Physics

Sound

3. The Particles of Matter are Constantly Moving

The best evidence that particles of matter are constantly moving comes from the studies of diffusion and Brownian motion (which we have already discussed). We will now describe two more experiments involving diffusion in gases and diffusion in liquids which will show us that the particles of matter (gases and liquids) are constantly moving.
(a) When we light (or burn) an incense stick (agarbatti) in one comer of a room, its fragrance (pleasant smell) spreads in the whole room quickly. This observation 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 (alongwith air). 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, the observation that the fragrance of a burning incense stick spreads in the entire room very quickly tells us that the particles of matter (here incense gases and air) are constantly moving.
(b) We will now describe the diffusion of copper sulphate into water.Copper sulphate crystals are blue in colour. When a few crystals of copper sulphate are placed at the bottom of a beaker (or a gas 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 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 called diffusion. This diffusion of copper sulphate particles and water particles into each other goes on until the whole water turns blue. Thus, 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.
If we carry out this experiment by using hot water in the beaker (or gas jar), we will find that the water turns blue at a faster rate. This is because, on heating, the particles of water and that of copper sulphate gain kinetic energy and move faster. And due to faster movements, they mix into each other more quickly.

Notes

Sound - Notes

1. 3. Humidity of Air