Question

You have three substances: `"lithium fluoride"`; `"carbon dioxide"`; and `"dinitrogen pentoxide"`. Which species should have the highest melting and boiling point?

Answer 1

You have 3 substances. One is ionic; and the other two are molecular..........

Explanation:

Ionic substances are intrinsically non-molecular, and (usually) exhibit exceptionally high melting and boiling points. And thus `"lithium fluoride"`, `i.e. Li^(+)F^-`, an ionic substance, with no discrete molecules, should have much higher melting and boiling points than MOLECULAR `CO_2`, and `N_2O_5`.

But, as I have said here before, you are a physical scientist. You are required to interpret data, not to remember them. Look up the melting points/boiling points of the three substances, and see if you can interpret the data on the basis of molecularity.