Give a reason for the following observations:

$(a)$ Naphthalene balls disappear with time without leaving any solid.

$(b)$ We can get the smell of perfumes sitting several metres away.

(a) Naphthalene balls undergo sublimation due to which it converts directly into vapours and disappear into air without leaving any solid.

(b) The particles of perfume diffuse rapidly into the air and its smell can be felt while sitting several metres away.