(a) The phenomenon of mixing of particles of different substances together is known as diffusion. Diffusion takes place because of the movement of particles (atoms and molecules) of matters. The mixing of different gases in air is a natural process that takes place continuously.

(b) The particles of a gas have maximum kinetic energy. They move with high speed in all directions. Gases thus can mix or diffuse very fast as compared to solids and liquids.

(c) Oxygen and Carbon dioxide are two gases of air which dissolve in water by diffusion. Aquatic animals take dissolved oxygen in water while breathing. Aquatic plants synthesize their food under water because of the dissolved carbon dioxide in water. This process is called respiration.