There’s a lot of ways that the word ’fluid’ has been used in science, and I’m really not sure which one you’re looking for. So I’ll try to hit as many as I can think of. The most common use of the word ’fluid’ is to mean the same thing as the word ’liquid’ (as opposed to solids or gases). You can find lots of information on liquids by doing a search of our answers list.
Another definition of ’fluid’ includes pretty much everything that’s not a solid. That is, it includes both liquids and gases, things that fill the shape of their containers.
It’s interesting that sometimes it makes sense to call something a fluid even though it wouldn’t make any sense to try to say specifically whether it was a gas or a liquid. Normally, when you take a liquid and you heat it, it will eventually boil, turning into a gas. This temperature is called the ’boiling point’. However, if you increase the pressure as well, the boiling point actually increases, too. This is why people cook foods in pressure cookers, since the higher temperature speeds the cooking.
But that doesn’t go on forever. If you keep raising the pressure higher and higher, you will eventually reach a point where the boiling point just ’disappears’. This is called the ’critical pressure’. The boiling point at this pressure is called the ’critical temperature’, and together they make the ’critical point’. Above the temperature of the critical point, you still have a fluid. At low pressure it’s similar to a typical gas, with the molecules not touching each other much.. At high pressure the molecules are crowded in and interact a lot, like in a liquid. But since there’s no sharp change from one to the other, it wouldn’t mean anything to try to pin down whether to call the medium-pressure fluid a liquid or a gas.
You can see what I’m talking about by looking at a phase diagram. The lines in a phase diagram tell you the boiling and freezing points for a substance at all different pressures. In this diagram, Tc is the critical temperature and Pc is the critical pressure. You can see that above the critical point, the line between liquid and gas just stops.
(republished on 07/25/06)