files/slm/BSCCH-204.pdf
2. [3] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures > 8.3.2 Compounds > p. 124