Which of the following is not a function of an impurity present in a crystal?

$NaCl$ is heated in an atmosphere of sodium vapour. The resultant yellow colour is due to the formation of

While writing the formula of ferrous oxide,it is written as $(FeO)$,because it is

How do defects arise in crystals? State the types of defects.

The formula of nickel oxide is $Ni_{0.98}O_{1.00}$. What is the approximate percentage of $Ni^{2+}$ in it?

Zinc oxide (white) is heated to high temperature for some time. Observe the following statements regarding the above process:
$I$. Zinc oxide colour changes to pale yellow
$II$. The type of defect formed is 'metal deficiency'
$III$. Some $Zn^{2+}$ and $e^{-}$ are present in interstitial places
The correct statements are: