What is the colour of anhydrous CuSO₄ ?
White
Blue
Green
Yellow
Answer is Wrong!
Answer is Right!
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UPSC Geoscientist – 2024
When copper(II) sulfate pentahydrate is heated, it loses its water of crystallization. The blue pentahydrate first turns into pale blue CuSO₄·3H₂O, then blue-green CuSO₄·H₂O, and finally becomes anhydrous copper(II) sulfate (CuSO₄), which is a white powder. In the anhydrous state, the crystal structure changes, and the Cu²⁺ ion is no longer surrounded by water ligands in the same way. This alters the energy levels of the d orbitals, and the absorption characteristics change such that no visible light is strongly absorbed, causing the compound to appear white.