I know that at very large scales, solids behave like liquids, which is why planets are round, rather than knobbly. I also understand that you can pour some gases, and, if I have this right, that the air around the Earth is like a like an additional layer of liquid, but very much lighter. You can still be crushed by enough air.

This makes me wonder, are the differences between solids, liquids, and gases a human distinction that is merely useful under certain common circumstances, or are they actually inherently different concepts?


In the case of liquids and gases, at least, there's no fundamental difference. To see this, take a look at Wikipedia's phase diagram for water.

enter image description here

Ignore the dotted lines for the moment, and note that the line between vapour (steam) and liquid stops at a certain point, called the critical point. What this means is that if you go through the following sequence of steps, you can turn liquid water into steam without ever noticing a sudden change:

  • increase the pressure above the critical point (about 218 atmospheres), keeping the temperature constant at a relatively high value, but less than $100^\circ C$

  • keeping the pressure constant at $>218$ atm, increase the temperature to above about $374^\circ C$

  • reduce the pressure back down to 1 atmosphere or below

  • reduce the temperature to somewhere closer to $100^\circ C$.

By doing this, you go from the region marked "liquid phase" to the region marked "gaseous phase" (or "vapour"), but you go around the critical point, thus avoiding going through the phase transition. So you never see the liquid boil, it just gradually changes its density until it becomes what we normall call a gas.

Now, you'll notice that as you do this you actually pass several of those dotted lines, so that you start with a liquid, then get a compressible liquid, then a supercritical fluid, then gas, then vapour. But these are, as you say, "human distinctions" - there's no directly observable change in the fluid when you pass the dotted lines, they just represent definitions that people sometimes find it convenient to make.

So for the liquid-gas transition, it's reasonable to say that although the phase transition (boiling or condensing) is real, measurable thing, the distinction between liquids and gases is not.

However, for liquids and solids there tends not to be a critical point - there's no path from one to the other that doesn't go through the phase transition. So in this case there's more motivation to say that they really are distinct. Though having said that, if you consider mixtures of several compounds it can get a bit less clear-cut. Some substances (perhaps most) can be a solid on small spatial scales but a fluid on larger ones - the Earth's mantle being a good example.

So basically I think you're right. In some cases the distinctions are real, and they're very often useful distinctions to make, but at a really fundamental physical level it's much more meaningful to talk about phase transitions than the reality of states of matter in themselves.

  • $\begingroup$ There is something of the oxymoron in your " much more meaningful to talk about phase transitions than the reality of states of matter in themselves." How can one have transitions from vaguely defined phases? The fact that there are similarities between some behaviors of two phases does not mean they can not be well defined, imo. They are defined by the dissimilar behaviors. $\endgroup$
    – anna v
    Jul 4 '13 at 4:02
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    $\begingroup$ Actually, it doesn't seem like an oxymoron. Brings to mind a cliff with what seems to be a clear line between the upper and lower ground, but if you walk the long way around, you find a nice smooth slope leading from one to the other. So there can be both a clear transition and a fuzzy one, allowing the existence of a vague definition, in spite of the clearer boundary. $\endgroup$ Jul 4 '13 at 6:44
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    $\begingroup$ @annav what AlbeyAmakiir said. The phase transition between steam and water is a genuine phenomenon that can be observed in any laboratory or kitchen, but the fact that you can turn steam into water without a phase transition implies that they are not fundamentally distinct things. The line on the phase diagram represents an observable phenomenon, but it does not divide the phase space into two distinct regions because it stops at the critical point. $\endgroup$
    – Nathaniel
    Jul 4 '13 at 9:18
  • $\begingroup$ Wondering if there is no real distinction between liquids and gases even from hydrostatics point of view (compressibility, stresses etc.). Do all the hydrostatics laws apply to liquids and gases in the same way? I know this may be worthy of an independent question, but just fleeting remarks/pointers from experts would be helpful here. $\endgroup$
    – manisar
    Jul 15 at 5:09
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    $\begingroup$ @manisar at everyday temperatures and pressures, for many purposes, water can be treated as incompressible whereas steam is compressible and to some degree of approximation follows the ideal gas law. So from that perspective they're clearly different - it's only once you start getting to crazy high temperatures and pressures that the distinction starts to become blurry. However, for liquid CO2 the critical point is around room temperature (at about 73 bars of pressure), so if you were working with that I guess you'd apply the hydrostatic laws to the liquid and the gas in the same way. $\endgroup$
    – Nathaniel
    Jul 15 at 6:16

