The movement of molecules from higher concentration to lower concentration is called diffusion. Diffusion occurs down the concentration gradient. It is a spontaneous process. Diffusion continues till dynamic equilibrium is achieved. It follows laws of thermodynamics. But it is a very slow process.

The rate and the direction of diffusion of different substances depend upon the difference in their concentration at different points. It is independent of the presence of other substances.

Examples

I. Oxygen is utilized and carbon dioxide is removed during respiration in a leaf. This CO, collects in the intercellular spaces of the leaf. Carbon dioxide diffuses from the intercellular spaces into the outer air. But oxygen diffuses in the reverse direction.

2. Petrol and perfume are volatile liquids. Camphor and naphthalene balls are volatile solids. They diffuse in the same way. Their odour can be detected from some distance away.
3. A crystal of copper sulphate is placed in a beaker of water. It slowly dissolves. The surface of the crystal has greater concentration. But other parts of water have lesser concentration of copper sulphate. Therefore, the molecules of copper sulphate move from surface toother part. After some time the crystal disappears. Its molecules are evenly distributed throughout the surrounding liquid.Factors affecting the diffusionFollowing factors are responsible for diffusion:
   1. Chemical potential

   All the three states of matter solid, liquid and gas are made up of molecules. These molecules possess kinetic energy or free energy at temperature above —273°C. The total kinetic energy of the molecules is called chemical potential. Therefore, the molecules are in constant motion. They collide with one another. Thus molecules of a substance try to move from the region of their higher concentration or higher kinetic energy to the region of their lower concentration or lower kinetic energy. Thus diffusion directly depends on the chemical potential. Temperature and pressure affects the chemical potential. Therefore they also affect the rate of diffusion.

   2. Density

   The density of the diffusion medium also affects the rate of diffusion. The molecules of diffusion medium produce resistance to the diffusing particles. Greater the density of the medium, greater will be this resistance. Therefore, it slower the rate of diffusion. For example. diffusion of sugar molecules in water. Sugar dissolves rapidly in hot water than in cold water. The water molecules become less dense at higher temperature. Therefore, its resistance to the diffusing sugar molecules decreases. Thus the sugar molecule can diffuse through water with greater velocity. The density of a substance depends upon its molecular weight and the distance between the molecules. The distance between the molecules is the greatest in gases. Therefore, gases have the lowest density. The solids have least distances. Therefore, they have higher density and low rate of diffusion.

   Graham’s law

   It states that the rates of diffusion of gases are inversely proportional to the square roots of their densities. Graham’s law is applied on the diffusion of gases.

   r, = Jd₂

   r₂             d_i

   Where r_i and r, are the rate of diffusion and d_i and d₂ are their densities.

   3. Diffusion pressure

   The pressure exerted by the diffusing particles is known as diffusion

   pressure. This pressure is directly -proportional to the concentration or the number of diffusing particles. Greater the concentration of diffusing particles in a system. the greater is their diffusion pressure. For example, the diffusion-pressure of copper sulphate particles is the highest near the dissolving crystal. But it is the low.est in the other regions. Therefore, the particles of copper sulphate diffuse from the region of their greater diffusion pressure to the region of their lesser diffusion pressure