Particles on the Move

Why it matters

Someone opens a bottle of perfume at the door. Minutes later you can smell it right at the back of the room, even though nobody fanned it your way.

Predict first

A drop of ink is placed in still water and slowly colours the whole glass. Why does this happen on its own?

Particles of matter are always moving. They wander into the spaces between other particles, so the ink and water mix by themselves. This intermixing is called diffusion.

• The water pushes the ink to the edges
• Tiny particles are always moving and mix on their own — correct
• The ink is lighter than water and floats up
• Only stirring can make the ink spread

What you do

Drag these from the FASTEST diffusion at the top to the SLOWEST at the bottom.

Check yourself

What do we call the intermixing of particles of two different substances on their own?

Diffusion is the spontaneous intermixing of particles of two substances because those particles are always moving.

In which state of matter does diffusion happen fastest?

In gases the particles are far apart and move very fast, so they mix quickly. That is why diffusion is fastest in gases.

You want a sugar cube to dissolve and spread in tea faster. What should you do?

Heating gives particles more energy so they move faster. Higher temperature therefore speeds up diffusion.

• Diffusion — correct
• Melting
• Filtering
• Freezing
• Gases — correct
• Solids
• Liquids
• It is the same in all states
• Use hot tea instead of cold tea — correct
• Use colder tea
• Cover the cup tightly
• Add more sugar