·         When chemicals like the smell of perfumes or burning toast are lit loose in a room, the particles mix with the air particles. The particles of smelly gas are free to move quickly in all directions. They eventually spread in all directions i.e. diffusion.

·          You don’t have to mix the gases by waving upon mass around; it mixes on its own. Diffusion in gases is quick because the particles in a gas move quickly it happens even faster in hot gases.

·         We have discussed that molecules in matter are always moving. The movement is very small in solids, relatively large in liquids and very large in gases. This movement of molecule gives rise to diffusion.

·         Diffusion will be more rapid when the differences in concentration are larger. Gas diffuses more rapidly than liquids. Solids arises slowest to diffuse, when the particles of diffusing substance get evenly distributed, a state of equilibrium is reached, it stops further movement.


·         Diffusion may be defined as the process of intermixing of two or more material, when placed in contact, due to their random movement of their respective molecule. OR

·         Diffusion is also defined as the movement of the particles of different substances from the region of their higher concentration, free energy or diffusion pressure to region of their low concentration, free energy or diffusion pressure. OR

·         The tendency on the part of molecules, atoms, ions etc of gases, liquids and solids to get evenly distributed throughout the available space on account of their random kinetic motion is called diffusion.

·         For example, when we light an agarbathi in one corner of a room, its fragrance spreads to all parts of the room. If it’s spread with the help of a moving fan on the room, then it is not diffusion. But if there is no circulation of artificial air in the room, then it is due to diffusion. The molecules of agarbathi fumes will mix with the air present in the room due to their own random movement. Thus the fragrance spread throughout the room by diffusion.

·         Diffusion of one substance is independent on the diffusion of particles of another substance is known as independent diffusion.

We can explain diffusion of gas process by several methods, let discuss them in detail.

·         Diffusion of gas occurs when there is a net movement of molecule from an area in which that particular gas exerts a high partial pressure to an area in which it exerts a low partial pressure. During diffusion, different gases move according to their own individual partial pressure gradient.

·         Gas transfer during diffusion occurs by random molecular movement. It is therefore dependent upon temperature, because molecular movement increases at higher temperature. Gases move in both directions during diffusion, but the area of higher partial pressure because of its higher greater number of molecule/ unit volume, has proportionally more random departures. Thus the net movement of gas is dependent on the partial pressure difference between the two areas.

·         In a static situation, diffusion continues until no partial pressure difference exist for any gases in the two areas; in the lungs, O2, CO2 continuously enter and leave the alveoli and so much an equilibrium does not take place.

·         Examples are perfumes in bottles, left open and fragrance evaporates and diffuses into the air creating a fragrance in the room, Gas stations are another example where gasoline evaporates and diffuses into the air creating the gasoline smell.

Graham’s law:

Scottish chemist Thomas Graham experimentally determined that the ratio of the rates of effusion for two gases is equal to the square root of the inverse ratio of the gases molar masses. This is written as follows:

Rate of effusion gas 1    =  ∫M1

Rate of effusion gas 2         ∫M2

Where M,  represents the molar mass of the molecules of each of the two gases.

The gases effusion rate is directly proportional to the average velocity at which they move, a gas is more like to pass through an orifice if its particles are moving at faster speeds.

Factors affecting diffusion:

There are following factors which affect the process of diffusion of gas:

·         Number of particles per unit volume.

·         Density of medium

·         Distance through which diffusion occur

·         Temperature

·         Pressure

Significance of diffusion:

·         Transpiration is a diffusion process between plants and dry atmosphere.

·         Exchange of CO2 and O2 During photosynthesis and respiration occurs through diffusion.

·         Osmosis is a diffusion process in which only solvent is permitted to diffuse.

·         It takes place in short distance translocation of substance inside the cell as well as between the cells.



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