Question

Which of the following is a Non – crystalline solid?
(A) \[CsCl\]
(B) \[NaCl\]
(C) \[Ca{{F}_{2}}\]
(D) Glass

Answer 1

Hint: In crystalline solid, the arrangement of building constituents is regular throughout the entire three – dimensional network. They are made up of a large no. of unit cells. On the other hand, amorphous or non-crystalline solids have irregular and haphazard building constituents.

Complete Solution :
We know that crystalline solids have a definite geometrical shape with flat faces and sharp edges. Also, they are very rigid and their molecules cannot be deformed by mild distorting force.
- From the given options, we see that, \[NaCl,CsCl,Ca{{F}_{2}}\] are regular throughout and hence are crystalline solids.
Glass is an amorphous or a non – crystalline solid.
- This is because when glass is made, the material that often contains silica is quickly cooled from its liquid state but it does not solidify when its temperature drops down below its melting point. Therefore, glass can also be referred to as a ‘supercooled’ liquid.
- Also, crystalline solids have a distinct melting point unlike amorphous solids. This is the reason why glass can be shaped into any form when heated.
So, the correct answer is “Option D”.

Note: Glass is often called a super cooled liquid because it has a tendency to flow (though it is very slow). This is evident from the fact that the glass panes in the window of old buildings are invariably found to be slightly thicker at the bottom than at the top.
- The intermolecular force of attraction in amorphous solids is weaker than in crystalline solids.
- Some other examples of amorphous solids are rubber and plastics.