$NaCl$ is an example of

Assertion : Stability of a crystal is reflected in the magnitude of its melting point.
Reason : The stability of a crystal depends upon the strength of the interparticle attractive force.

$A$ clock spring is heated to redness and then plunged into cold water. This treatment will cause it to become

Which of the following is a non-crystalline solid?

Which among the following is $NOT$ a polar molecular solid?

Explain crystalline and amorphous solids.