What is the order of arrangement of constituent particles in a crystalline solid?

Which of the following is isotropic?

Match the column $A$ (type of crystalline solid) with the column $B$ (example for each type).

$P$. Molecular solid	$i$. $SiC$
$Q$. Ionic solid	$ii$. $Mg$
$R$. Metallic solid	$iii$. $H_2O$
$S$. Network solid	$iv$. $MgO$

What happens when glass is heated?

'Stability of a crystal is reflected in the magnitude of its melting points'. Comment. Collect melting points of solid water,ethyl alcohol,diethyl ether and methane from a data book. What can you say about the intermolecular forces between these molecules?

Refractive index of a solid is observed to have the same value along all directions. Comment on the nature of this solid. Would it show cleavage property?