In this lesson, we are going to learn about
diffusion of gases. This will help you to
understand how gases behave and also explain
why smells spread.
Before we start, you need to recall what solids,
liquids and gases are like in terms of their
particles. In particular, I'd like you to
ask yourself, how are the particles arranged
in a gas? Pause the video and restart it when
you think you have the answer.
The answer is: The particles in a gas are
spaced out and move about very quickly in
random directions. Did you get it right?
This, of course, explains why a gas takes
the shape of it's container. It's because
the particles will each continue to travel
in a straight line until they hit the wall
of the container. So, the smelly gases given
off by rotting vegetables in a sealed box
will fill the box with rotting vegetable smell,
but you won't be able to smell them outside
the box.
However, take the lid off the box, and those
particles will no longer bounce off the lid
and be contained. They will start to escape
into the air around the box. You'll soon notice
the effect and will be able to smell the rotting
vegetables; that's those gas particles reaching
your nose by diffusion!
Diffusion is the random movement of gas through
another, from a region of high concentration
(in this case, inside the box) to a region
of low concentration (in this case, outside
the box).
It is a process that doesn't rely on wind
or air currents; it will happen even if you
are inside a building and the air is completely
still.
There are many examples of diffusion; but
smell is probably the most obvious one. These
could be food smells, body smells from other
people, the scent that a plant gives off to
attract pollinating insects. The same even
happens with the smoke from a candle after
it has been extinguished - it spreads out.
It's all due to diffusion.
Here's challenge for you. If someone walks
into a room with some fresh flowers, why does
it take a while for the smell of the flowers
to fill the room? Why can't you smell them
straight away? Pause the video and restart
it when you think you have the answer.
Just as the gas particles can hit the walls
of a container and rebound, the particles
can also hit each other. It doesn't matter
if there is only a single gas or a mixture
of gases, these collisions between particles
are always taking place. It's very similar
to trying to walk in a straight line through
a busy crowd of people - it's very difficult
to do without bumping shoulders with someone
and changing direction.
This explains why it takes a while for a smell
to get from one part of a room to another.
The high concentration of gas (or smell) particles
takes time to spread out due to the collisions
with the other gas particles in the air.
And that... is diffusion.
