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For the exams, the definition you will need to know is that "diffusion is the net movement of a substance from an area of high concentration to an area of low concentration, down a diffusion gradient".
But what does this actually mean?
Imagine a box filled with many butterflies, separated by a wall in the middle. The ones on the left are red, and the ones on the right are blue. The butterflies are flying around at random on their respecive sides. At this point, the concentration of red butterflies on the left is high, and on the right is low. The concentration of blue butterflies on the right is high, and on the left is low.
The wall is removed. The red butterflies are free to fly to the right hand side, which they will, to avoid being bunched up together. In other words, they move from an area of high concentration of red butterflies to an area of low concentration of red butterflies. The same goes for the blue butterflies on the right - they will move onto the left hand side when the wall is removed.
It is important to note that the butterflies do not "decide" to move away from the others of their colour. They simply move at random into all the space available, which naturally means they will spread out into the new space made by removing the wall. Some may go back to their original side, then maybe cross back over again, it doesn't matter. Until there are equal numbers of blue and red butterflies on each side, each colour will show a net movement from high to low concentration (in other words, more butterflies are moving from high to low than move from low to high concentration). This is called moving down a diffusion gradient, in the same way that moving from high land to low land would be moving down the gradient of a hill.
The same happens with particles. Because they move at random, they will spread out from an area where there is lots of them, to an area where there are not as many. Think of ribena - if you have a glass of water and add ribena, you can see the two different colours separately. But if you come back in a few hours, the ribena will have completely mixed with the water so it all looks the same colour. This is diffusion, as the two different types of particles have spread out into each other (from a high concentration to a low concentration) until their concentrations are equal.see more