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  4. Assertion: Cuprous ion (Cu+ ) is colourless whereas cupric ion (Cu++ ) is blue in the aqueous solution. Reason: Cuprous ion (Cu+ ) has unpaired electrons while cupric ion (Cu++ ) does not.

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Q. Assertion : Cuprous ion $(Cu^+ )$ is colourless whereas cupric ion $(Cu^{++} )$ is blue in the aqueous solution.
Reason : Cuprous ion $(Cu^+ )$ has unpaired electrons while cupric ion $(Cu^{++} )$ does not.

AIIMSAIIMS 2002The d-and f-Block Elements

Solution:

Cuprous ions $\left( Cu ^{+}\right)$ is colourless because it has no unpaired electrons in the $3d$ sub-shell as it has $3 d ^{10}$ configuration. The cupric ion $\left( Cu ^{++}\right)$ has one unpaired electron as it has configuration $3 d^{9}$. By absorbing the visible light, the unpaired electron can migrate from one set of $d$- orbitals to another under the applied field of water molecules. Hence, cupric ion $\left( Cu ^{++}\right)$ in aqueous solution shows blue colour.