Particles never stop moving
Every thing around you is made of very tiny pieces called particles (atoms and molecules). They are far too small to see. The big idea: these particles are always moving. In a gas they fly about fast. In a liquid they slide past each other. In a solid they only shake in their places.
Why do we not see this motion? Because each particle is so small. We only see what many of them do together.
Brownian motion
In 1827 the scientist Robert Brown looked at tiny pollen grains in water under a microscope. The grains kept jiggling in zigzag paths, and they never stopped. Today we know why: the water particles are invisible, but they hit the grain again and again from all sides. At each moment, a few more hits come from one side than from the other, so the grain gets a small push in a random direction.
- The path is random and zigzag.
- A smaller grain jiggles more. A big stone does not move at all, because the hits cancel out.
- A hotter liquid makes the grain jiggle faster.
Brownian motion is the proof that matter is made of moving particles. The grain is the thing we can see. The moving water particles are the cause we cannot see.
Diffusion
Diffusion means that particles of one substance spread by themselves into another substance until they are mixed evenly. No stirring is needed. Particles move from a place where there are many of them to a place where there are few.
- In gases it is fast: the smell of perfume crosses a room.
- In liquids it is slower: ink spreads in water over minutes.
- In solids it is extremely slow: it takes years for two metal blocks pressed together to mix a thin layer.
Diffusion is faster when it is hotter, because the particles move faster. It is also faster when the particles are lighter and smaller.
Attraction and repulsion
Particles also act on each other with forces. When two particles are a little apart, they pull each other (attraction). This is why a drop of water holds together and why a solid keeps its shape. When they are pushed very close, they push each other away (repulsion). This is why you cannot squash a stone or a block of iron easily.
The pull is strong in solids (particles are close), weaker in liquids and almost zero in gases (particles are far apart). That is why gases spread to fill any container.
Try it at home
Take two clear glasses, one with cold water and one with warm water (not boiling). Put one drop of food colour or ink in each, without stirring. Watch with a timer. Guess first: which spreads faster? Then check. Write down the time for the colour to reach the whole glass.
Key formulas and definitions
- Brownian motion = random zigzag motion of a visible grain pushed by invisible moving particles
- Diffusion = spreading of particles from many-place to few-place until mixed
- Hotter means faster particles, so faster diffusion and stronger Brownian motion
- Far apart: attraction (pull). Very close: repulsion (push)
Worked examples
1. A pollen grain in water moves in a zigzag path. What makes it move?
Invisible water particles keep hitting it from all sides. The hits are not equal at every moment, so the grain is pushed in random directions.
2. A perfume bottle is opened in one corner of a room. After some time everyone smells it. Name the process and explain it.
It is diffusion. Perfume particles move randomly, spread from where they are many to where they are few, and mix with the air.
3. Why does sugar dissolve faster in hot tea than in cold tea?
Particles in hot tea move faster. They collide with sugar more often and carry it away, so the sugar mixes quickly. Diffusion is faster at higher temperature.
4. Why can gas be squeezed into a small cylinder easily but a solid cannot?
In a gas the particles are far apart and the force between them is tiny, so there is a lot of empty space. In a solid the particles are already very close, so pushing them closer makes them repel strongly.
5. A big iron ball in water does not show Brownian motion. Why?
The ball is so large that the hits of the tiny particles come from all sides almost equally and cancel out. Only very small grains feel a noticeable net push.
6. Two particles are 5 units apart and then moved to 1 unit apart. What does the force between them do?
At 5 units, they pull each other (attraction). When pushed to 1 unit, they are very close and push each other apart (repulsion).
Common mistakes
- Thinking the grain moves by itself. It is pushed by invisible particles hitting it.
- Thinking particles stop when it is cold. They slow down, but they never fully stop.
- Saying diffusion needs stirring or wind. It happens on its own, stirring only helps.
- Mixing up the two: Brownian motion is what the grain does; diffusion is the mixing of two substances.