A colour that spreads by itself

The idea

Particles of liquids and gases are in constant motion; they hit the particles of another substance and spread them throughout, which is why a colour spreads through water and a fragrance through a room.

Nobody stirs it

A few grains of potassium permanganate are put in a tumbler of water. Nobody stirs it.

What will the water look like after some time?

• Uniformly pink all over
• Pink only at the bottom, for ever
• Still colourless

Wait and watch

Drop the grains and wait. Then light an incense stick in the room and wait. Watch the particles too.

Particles keep moving

Particles of liquids and gases are in constant motion. They hit the particles of another substance and spread them throughout. That is why a colour spreads through water and a fragrance spreads through a room.

This is also how the fragrance of perfume reaches us. Use potassium permanganate only with a teacher.

• What spreads | Moving particles that spread it
• Pink colour of potassium permanganate | Water particles
• Fragrance of an incense stick | Air particles

Notes

Particles of liquids and gases are in constant motion. They hit the particles of another substance and spread them throughout: a colour spreads through water, a fragrance through a room.

Check yourself

Why does the pink colour spread through the whole tumbler without stirring?

An incense stick burns in one corner of a room. Soon you smell it in the far corner. Why?

Which of these shows that the particles of a gas are always moving?

• The pink particles are heavy and sink. If they only sank, the colour would stay at the bottom.
• Water particles are in constant motion and hit the pink particles — correct. Yes. They spread the pink particles throughout.
• The tumbler keeps shaking. Nobody shakes it. The water particles themselves are always moving.
• A smell can spread only when a fan is on. It spreads even in still air, because air particles are always moving.
• The fragrance particles stay near the stick. Then you could not smell it in the far corner.
• Air particles keep moving and hit the fragrance particles, spreading them — correct. Yes. The fragrance spreads through the whole room.
• The smell of perfume reaches us from across a room — correct. Yes. Moving air particles spread the perfume particles.
• A stone keeps its shape. That is about solids keeping their shape.
• Water keeps its volume when poured. That is about liquids keeping their volume.