What do we mean when we say diffusion? You probably already have some idea of what the word means, you've probably had those air fresheners in your house where you have a bunch of sticks sticking out of a bottle, these are called "reed diffusers" and they work by soaking up the perfume and allowing it to evaporate off into the air.


When the perfume evaporates, it spreads out into the room. If you are right by the reed diffuser you will probably notice the smell quite quickly, but if you're on the other side of the room it might take a while to get to you. So, effectively diffusion is just the natural way in which the smell spreads through the room, but of course for the benefit of your learning there is a very formal way of putting this, and you should remember this:


"Diffusion is the spreading out of particles from an area of high concentration to an area of low concentration"


So how do the particles actually diffuse?

Think of the particles as little balls floating around in the air, think of the particles making up the gases in the air in the same way.


Lots and lots of little particles floating around in the air, some of them are just gas and some of them are your perfume. 


Air particles (gas molecules) are constantly moving, and the more energy that they have (for example if they get warmer) then the faster they will move. If a rapidly moving air particle hits one of your perfume particles it will knock it elsewhere, possibly into the path of another perfume particle which will then be knocked elsewhere or into the path of another air particle which will knock it somewhere else.



The overall effect is that the air particles and the perfume particles are in constant collision with each other, and as they collide they will gradually spread out to fill out the available room (as gases do). Now think about when the perfume particles originally started their journey, there were lots and lots of them in one place, but as they get hit by a particles they spread out and gradually get thinner and thinner. You will see this as the perfume getting weaker and weaker and someone on the other side of the room might not even be able to smell it very well at all.


Another way of restating that last paragraph is that originally the perfume particles are in a high concentration, and by diffusion (collision with air particles) the concentration gets lower and lower. This is perhaps the simplest type of diffusion, when different types of gas particles spread out through each other. Another type of diffusion which is very important to us is when gas particles diffuse through cell membranes, such as when oxygen molecules diffuse into our blood.


Now diffusion is a random process, the particles don't know that they are in a higher concentration in any particular place and therefore have no idea that they will diffuse into any area with lower concentration, it just happens that way because diffusion is a "net" process. If you have for example more particles on one side of the fence than the other, but the particles can pass through the fence in either direction you will have a "net" overall movement from the higher concentration to the lower concentration simply because there are more of them on the higher side and therefore the likelihood of them moving across is greater.


The greater the difference in concentrations either side of the fence, the greater will be the overall diffusion or the "net" diffusion. The scientific way to describe this is to say that the "concentration gradient" is greater, and the greater it is, the faster the diffusion will take place from the high to low concentrations.


There are three things that will affect the rate of diffusion:


  • The concentration gradient (as we have discussed above the greater this is the faster the diffusion will take place)

    Think of it this way, there is a massive crowd outside the shop because the sale is about to start. At 9 o'clock one of the staff is sent to unlock the doors, I think you can understand that the more people trying to get through the door then the faster the influx of people looking for bargains will be. As the day progresses and the overall rush slows down, the movement of people to and from the shop will slow down (this is like a diffusing perfume moving through the room, eventually the smell will get weaker and weaker).

  • The temperature (the higher the temperature, the more energetic the particles will be and so they will move around faster which means that the rate of diffusion will increase)

    The more energy a particle has, the more it will vibrate. You probably know from states of matter that particles inside solids, liquids and gases are vibrating and that if they have enough energy they can move apart from their "neighbours", in the case of solids they will be able to move sufficiently far enough apart to be able to slide around each other (this is when the solid starts to melt and become a liquid) and when the liquid particles have sufficient energy they can completely break away from their "neighbours" and move wherever they want to (this is when a liquid boils and turns into a gas).


  • The available surface area of the membrane that they are passing through.

    The bigger the surface area of the membrane, the more diffusion can take place in any particular time. If you think of the particles diffusing through a cell membrane as people inside a crowded shop with their bargains, now queueing up at the tills to pay so that they can leave, then you would understand that the more tills that are open (the larger the surface area) than the faster the "diffusion of people out of the shop" will be. This last point is very important when we start to look at diffusion of substances across cell membranes (exchange surfaces).


A similar type of diffusion, osmosis deals with the movement of water through membranes. A very important process for us as humans as we need water to survive. We'll move on to osmosis in another section.


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