Is there a difference between solids, liquids, and gasses?

The above is the original heading of the question and this answer is adressed to that question.

The above is a question from somebody who has very limited knowledge of physics. Do you know that all matter is composed of atoms and molecules, which are combinations of atoms?

These are tiny particles, much smaller than the dust motes you see in sunlight, and have interactions between them which depend on the proximity they have to each other and the varying attractive forces they feel with each other, and on macroscopic thermodynamic measures like temperature and pressure and the universal gravitational force.

Here are what the various states of organization that atoms and molecules have :

phases of matter

Solids are tightly packed like lego constructs, held together by attractive electromagnetic forces, called generically Van der Waals forces . Their temperature is due to vibrations in place . As the temperature goes up the kinetic energy of each atom gets larger, but still the attractive forces exist between them and we get the liquid phase. With higher temperatures the kinetic energy dominates and the atoms/molecules are free to bounce around as a gas state. Even higher and electrons are kicked off from the collisions between atoms/molecules and we get plasma, where there are electrons and ions, a fourth state.

A solid behaves as one body under forces, including gravitational ones, although it may display some elasticity. A liquid follows the equipotential of the gravitational field A gas is much freer from the gravitational field since it can move in three dimensions giving up a bit of kinematic energy against gravity. Effects of gravity can be seen over very large volumes, for example light atoms/molecules going to the top of the atmosphere and heavier at the bottom.

These differences in the behavior of matter have been quantified, and the transition between phases is called a phase transition.

phase transition

and for different types of atoms/molecules the phase diagram exists

phase diagram

A typical phase diagram. The dotted line gives the anomalous behavior of water.

So the phases of matter are well defined and there exists a mathematical theory, thermodynamics , that describes their behavior very well.

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    $\begingroup$ This is actually a question from someone who knows all that, but has noticed inconsistencies, and wants to know the nature of the definitions, rather than the definitions themselves. I have updated the question title to make it clearer. $\endgroup$ Jul 3 '13 at 23:00

Solid, liquid and gas are well-defined states of matter in nearly all circumstances. That planets are round is not an indication that solids are just very slow-moving liquids. Planets actually get very hot inside during formation, and the solid mineral material melts. It acts liquid because it is liquid.

Glaciers are harder to explain. They're solid, but we've all seen photos from the arctic and antarctic of beautiful flows of ice through huge rocks. Parts of glaciers actually melt, and the solid parts can crack, melt, refreeze. (I'm not expert enough on ice to go into any detail.)

Gas can be poured? Well, a dense gas (or cold gas) can pour and settle in the bottom of a container otherwise filled with a less dense or hotter gas. It's all still gas, but we're looking at more complex interactions requiring concepts of energy, entropy, and equilibrium. Through in diffusion and heat transfer, too, just for fun. Gases can do interesting things - like make all kinds of weather.

Liquids go to the bottom of a container, while gases fill the whole thing. But put several substances together, and you have to allow for differing densities, solubilities, and so forth. Still the distinctions hold up. Solid, liquid and gas are well-defined.

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    $\begingroup$ Your explanation of why planets become round is quite wrong. A solid cube of cold steel the size of the Earth would become a sphere, not because it would melt but because steel is not strong enough to withstand the gravitational forces involved. It would deform continuously under the gravitational stress, satisfying the definition of a fluid. $\endgroup$
    – Nathaniel
    Jul 3 '13 at 8:29

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