Scattering
General physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass.
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 something, in this case 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 one 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, have a think 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 or a market place - 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.