Question 6
Why do most covalent compounds have low melting and boiling points?
The right answer is the one marked above.
Only weak attractions between separate molecules have to be overcome. Melting separates whole molecules from one another; it does not touch the bonds inside them.
The covalent bonds themselves are strong — that is exactly why they survive boiling. Covalent compounds contain no ions, and heavy covalent molecules still melt low.