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What is diffusion? Give examples.

Class 8 ScienceNCERT chapter: Particulate Nature of Matterइसे हिंदी में पढ़ें

Exam-ready answer

Diffusion is the mixing of particles of two or more substances on their own, due to the constant motion of their particles. Particles move from a region where they are more crowded to where they are fewer. Examples: the smell of a burning agarbatti spreading through a room, and the colour of a potassium permanganate crystal spreading through water.

Explanation

The particles of every substance are always moving, and there are spaces between them. When two substances are brought in contact, their moving particles slip into the spaces between each other's particles. Nobody stirs them; the mixing happens by itself. The particles keep spreading from the region where they are more concentrated towards the region where they are fewer, until they are evenly mixed.

Diffusion is fastest in gases, slower in liquids and extremely slow in solids. Gas particles move fast and have large spaces between them, so the smell of an agarbatti or of food cooking in the kitchen reaches other rooms quickly. Liquid particles move more slowly and are closer together. Solid particles only vibrate in fixed positions, so diffusion in solids is extremely slow.

Temperature also affects diffusion. Put a crystal of potassium permanganate in a glass of cold water and another in hot water without stirring. The purple colour spreads faster in hot water, because heating makes the particles move faster. Other examples include a drop of ink spreading in water and the aroma of hot food being noticed from farther away than that of cold food.

Points to remember

  1. Diffusion: the spontaneous mixing of particles of different substances due to the motion of their particles.
  2. It happens because particles are always moving and have spaces between them.
  3. Rate of diffusion: gases > liquids > solids.
  4. Diffusion becomes faster when the temperature rises.
  5. Examples: smell of agarbatti spreading, potassium permanganate or ink colouring water, aroma of cooking food.

Exam tip

Give the definition first, then at least two examples from daily life, one in a gas and one in a liquid. Write 'gases > liquids > solids' for the rate and mention that heating increases diffusion.

Questions students also ask

Why is diffusion faster in gases than in liquids?
In gases the particles move very fast and the spaces between them are large, so particles of another substance can easily move into those spaces. In liquids the particles are closer and move more slowly, so mixing takes longer.
Why does hot food smell stronger than cold food?
At a higher temperature the particles of the aroma move faster and more of them escape into the air. They diffuse more quickly through the air and reach our nose sooner, so we notice the smell of hot food from farther away.
Does diffusion happen in solids?
Yes, but extremely slowly. Particles in a solid are packed closely and only vibrate about fixed positions, so very few particles can move into the spaces of another solid. It can take a very long time to notice any mixing.

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