Diffusion

If a bottle of perfume is opened in one corner of a room, it spreads in the whole room by diffusion. If you drop a little ink in a beaker of water it will spread by itself in the beaker of water and the color spreads uniformly.

Diffusion is the natural process of intermingling of the molecules of two substances. The rate of diffusion in gases is inversely proportional to the square root of their densities.

Let us study diffusion of liquids with the help of the following experiments:

Experiment 1:

Place a layer of deep blue CuSO₄ solution in a tall beaker and allow it to settle. CuSO₄ is a salt which ionizes completely when placed in water into Cu²⁺ and ions. Then carefully transfer a layer of water on to the CuSO₄ solution layer using a pipette. Cover the beaker with a lid and leave it undisturbed for several hours as shown in figure.

Initially water floats on top of the CuSO₄ solution as it is less dense. The boundary line very soon disappears and after a few hours the solution becomes a homogeneous pale blue color as shown in figure. The intermingling can be explained by describing the motion of molecules of water, copper and sulphate ions moving randomly in all directions.

Sub Topics

Dissolving of Crystals in a Liquid due to Diffusion
Diffusion in Solids
Diffusion in Gases
Assumptions of Kinetic Theory of Matter

Dissolving of Crystals in a Liquid due to Diffusion

Experiment 2:

Place some crystals of potassium permanganate at the bottom of a beaker. Slowly transfer water into the beaker with the help of a pipette. After sometime a dense purple solution is formed at the bottom and you will notice that it gradually spreads with the rest of water. After a few hours, it will be seen that the purple solution of a uniform concentration has formed throughout the solution.

This is because of the random motion of the potassium permanganate crystals in the water and by diffusion a homogeneous solution of potassium permanganate and water is formed.

Diffusion in Solids

When a lead plate and a gold plate are placed one over the other if they are kept under pressure for a period of one or two years. It will be observed that the lead particles have diffused into gold and gold particles have diffused into lead plate.

It is believed that fossils of animals like reptiles and leaves lie in contact with stones and rocks for number of years, which leaves an impression on the stones or rocks. This is because of the diffusion of the particles of the organic matter into the stones, which are in contact.

Diffusion in Gases

If a bottle of perfume is opened in one corner or a room, it spreads in the whole room by diffusion.

Experiment 3:

A gas jar of bromine and a jar of air are placed, separated by a cover plate. The cover plate is removed so that the two open ends of the jars are in contact. The bromine gas spreads (diffuses) rapidly into the air to produce a uniform pale brown colour in both jars.

Experiment 4:

A porous pot containing air is fitted with a one-holed rubber cork and a glass tube which dips in a beaker of water. The porous pot is enclosed in another container filled with hydrogen as shown in figure.

In the second case, the porous pot is enclosed in another container filled with carbon dioxide.

In both the cases, diffusion of gases takes place. The rate of diffusion depends upon the density of the gas. Hydrogen being lighter than air, the volume of the hydrogen that diffuses into the porous pot is more than the amount of air that diffuses out of the pot. As a result, the volume of the mixture in the pot increases and the excess air comes out in the form of air bubbles.

In the second case, air being lighter, volume of the air that diffuses out is more than the volume of carbon dioxide that enters the pot through the process of diffusion. As a result, a partial vacuum is created in the pot. Hence the level of water rises in the glass tube.

Gases move in and out of living things by diffusion.

Oxygen diffuses into your blood form your lungs, similarly carbon dioxide diffuses from your blood into your lungs.

Diffusion is a process of great importance in manufacture and operation of transistors.

Assumptions of Kinetic Theory of Matter

All matter is made up of particles called molecules. In normal circumstances it exists in three states.

All the molecules of a given substance are alike in all respects.

The molecules are separated from one another by the intermolecular space which is more than the diameter of the molecule itself.

The molecules attract each other with a force called inter-molecular force, which is strongest in solids and weakest in gases.

Molecules are in constant motion.

The temperature of a substance is proportional to the average kinetic energy of all the molecules of the substance.