Question

ZnO is white when cold and yellow when hot because it____.

• A. sublimes
• B. melts
• C. forms a non-stoichiometric compound at high temperatures
• D. all of the above

Answer 1

Answer: (C)

forms a non-stoichiometric compound at high temperatures

Zinc oxide is white in colour at room temperature. On heating, it loses oxygen and turns yellow.
$ZnO\underrightarrow{\text{Heating}}{Zn}^{2+}+\frac{1}{2}{O}_2+2e^{-}$
Now, there is an excess of zinc in the crystal and its formula becomes $Z{n}_{1+x}O$. The excess $Z{n}^{2+}$ ions move to the interstitial sites and the electrons to the neighbouring interstitial sites. This gives rise to colour.

Hence, $ZnO$ is white when cold and yellow when hot because it forms a non-stoichiometric compound at high temperatures.