Question

Colour change of blue vitriol on heating is a:
A. chemical property of vitriol
B. physical property of vitriol
C. cannot be predicted
D. none of these

Answer 1

Think about the formula of blue vitriol and the properties of it which give the compound a blue colour. Consider the property that will change when the compound is heated and answer the question accordingly.

Complete solution:
-We know that blue vitriol is the common name for hydrated copper sulphate. The formula of this compound is $CuS{{O}_{4}}\cdot 5{{H}_{2}}O$. Here, we can see that the compound has a water of hydration. This water of hydration is an integral part of the crystalline structure of any given compound. It is this water of hydration that gives the crystals their blue colour.
-When the crystals are heated, the copper sulphate loses its blue crystalline structure and instead turns into a white amorphous powder. This amorphous powder has different chemical properties than hydrated copper sulphate. The water molecules are suspended within the crystal structures and form bonds with them. These bonds are broken when the crystal is heated and thus, this is a chemical change. The reaction of this process is as follows:
$CuS{{O}_{4}}\cdot 5{{H}_{2}}O\xrightarrow{\Delta }CuS{{O}_{4}}+5{{H}_{2}}O$
These five molecules of water evaporate immediately due to the heat that is provided and we are left with a white powder.

Hence, the answer to this question is (A), 'chemical property of vitriol’

Note: Although this might seem like a physical change since just the colour and structure of the compound is changing at first glance and all the bonds are remaining intact, that is not the case. When the crystals are heated, the bond between the water molecules and the atoms in the crystal do get broken and the properties of both the reactants and the products are also different. Remember that this change is not easily reversible either. Just adding water to the amorphous powder does give the blue colour back, but the compound does not regain its crystalline character.