a) Naphthalene balls disappear with time without leaving any solid.
Naphthalene undergoes sublimation easily i.e., the change of state of naphthalene from solid to gas without the intervention of the liquid state. Thus, naphthalene balls keep on forming naphthalene vapours which disappear into the air with time without leaving any solid.
b) We can get the smell of perfume sitting several metres away.
Perfume has a higher degree of vaporization and its vapour diffuse into the air easily. Gaseous particles possess high speed and move very rapidly in all directions. When perfume is sprayed, its particles diffuse into the particles of air at a very fast rate and reach our nostrils. This enables us to smell the perfume from a distance.