9th science

Matter In Our Surroundings

Diffusion in Matters


The phenomenon of mixing of particles of different substances together is known as diffusion. Diffusion takes place because of the movement of particles of matters.

Diffusion is a natural process. The mixing of different gases in air is a natural process that takes place continuously.

Diffusion takes place from higher concentration to lower concentration.

Diffusion comes from Latin word "diffundere". The meaning of "diffundere" is "to spread out".

It was Robert Brown who discovered the random moment of particles of matter. Albert Einstein developed the theory of Brownian motion based on the discovery of Robert Brown.

Diffusion takes place in solid, liquid and gases.

Diffusion in gas:

Diffusion takes place most rapidly in gasses.

The particles of gas have more kinetic energy due to that they move with high speed. Because of the movement of particles with high speed gas diffuses most rapidly.

Example: Mixing of gases in air.

Smoke coming out of factories is seen only near the mouth of chimneys. After rising high it mixes up with air and vanishes out. This happens because of diffusion of gases.

Diffusion in liquid:

Diffusion takes place in liquids. The rate of diffusion is less in liquid than that of gases.

The particles of liquid have enough movement because of kinetic energy for diffusion. When two liquids are kept together they are mixed because of the diffusion.

For example; When salt solution is kept in water this makes whole the solution salty after some time. This happens because of diffusion in liquids.

Diffusion in solid:

Diffusion takes place with slowest rate in the case of solids. The rate of diffusion is almost negligible in solids.

Since the particles of solid have lowest kinetic energy so they do not move. Hence, diffusion takes a long time in the case of solid.

Example – Some time we observe that the marks of chalk on the black board could not be wiped out easily after a long time. This happens because of the diffusion of particles of chalks with the particles of black-board and marks of chalk could not be wiped out.

Alloys are the mixture of two or more than two metals. Alloys are possible to get prepared because of the diffusion of solid into solid. Bronze, steel, brass are the examples of alloy.

Diffusion in everyday life:

  • We get the smell of our favourite food sitting in the drawing room even when the food is being cooked in kitchen. The vapor coming out from the food reaches to us because of diffusion in air and we get the smell of food.
  • We get the smell of burning incense stick sitting far from it. When an incense stick is burnt, the incense present in it vaporizes because of heat. The vapor coming out from the incense stick mixes with air and reaches to us.
  • In similar way we sense the good or bad smell from a distance.
  • When sugar is added in water, it is mixed because of the diffusion. The particles of sugar collide with the particles of water because of their continuous movement. And finally they are mixed together because of diffusion.
  • Carbonated drinks are prepared by the diffusion of gas in water. We notice a hiss sound when the cap of bottle of a carbonated drink is open, the hiss sound comes because of the diffused gas coming out of water. Soda, Soft drinks (coca-cola) etc. are the carbonated drinks and prepared mainly because of the diffusion of carbon dioxide in water.
  • Aquatic animals take dissolved oxygen in water while breathing. Aquatic plants synthesize their food under water because of the dissolved carbon dioxide in water. These gases dissolved in water because of the diffusion.

Temperature and Rate of Diffusion:

The rate of diffusion increases with increase in temperature and decreases with decrease in temperature.

Diffusion takes place because of the movements of particles of matters. The increase in temperature increases the kinetic energy, which increase the speed of movement of particles of matter. The increase in the speed of movement increases the rate of diffusion.

Example: We do not get the smell of cold food from a distance while we get the smell of hot food from a distance. Because smell coming out from the cold food in the form of gas do not have enough kinetic energy to cover more distance. But when food is hot the vapor coming out from it moves more rapidly because of more kinetic energy and reaches to us even if we are at a distance from it. This shows that temperature increases the rate of diffusion.