Why does white Zn 0 (s) becomes yellow upon heating?
On heating ZnO loses oxygen as follows
$$\mathrm{ZnO} \xrightarrow{\text { Heat }} \mathrm{Zn}^{2+}+\frac{1}{2} \mathrm{O}_2+2 e^{-}$$
$\mathrm{Zn}^{2+}$ ions and electrons move to interstitial sites and F -centres are formed which impart yellow colour to $\mathrm{ZnO}(\mathrm{s})$.
Why does the electrical conductivity of semiconductors increase with rise in temperature?
The gap between conduction band and valence band is small in semiconductors. Therefore, electrons from the valence band can jump to the conduction band on increasing temperature. Thus, they become more conducting as the temperature increases.
Explain why does conductivity of germanium crystals increase on doping with galium?
On doping germanium with galium some of the positions of lattice of germanium are occupied be galium. Galium atom has only three valence electrons. Therefore, fourth valency of nearby germanium atom is not satisfied and the place remains vacant. This place is deficient of electrons and is therefore called electron hole or electron vacancy. Electron from neighbouring atom comes and fills the hole, thereby creating a hole in its original position. Under the influence of electric field electrons move towards positively charged plates through these and conduct electricity. The holes appear to move towards negatively charged plates.
In a compound, nitrogen atoms $(\mathrm{N})$ make cubic close packed lattic and metal atoms (M) occupy one-third of the tetrahedral voids present. Determine the formula of the compound formed by M and N ?
Let the number of N atoms in ccp is $x$
$$\begin{array}{lrl} \therefore & \text { Number of tetrahedral voids } & =2 x \\ \therefore & \text { Number of } M \text { atoms }=\frac{1}{3} \times 2 x \\ & \frac{\text { Number of } N \text { atoms }}{\text { Number of } M \text { atoms }}=\frac{3 x}{2 x}=\frac{3}{2} \end{array}$$
So, the formula of the compound is $M_2 N_3$.
Under which situations can an amorphous substance change to crystalline form?
On heating, amorphous substances change to crystalline form at some temperature some objects from ancient civilisation are found to be milky in appearance. This is due to crystallisation.