(a) Naphthalene is a volatile solid organic compound which can undergo sublimation (change from solid state directly into vapour state). The solid naphthalene balls keep subliming slowly (keep changing into vapours slowly). And after a certain time, the naphthalene balls sublime completely forming naphthalene vapours (which go into air), and hence they disappear without leaving any solid residue.
(b) We can get the smell of perfume sitting several metres away due to the diffusion of perfume vapours into air. This can be explained as follows : Perfume is a volatile liquid. When liquid perfume is applied by a person, it quickly changes into vapours (or gas). The perfume vapours move very rapidly in all directions in air, mix up with air particles and spread in the air by diffusion. When this air containing perfume vapours reaches several metres away, we can smell the perfume.
