The entire universe is made of tiny little particles all whizzing round each other, and banging into one another, and vibrating wildly at incredible speeds. Of course when you look at Dad sitting on the sofa in front of the telly with a can of beer sitting on his belly it may be hard to believe that all this activity is going on, but it is, right there, even in his body.
All of this movement can be distilled down to a single generalised ‘thingie’ called Brownian Motion. Brownian Motion is essentially the continuous random movement of atoms and molecules in anything that allows atoms and molecules to move around (i.e. not a solid, but something like a liquid or a gas or fizzy drink for instance). This motion is caused by all the molecules jiggling around and banging each other around. Molecules generally jiggle as a rule anyway (because mass is energy and energy is mass and mass is therefore jiggling and without jiggling you do not have mass....ask Einstein, I don’t want to explain this right now).
Anyway...Brownian Motion. It may sound dull and pointless, but it is this movement that causes DIFFUSION for instance, the stuff that dictates that when you fart on one side of the room, it is inevitable that the people at the other end are eventually going to smell it and start asking questions.
To illustrate more clearly what Brownian motion is like, please assemble the following:
some butter or other slippery substance
a polished floor
a few hundred tennis balls
Now grease your hamsters feet thoroughly and place it on a polished floor, and let it slip and slide around for a moment to get its bearings.
Get all of your mates, give them a big armful of tennis balls and arrange them all around the edges of the polished floor.
Now get all of your mates to pelt your hamster viscously with the tennis balls.
You will note that for every time that Hammy gets hit, he will start to skid off in the direction the ball was going in as it hit him. As he gets hit by another ball from a different angle, he gets sent careering off in a different direction. If he hits any balls just lying on the floor, he will send them spinning off in another direction with his path being deflected until he is pelted again.
This is what life is like as an atom (or molecule). Every single atom is being pelted wildly all the time by lots of other atoms, being pushed and shoved every which way hundreds of thousands of times a second. Its like living in the middle of New York City Underground in Rush hour for every minute of your life. It must be hell.
Now all of this motion is what causes things to move around, like hammy on the floor, or a fart particle journeying to one of your work colleagues nostrils, or any other atom in any other vaguely fluid substance.
Oh, you can retrieve your hamster now, revive him from his concussed state and give him a handful of seeds as a reward.
Now you may have noticed when you were pelting hammy that his movement around the floor was quite a slow process. Meanwhile the progress of a fart particle is really quite terrifyingly fast. This all comes down how fast your particles are jiggling, and this is controlled by two simple factors
Density - The amount of particles per square whatever bouncing each other around.
In terms of your greased hamster on a slippery floor, imagine you had more tennis balls lying around on the floor, or a slightly smaller room so effectively there were more balls per square foot in the air at any one time. In this case, there would be more things to bounce hammy around so his sliding around the floor would be more random, HOWEVER, his progress across the slippery floor would be slower because there would be more to bump into.
THIS is why if you drop blue ink into a glass of water, it takes ages to mix itself in, and yet why a fart particle takes only a few seconds to cross a room. In water, there are more atoms per square whatever, so it takes longer for any atom to get anywhere because it is being bumped into so much. In the air, there are less atoms to bump into, so your fart atom can cross a room pretty darn quickly.
Temperature. The amount of energy that the particles are given to jiggle around with.
Again in terms of your greased hamster, imagine that you had the same number of balls as before, and the same sized room, but your friends threw the balls much quicker. Now hammy is sliding really quickly around the room, and though he is going to bump into the same number of balls on average as he did the last time, it will be in a shorter space of time, so obviously he is going to get to wherever he is going quicker.
THIS is why if you drop blue ink into a glass of HOT water, it mixes quickly. Because fart particles take such a short time to travel across a room anyway, you probably WOULDN’T see the difference if you warmed the temperature of the room up, but you can see how this is very important with denser things like liquids.
All of this is called the "Kinetic theory of matter" - describing the physical properties of matter in terms of the movement of its component atoms. It dictates that if you warm things up, the particles jiggle more, if you cool things down they jiggle less until you cool things to a temperature called Absolute Zero when things stop jiggling all together.
As gases are incredibly difficult things to weigh, count and generally measure - plus they are tricky swines in terms of working out what temperature and volume they are...Kinetic theory has formed the basis for a whole bunch of theories which use molecule jiggle and related stuff to quantify gases, including Avogadro's hypothesis (stating that equal volumes of all gases, when at the same temperature and pressure, have the same numbers of molecules), Boyle's law (stating that the volume of a given mass of gas at a constant temperature is inversely proportional to its pressure, e.g. if the pressure of a gas doubles, its volume will be reduced by a half, and vice versa), and Charles's law (stating that the volume of a given mass of gas at constant pressure is directly proportional to its absolute temperature in kelvin). This all comes down to same basic logical principles you would observe whilst pelting a greased hamster on a slippery floor....but people tend to get scared by the names....
Bet you never thought about all THAT when you last farted....