If we keep hydrogen sulphide gas (H2S) exposed in any comer of the room, after a very short time the whole room is filled up with bad odour of the gas. In fact, this happens because the molecules of hydrogen sulphide gas gradually spread in the whole room. In other words, it can be said that the molecules of the gas gradually mix with the molecules of the air present in the room, uniformly. This process is called diffusion.
During respiration the oxygen is used in the leaf, and the carbon dioxide is accumulated in the intercellular spaces of the leaf.
Now this carbon dioxide comes outside in the atmosphere, by diffusion and the oxygen is taken in the cells by diffusion. This process of diffusion may take place in all kinds of matters such as solid, liquid and gas. In other words, it can be said that the molecules of these matters may spread uniformly in the solvent.
If the volatile substances, such as petroleum, alcohol, ether and perfumes, etc., are kept exposed in a room, then in a meanwhile, the whole room fills up with the odour of the volatile substance.
It means, the molecules of these substances spread in the whole room, and gradually mix with the molecules of the air present in the room. Besides these, if the solid substances such as naphthalene balls and camphor are kept exposed in the room, very soon their molecules spread, and mix with air molecules, and the whole room fills up with their odour.
The solid substances are also diffused in the liquid substance, but with a slow speed. For example, a crystal of potassium permanganate (KMnO4) is dropped in a beaker filled up with water. In a mean while the water of the beaker around this crystal becomes deep red, and after some time the whole water of the beaker becomes of uniform red colour, and the crystal is fully dissolved.
In the beginning, the molecules of potassium permanganate, around the crystal are in greater concentration, but very soon, the crystal dissolves, and the molecules spread uniformly in whole of the water of the beaker and the water becomes of uniform red colour, because of uniform distribution of crystal molecules, in the solvent, by diffusion.
This experiment may also be done by the soluble crystal of copper sulphate. If sugar crystals are added to water and kept for some time, then after some time the water becomes sweet — this is also a good example of diffusion.
In this way, the solid, liquid and gases diffuse in different media. The above mentioned examples prove that, the gases may be diffused in other gases; the liquids may diffuse in the gases or other liquids; and the solids may also diffuse either in gas or in liquid.
The above mentioned examples of diffusion may be summarized as follows:
1. That diffusion is a random movement of individual molecules from a region of higher concentration to a region of lower concentration.
2. Larger the difference in concentration more rapid is the flow of molecules.
3. Diffusion is more rapid in gases than in liquids.
4. When there is no further movement of molecules, a state of equilibrium is acquired, which means that the diffusing substance is evenly distributed.
5. In diffusion, the movement of molecules is random and independent of each other.
6. Suitable examples of diffusion in plants are: supply of carbon dioxide from atmosphere to the leaves for photosynthesis, and loss of water vapour from leaves to the atmosphere.