Question

What is the colour of $CuS{O_4}.5{H_2}O$?
A) Blue
B) Black
C) Green
D) Brown

Answer 1

As we know that copper sulphate which is an inorganic compound having molecular formula given as $CuS{O_4}{({H_2}O)_x}$ where number of water molecules can vary from $0 - 5$. We also know that nascent copper is present in blue colour and anhydrous salts basically removes the water molecules from the compound.

Complete step by step answer:
As we know that copper is present in blue colour crystals and the compound $CuS{O_4}.5{H_2}O$ is commonly known as blue vitriol or bluestone which basically corresponds to its blue colour. $CuS{O_4}.5{H_2}O$ is a hydrate and metal sulphate where five water molecules acting as ligand are present in blue colour that causes splitting of d-orbitals of copper which basically facilitates the d-d transition of the compound thereby imparting a blue colour.
On decomposition, copper sulphate pentahydrate, loses two water molecules when it is subjected to heating at ${63^\circ }C$ and three more water molecules are lost on further decomposition which results in the white colour of the solution of copper sulphate called anhydrous $CuS{O_4}$, this is due to the absence of water molecules acting as a ligand which makes it impossible for splitting the d-orbitals and thus no d-d transition takes place.
Iron is present in green colour in various inorganic compounds, boron is present in black colour and titanium is present in brown colour in their corresponding compounds.

Hence the correct answer will be Option B.

Note: Always remember that when an electron from a lower energy of d-orbital is excited to higher energy d-orbital, the energy of excitation corresponds to the frequency of light absorbed and this frequency of light is determined by the nature of ligand, size of metal ion and the oxidation state of metal.