Chemical properties and color changes of copper sulfate

The systematic name for CuSO4 is copper(II) sulfate, but it is also referred to as blue vitriol, Roman vitriol, the vitriol of copper, and bluestone. Copper sulphate is an inorganic salt generated by the reaction of sulphuric acid and copper. This inorganic substance is a crystalline, odorless powder that absorbs water well. Anhydrous copper (II) sulfate is white and only turns blue when it comes into contact with water molecules. Copper sulphate pentahydrate is the most prevalent form, with the chemical formula CuSO4.5H2O. This type is distinguished by its intense blue color.

The CuSO4 molecule is an ionic bond between the copper cation (Cu2+) and the sulfate anion (SO42-). Copper sulfate can be prepared by treating metallic copper with heated and concentrated sulphuric acid or by treating the oxides of copper with dilute sulphuric acid. It can be noted that the oxidation state exhibited by the copper atom in a CuSO4 molecule is +2.

The anhydrous and pentahydrate forms of copper sulphate have molar masses of 159.609 and 249.685 grams per mole, respectively. The densities of the anhydrous and pentahydrate forms are 3.6 g/cm3 and 2.286 g/cm3, respectively. Copper sulphates, both hydrated and anhydrous, decompose when heated. Hence, their boiling points aren’t exact. CuSO4.5H2O crystals contain orthorhombic crystal structures, whereas anhydrous CuSO4 crystals have triclinic structures. Copper sulfate is highly soluble in water, with solubility values of 1.055 molal and 1.502 molal at 10 ℃ and 30 ℃, respectively.

So, why is anhydrous copper sulphate white and pentahydrate blue?

In hydrated CuSO4, the water molecules surrounding the Central Metal (Cu) act as ligands, resulting in a d-d transition and, therefore, emitting a blue color in the visible region, due to which hydrated CuSO4 appears blue. Since anhydrous CuSO4 does not hold any water of crystallization, It retains its white color.