Among solids,the highest melting point is established by

What happens when glass is heated?

$A$ group $14$ element is to be converted into $n-$type semiconductor by doping it with a suitable impurity. To which group should this impurity belong?

Explain:
$(i)$ The basis of similarities and differences between metallic and ionic crystals.
$(ii)$ Why are ionic solids hard and brittle?

$A$ hard substance melts at high temperature and is an insulator in both solid and in molten state. This solid is most likely to be a / an :

Classify the following materials into superconductors,conductors,semiconductors,and insulators: $Cu$,$Al$,$Si$,$Ge$,glass,Teflon,$Au$,$Pt$,$CuO$ (at low temperature),graphite,carbon black,and alloys at low temperature.