Transition metals form coloured ions or compounds due to the partially filled d-orbitals. In the presence of solvent molecules (in solutions) or ligands (in complexes) or counter ions (in crystals), the d-orbitals split into two sets. The electrons in transition metal ions which occupy one set of d-orbitals having lower energy can be excited to another set of d-orbitals having high energy by absorbing energy from visible light. Since the energy difference \((\Delta E)\) is small between the two sets of d-orbitals, the light in the visible region only is absorbed by the electron during its excitation. The colour of the transition metal ion is due to d–d excitation or d–d transition of the electron. During d–d excitation the electron absorb one colour in the visible light and thus it appears in the complimentary colour of the absorbed light. The complimentary colours can be identified using Munsell colour wheel (as depicted below).

The number of electrons undergoing d–d transition and the energy difference between the two sets of orbitals decide the colour. The colour of particular transition metal ion, e.g., copper ion which is blue in aqueous solution changes to dark blue in the presence of sufficient ammonia and to green if sufficient chloride ions are added. \(\Delta\)E value depends on the nature of metal ion, the nature of ligands and several other factors. Some metal ions exhibit different colours in different oxidation states.

The hydrated copper ion appears blue because it absorbs the light of

orange red colour

The correct answer is option 3. orange red colour.

The hydrated copper ion (Cu²⁺) appears blue because it absorbs light in the orange-red region of the spectrum.

The blue color observed in hydrated copper ions (Cu²⁺) is due to the absorption of light in the visible region of the electromagnetic spectrum. When white light (which contains all the colors of the spectrum) passes through a solution containing hydrated copper ions, certain wavelengths of light are absorbed by the Cu²⁺ ions. In the case of hydrated copper ions, the absorption spectrum shows strong absorption in the orange-red region of the spectrum. The remaining light that is transmitted through the solution appears blue to our eyes because it consists of wavelengths that are not absorbed by the Cu²⁺ ions. Therefore, the hydrated copper ion appears blue because it selectively absorbs light in the orange-red region of the spectrum, leaving the transmitted light to be perceived as blue.