# What's the difference between liquid and gas?

This seemed like an easy question, but the more I think about it, the more stumped I am. What’s the difference between liquid and gas? I realize that the simple answer is “heat”, but why?

Let’s take water. I get that when it is cold, it sticks together, and forms something that appears to be rigid. Fine. Heat that up, and it breaks apart, and suddenly the molecules can all roll around independently as a liquid. Still fine. Apply more heat and now…what? They’re still in basically the same state, it seems. How come all of a sudden, they float up into the air and no longer dissolve salt and fish can’t breathe it and so on?

Steam and liquid water seem like they should be more or less the same to me, molecules that are not sticking together. Educate me as to why that isn’t the case.

Well, a quick and dirty physical answer is:

Solid - definite shape, definite volume
Liquid - no definite shape (i.e - conforms to container, flows along surface, forms a ball under microgravity), definite volume
Gas - no definite shape or volume, conforms to container, takes up as much space as is available, likewise forms a ball in microgravity

Why the transition you ask? Quick and dirty again, thermal energy. At some fixed point, the boiling point, the attraction molecules used to have, has been overwhelmed by molecular motion.

There’s your problem. Heat up ice, and the molecules don’t break apart. They remain bonded and in contact, and form crystals over the short range. But the crystal boundaries move around, and molecules can slide over each other without losing contact.

When water goes to steam, the weak bonds between molecules are broken, and molecules fly apart to enormous distances.

Heat is a factor, but so is pressure in the atmosphere (higher pressure atmospheres would create a higher boiling point). Plus in water you have the hydrogen bonding playing a role. Molecules like methane which are roughly the same size as water do not have hydrogen bonding and boil at -161C as a result.

Either way the kinetic energy in the molecules makes it hard for the molecules to exist in a water state. They bounce off each other too hard for them to exist in that state. That is basically what much of it comes down too, the more kinetic energy in the molecules (via heat) and the fewer forces holding them together (hydrogen bonding or pressure) the more unstable they become. Keep in mind that absolute zero is -273C, and water boils at 100C so that is actually a 373C increase in heat from absolute zero. The energy has to go somewhere, and the molecules cannot stick to each other.

If you increase atmospheric pressure you can heat water to 300F and it still will not boil because the molecules have nowhere else to go.

Think of a lotto machine, with the ping-pong balls all bouncing around: That’s a gas. Now turn off whatever’s adding energy to them to make them bounce around, and let them all fall to the bottom of the container, where the pile-o-balls conforms to the shape of the container: That’s a liquid. Now cover all the balls with glue and let it set, so even if you took away the container, the balls would stay stuck together in the same shape: That’s a solid.

The best definition I know is kind of…ambiguous. Hahahaha. As the temperature and pressure of a saturated liquid vapor mix are increased towards the critical point the specific volume of the saturated liquid (all liquid at the saturation temp and pressure) and the saturated vapor (all vapor at the saturation temp and pressure) get closer and closer until they are the same (w00t you’ve reached the critical point). At this point you cannot distinguish any difference between the liquid and the vapor, they both have the same properties. I’ve seen a video of this process, but can’t find it. Basically the meniscus that you see in a liquid/vapor interface disappears. It’s pretty cool.

Lest you think some of the definitions are a bit ambiguous, know that a lot of important physical properties vary significantly with the liquid to vapor transition.

Here’s a quick and dirty example:

Constant pressure heat capacity of water, at 1 ATM, temp varies from 80 C to 120 C.

Reference - NIST Webbook. Requires Java VM.

Notice the huge step change at about 100 C. For a lot of physical properties, the liquid-vapor transition is anything but ambiguous.

edit - gotta pick Cp on the drop down, as the link goes it shows density. Internal energy is another good value to look at to see the step change. For Internal Energy, the huge step change represents the energy you need to put in to accomplish said liquid to vapor transition.

I was talking about the critical point transition in particular not the difference at 100 C & 100kPa. The question was, what’s the difference between a liquid and a vapor.

Ah, my bad. I wasn’t necessarily calling you ambiguous, more the standard high school texts that just say “liquid - molecules have more order. Vapor - molecules have less order.” I was responding more to the OP though.

You are correct about critical points though - there is no longer any definition between liquid and vapor.

One of my professors once said that, if you didn’t have the physical examples in front of you, and you were developing the ideas of states of matter from the known behavior of the constituents, you’d come up with a gas – essentially noninteracting particles far from each other – and you’d come up with a solid – particles forming crystalline structures based on interacting forces and packing – but you probably wouldn’t come up with a liquid. Liquids have the particles close and interacting, but free to slide around each other. They can overcome the attractive forces enough to not be packed tightly, but not enough to be free from each other, and the aggregate mass is held down by gravity. Liquids do have short-range structure, especially in the vicinity of a forming crystal.

Incompressible newtonian fluid dynamics does not distinguish between liquids and vapors, they are both treated as fluids. The properties are different but the behaviour is the same.

If it’s incompressible fluid dynamics, doesn’t that pretty much rule out gases anyway?

Not in low speed external flows, where “low” means Mach numbers of less than about 0.3. Compressibility effects are generally small enough to neglect in such situations.

Can anyone show an example derivation of a transition point? I think that would help me understand the factors involved.