Spreading out
Particles drift from crowded to less crowded places.
A drop of ink
Drop a spot of ink into still water. Slowly the colour spreads through all the water, even if you do not stir.
Why does the ink spread?
What this lesson covers
The idea
Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration, caused by the concentration gradient; it occurs even without a membrane.
A drop of ink
Drop a spot of ink into still water. Slowly the colour spreads through all the water, even if you do not stir.
Why does the ink spread?
- Particles move from where they are many to where they are fewer
- The water pushes the ink away deliberately
- The ink gets used up
Remove the partition
Remove the partition, then slide time forward and watch where the particles go.
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by the concentration gradient. It occurs even without a membrane.
The smell of fragrance spreading across a room, and dye spreading in water, are both diffusion.
- Start | Over time | End
- crowded on one side | particles spread out | same concentration everywhere
Notes
Diffusion is the net movement of particles from higher to lower concentration, driven by the concentration gradient. It needs no membrane.
Check yourself
Diffusion is the net movement of particles from…
Does diffusion need a membrane?
A smell spreads from the kitchen to the whole house. This is…
What is the concentration gradient in the tank at the start?
- higher concentration to lower concentration — correct. Yes.
- lower concentration to higher concentration. That is the wrong way round.
- a membrane to the wall. Diffusion does not need a membrane.
- No. It happens even without one — correct. Yes. Think of ink in water.
- Yes, always. Ink spreads in water with no membrane.
- Only in living things. It happens in any liquid or gas.
- diffusion — correct. Yes. The smell particles move from where they are many to where they are fewer.
- osmosis. Osmosis is the diffusion of water across a membrane.
- a membrane. There is no membrane involved.
- More dye particles on the left than on the right — correct. Yes. That difference drives diffusion.
- The same number everywhere. Then there would be no gradient and no net movement.
- More dye on the right. The dye was crowded on the left.