Smells on the Move

Why it matters

Someone opens a bottle of perfume in the corner of a room, and soon you can smell it from across the room, without any fan. The tiny particles travelled to you all by themselves.

Predict first

You place a drop of ink in still water and an open perfume bottle in still air. Which smell or colour spreads through the whole space faster?

This spreading of particles is called diffusion. Gas particles are far apart and move very fast, so a smell diffuses through air much faster than colour spreads through water.

• The ink in water, because liquids are lighter than air
• The perfume in air, because gas particles are far apart and move very fast — correct
• Both spread at exactly the same speed
• Neither spreads unless you stir it

What you do

Order these from the fastest spreading to the one that barely spreads at all.

Check yourself

What do we call the spreading of particles from where there are many to where there are fewer?

Diffusion is the movement of particles from a region where they are crowded to a region where they are fewer, until they are spread out evenly. Smells and ink spread by diffusion.

In which state of matter does diffusion happen FASTEST?

Gas particles are far apart and move quickly in all directions, so they mix and spread the fastest. In liquids diffusion is slower, and in solids it is slowest of all.

Why does almost no diffusion happen through a solid wall?

In a solid the particles are held closely together in fixed positions and can only vibrate. Because they cannot move past one another, particles from outside cannot diffuse through.

• Diffusion — correct
• Freezing
• Melting
• Filtering
• In gases, because the particles are far apart and move very fast — correct
• In solids, because the particles are packed tightly
• Diffusion happens at the same speed in every state
• In liquids, faster than in gases
• The particles in a solid are packed tightly and only vibrate in fixed positions — correct
• Solids have no particles at all
• The wall is too colourful
• Smells are too heavy to move