View Full Version : Why do states of matter exist at all? Why not just a continuous range?
08-24-2003, 05:06 AM
Why are there solids, liquids and gasses at all? (plasma I can understand, with the electrons not being attached to the molecules). Why isn't there just a continuous range of states? Are there any compounds that *do* just have a continuous range of states?
08-24-2003, 11:20 AM
Well, I'm no expert, but here's some speculation.
There exists, actually, a point beyond which you can't tell which state something is in. As I recall, these are 'supercritical' states. There's a 'critical' temperature above which you can't tell the difference between liquid water and steam. It's a supercritical fluid with properties that are sort of like a gas and sort of like a liquid.
Here's a phase diagram (http://www.astro.uwo.ca/~jlandstr/planets/webfigs/matter/slide5.html) for water, with temperature on the horizontal axis.
Below the critical temperature, there's a difference between steam and liquid water. You know the properties of those two as well as I. I'm not really familiar with the properties of supercritical fluids, but this page (http://www.cfdrc.com/research/supercritical/) talks about them at the start.
Now, why is matter so contrived to fall into specific phases? I'm not sure. I do know that the molecules of a gas generally have much more energy on hand than if they were in a liquid state. Take water again... if you heat liquid water up, it gets warmer, but its physical propereties don't change a whole lot. At about 100 degrees Celsius (212 F) it stops getting warmer, even if you keep adding heat to it. For a while, it will take in all kinds of heat, and not change in temperature. All that heat is being used to excite molecules. And as the molecules get to a certain threshold of energy, they can escape from the intermolecular forces that hold them close to the rest of the molecules, and run around freely. They become a gas: they boil.
So, there is a threshold of energy that a molecules need to overcome the attractions to other molecules. Think of it in similar terms to 'escape velocity' for a spaceship. Below that energy, a molecule can't get away from its neighbors. With the requisite energy, it can overcome the molecular forces and do its own thing. When most of the molecules have this energy, the whole liquid has evaporated.
A similar thing happens when stuff melts from solid to liquid. Or, under very low pressures or other conditions where the liquid state is unstable, sublimate from solid to gas.
My guess is that it's just a matter of having the energy to break intermolecular forces of fixed magnitude.
But I'm not a professional.
08-24-2003, 12:37 PM
dolphinling: What if I asked you to describe these continuous state? How exactly would you propose a material that is not quite a liquid yet not quite a gas behave? This is just a thought experiment, but I think you'll see it'd be pretty hard to have any intermediate states.
Having said that, there ARE intermediate states. Look at something like Jello. Its a solid, but it can flow.
08-24-2003, 12:49 PM
This question seems to me more a philosophical one than one of the details of physics and chemistry.
The phases of matter are basic phenomena of our universe. Why that happens to be the case is probably noumena.
<gamer>Maybe our GM just rolled it up that way from the table of properties of matter in the GM's "Universe Construction Manual."</gamer>
08-24-2003, 12:59 PM
wolfstu hit it pretty much on the head.
If there was only one force that governed molecular attraction, then matter would behave very much like the "continuous range" speculated in the OP. In reality, intermolecular attractions (van der Waal's forces) consist of a number of different processes. The distinctions we see between states of matter are a result of a shift in dominance from one force to another as energy is added or removed.
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