Question

Why ${\text{ZnO}}$ turns yellow on heating?

Answer 1

${\text{ZnO}}$ is a chemical compound. The chemical name of ${\text{ZnO}}$ is zinc oxide. Initially, zinc oxide is white in colour. When zinc oxide is heated at high temperature, it changes colour from white to yellow.

Complete step-by-step answer:
We are given zinc oxide. Zinc oxide is a white powder. It is insoluble in water. Zinc oxide is found in the earth’s crust in the form of zincates.
Zinc oxide is an amphoteric oxide. It reacts with acids to produce zinc salts and it reacts with bases to form zincates.
When zinc oxide is heated strongly above ${800^ \circ }{\text{C}}$ it turns yellow. After cooling down, the colour of zinc oxide changes back to white. Thus, it is a reversible process.
The hot zinc oxide absorbs blue wavelength of light and thus, the reflected light is observed as yellow.
On heating, zinc oxide also loses a minute amount of oxygen. When oxygen is lost from the crystal lattice and Frenkel defect is created.
The reaction when zinc oxide is heated is as follows:
${\text{ZnO}}\left( {\text{s}} \right) \rightleftharpoons {\text{Z}}{{\text{n}}^{2 + }} + \dfrac{{\text{1}}}{{\text{2}}}{{\text{O}}_{\text{2}}} + {\text{2}}{{\text{e}}^ - }$
The zinc ion formed then enters the voids formed due to heating. Thus, a non-stoichiometric defect is created.
The released electron gets trapped in the neighbourhood. This increases the conductivity of zinc oxide.
Thus, ${\text{ZnO}}$ turns yellow on heating because it absorbs blue wavelengths of light and a non-stoichiometric defect is created.

Note: Zinc oxide can be used for various applications:
1. In the rubber industry.
2. In manufacturing of concrete to improve its water resistance property.
3. As an antibacterial agent.
4. As a deodorizer.