Classify the following as amorphous or crystalline solids: Polyurethane,naphthalene,benzoic acid,teflon,potassium nitrate,cellophane,polyvinyl chloride,fibre glass,copper.

Which one of the following is the most correct statement?

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

What is the main use of $Quartz$?

'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?

The one that is extensively used as a piezoelectric material is