Question

Which of the following is blue in colour?
(A) Aqueous $CuS{O_4}$
(B) $ZnS{O_4}$
(C) $FeS{O_4}$
(D) ${(Al)_2}S{O_4}$

Answer 1

Hint : Chemical colour is a physical feature that results from the excitation of electrons caused by the chemical's energy absorption. The complementary colour resulting from the elimination of the absorbed wavelengths is what is perceived by the eye, not the colour absorbed. Atomic spectroscopy was the first to notice this spectral view.

Complete Step By Step Answer:
All atoms and molecules may absorb and release energy in the form of photons, which is followed by a quantum state transition. The difference between the energies of the two quantum states is the amount of energy received or released. There are many other kinds of quantum states, such as the rotational and vibrational states of a molecule. However, visible light, or energy released visible to the human eye, has wavelengths ranging from 380 nm to 760 nm, depending on the person, and photons in this range are frequently associated with a change in atomic or molecular orbital quantum state. Simple inorganic (e.g. sodium chloride) and organic (e.g. ethanol) substances are almost all colourless. Transition metal complexes are often coloured due to electron transitions between d-orbitals of varying energies. When there is substantial conjugation in organic molecules, the energy gap between the HOMO and LUMO decreases, moving the absorption band from the UV to the visible range. Similarly, when an electron moves from the HOMO to the LUMO, the energy absorbed by the molecule is responsible for colour.
$CuS{O_4}$ is a kind of copper salt. It is blue in hydrated form (aqueous) and white in anhydrous form. ${(Al)_2}S{O_4}$ has no colour. Zinc sulphate $ZnS{O_4}$ is white in colour. $FeS{O_4}$ is an acronym for ferrous sulphate which is a greenish blue colour.
Hence option A is colour.

Note :
Atomic orbital, molecular orbital, Ligand Field Theory, and Crystal Field Theory all deal with the connections between the energies of distinct quantum states. When photons of a certain wavelength are absorbed by matter, we perceive the complementary hue, which is made up of the remaining visible wavelengths, when light is reflected from or transmitted through that matter. Because beta-carotene absorbs the most light at 454 nm (blue light), the remaining visible light looks orange.