Give reason for the following :
(a) Gases fill up completely the vessel in which they are kept.
(b) Gases exert pressure on the walls of the containing vessel.

Explanation

(a) Particles constituting gases are very fast moving and diffuse at a fast speed. They therefore, readily fill up the vessel completely in which these are kept.
(b)j Particles of gases collide against the walls of the containing vessel and impart momentum to them. This is responsible for the pressure of the gas.