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Q.
Cuprous ion is colourless while cupric ion is coloured because,
The d-and f-Block Elements
Solution:
$Cu ^{2+}$ forms $\left[ Cu \left( H _{2} O \right)_{6}\right]^{2+}$ complexes. If white light (ordinary sunlight, for example) passes through copper(II), some wavelengths in the light are absorbed by the solution. Copper(II) ions in the solution absorb light in the red region of the spectrum. Hence, it looks blue. The origin of this is d-orbitals being split by the water acting as strong ligands. $Cu ^{+}$being less charged has a smaller ligand field effect and the transition is in the infrared and does not change our human perceived color.