Diffusion is the spreading out or mixing of a substance with another substance caused by the motion of its particles. You may smell food being cooked in a kitchen while you are outside the kitchen. Because of diffusion in the air, the vapour from the food reaches you and gives you the fragrance of food. Salt, dissolved at the bottom of glass bottle, changes the taste of the whole water due to diffusion. Carbonated drinks are made by allowing gas to diffuse through water. Aquatic animals, living inside water, breathe dissolved oxygen in the water.

The process of diffusing a substance into another substance continues until a homogeneous mixture is achieved. Diffusion is a phenomenon that occurs in gases, liquids, and solids, that is determined by the movement of its particles. In gases, diffusion is the fastest, whereas in solids, it is the slowest. Diffusion in liquids is slower than that in gases and faster than that in solids. Various gases have different diffusion rates depending on their density. Denser gases mix together at a slower rate than lighter gases. Density and temperature are the major factors that affect diffusion.

• The higher the density of the diffusing substance, the lower the rate of diffusion.

As the temperature rises, the constituting particles of the diffusing substance travel faster, resulting in an enhanced rate of diffusion. For example, one can smell cold food from a short distance, whereas can smell hot food from a long distance.