Question

On heating, the colour of zinc oxide changes from white to yellow. This is a/an:
a.) physical change
b.) chemical change
c.) photochemical change
d.) irreversible change

Answer 1

Hint: This reaction is based upon one of the important chemical properties of zinc oxide. It is a type of an inorganic compound having chemical formulas like ZnO. The reaction is related to the thermo-chromic nature of zinc oxide.

Complete step by step answer:
First, let us discuss the reaction of heating the zinc oxide. It is a white powder that occurs in the form of zincite.
As mentioned, the zinc oxide is thermo-chromic, it means there is a change in colour on heating the compound.
Thus, when zinc oxide is heated in air, there is a change in colour from white to yellow, and when we cool it, it reverts back.
The zinc oxide heated will absorb blue light wavelength, and reflected light will turn yellow.
The colour change occurs due to loss of oxygen.
Thus, the change of colour to yellow on heating, and becomes white on cooling depicts reversible chemical change.
In the end we can conclude that this is a chemical change. The correct option is (B).

Additional information:
Zinc oxide acts a wide gap semi- conductor for the group II – VI. It also exhibits the property of high electron mobility, and strong room – temperature luminescence.

Note: Don’t get confused while defining the change that occurred in reaction. It is reversible but can’t be termed as physical change because the chemical reaction occurs on heating zinc oxide, so we termed it as reversible chemical change.