Why do particles of matter constantly move?

Everything around us is made up of tiny pieces or particles. The particles which make up matter are atoms or molecules. Our body, chair, table, book etc. all are made up of particles.

The evidence of the existence of particles in matter and their motion comes from the experiments on diffusion i.e. mixing of different substances on their own and Brownian motion. The zig-zag movement of the small particles suspended in a liquid or gas is called Brownian motion.

First we will study about matter.

Anything which occupies space and has mass is called matter. There are large number of things present in different shapes, sizes and textures. Like sand, sugar, hydrogen, oxygen, air, water, petrol etc. are all different kinds of matter because they occupy space, have volume and mass.

What are the characteristics of Particles of Matter?

- The particles of matter are very, very small.

- The particles of matter have spaces between them.

- The particles of matter are constantly moving.

- The particles of matter attract each other.

Now, let us understand that how particles of matter are constantly moving?

With the help diffusion and Brownian motion we can prove that particles of matter are constantly moving.

First, we will describe two experiments which involve diffusion in gases and diffusion in liquids.

(i) When we burn an incense stick say agarbatti in one corner of the room, its fragrance or the pleasant smell of it spreads in the whole room quickly. This can be explained as follows: The burning of incense stick produces gases or vapours having pleasant smell. The particles of gases produced by the burning of incense stick move rapidly in all directions, mix with the moving particles of air in the room and reach every part of the room quickly along with the air.

Do you know why Oil and Water does not mix together?

When the gaseous particles from the incense stick reach our nose with air, we can smell the fragrance. If, however, the particles of gases produced by the burning of incense stick and the particles of air were not moving, then the fragrance of incense stick could not spread in the whole room quickly. So, we can say that the fragrance of a burning incense gases and stick spreads in the entire room very quickly which shows that the particles of matter i.e. incense gases and air are constantly moving.

(ii) Now, we will study the diffusion of copper sulphate into water. Do you know that the crystals of copper sulphate are blue in colour? When a few crystals of copper sulphate are placed at the bottom of a beaker or a jar containing water, then water in the whole beaker turns blue slowly.  This can be explained on the basis of motion of copper sulphate particles and water particles as follows:
The crystals of copper sulphate dissolve in a little of water around them to form fine particles of copper sulphate in the solution. Now, the particles of copper sulphate in solution are in motion and the particles of clear water are also moving. Due to the motion of their particles, the blue copper sulphate particles in solution move upwards in the beaker and the colourless water particles move downwards and mix with each other. This is known as diffusion.

This process will continue until and unless the whole water turns blue. Therefore, we can say that the spreading of blue colour of copper sulphate crystals in water is due to the movement of both copper sulphate particles as well as water particles.

There are several examples that show particles of matter are constantly moving:

- The smell of food being cooked in the kitchen reaches us even from a considerable distance.

- The smell of perfume spreads due to the diffusion of perfume vapours into the air.

- Dissolution of a crystal of potassium permanganate into a glass of water. The solution turns pink.

- When we shout loudly a person standing far from us can hear it as the sound propagated through the air. Sound travels when its vibrations passes from one molecule to another.

So, we can say that particles of matter are constantly moving.

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