In this YouTube video it argued that:

  • Solids have constant volume, and constant shape
  • Liquids have constant volume, but flexible shape
  • Gases have flexible volume, and flexible shape (in fact they don't have volume and shape at all)

So, this table came to my mind: enter image description here

Do we have a phase of matter that fits in the red cell? In other words, do we have a phase of matter that keeps its shape, and only changes its volume? If so, what is the name of that phase?

  • $\begingroup$ Start by defining what these terms of mean, shape and volume... $\endgroup$ – Zhe Jan 4 at 13:26
  • $\begingroup$ Shape is quite a straightforward term but it is not a property of matter per sé. Replace it with the strength of the intermolecular forces responsible of shape and you will see that the red cell has not reason to be. $\endgroup$ – Alchimista Jan 4 at 13:45
  • $\begingroup$ Your definitions are problematic except as very broad approximations. Consider a rubber band: it is clearly a solid, but also clearly not a substance with a constant shape; less clearly, but still true, it doesn't have constant volume either. $\endgroup$ – matt_black Jan 4 at 13:57
  • 2
    $\begingroup$ Doesn't the shape change with volume? $\endgroup$ – lee Jan 4 at 15:43
  • $\begingroup$ You can't really retain shape and change volume in a significant way (certainly not in a comparable way to a gas). In any case, the properties in question are akin to compressibility and Young's modulus. $\endgroup$ – Buck Thorn Jan 4 at 16:59

There is a mathematical argumentative approach called reductio ad absurdum...i.e., bring your own logical reasoning to the point where the result is an absurd statement if you accepted the original statement as true. In this case you can accept that there is a phase which can change its volume but keeps it geometrical shape intacr. You can try disproving your own statement using this method for solids, liquids, and gases.

Do we have a phase of matter that fits in the red cell? In other words, do we have a phase of matter that keeps its shape, and only changes its volume? If so, what is the name of that phase?

a) When solids experience stress, there is strain = deformation, change in volume.

b) Liquids are almost incompressible and they have no shape, but under extreme extreme pressures their volume does change, typically, > 100,000 psi they can become solids. The solid will have the shape of container in which it became a solid.

c) Gases acquire the shape of the container (actually a bad statement, i.e., gases occupy the space provide by the container), and their volume can be easily changed by stress.

d) The fancier state of matter, plasma, it is also an ionized gas, so it does not have a geometrical shape. However, it can be confined by using magnetic fields so that it does not touch the walls of the container, and perhaps its "volume" and "shape" can be manipulated independently. A true physicist can better answer it.


Note that none of these characteristics is absolute—with enough pressure change, all phases of matter have measurably varying volumes. And with enough force, all phases of matter have varying shape.

Rather, these are general characteristics that provide guidelines to distinguish solids, liquids, and gases. And these general characteristics are best understood by seeing how they result from the microscopic differences between solids, liquids and gases.

The following is a simplification, but it's an appropriate simplification for a beginning student of chemistry:

Atoms/molecules in solids and liquids are very close together, so they can't be readily compressed. The atoms/molecules in gases are far enough apart that they can be readily compressed—all you're doing is taking up the empty space. Atoms/molecules in solids are rigidly connected. Atoms/molecules in liquids and gases aren't. Thus the shape of solids is fixed, while that of liquids and gases is not.

Now we can ask: What microscopic properties would give you constant shape, but varying volume? Well, in order to have varying volume, you'd need the atoms/molecules to be far apart (like in a gas). This means they're disconnected from each other, and moving approximately independently. If they're disconected from each other, they can't hold a structure, and thus can't have have a fixed shape.

A gas contained within a perfectly uniform spherical balloon could have fixed shape but varying volume as you change the external pressure, but that's a property imposed by the balloon and the surroundings; it's not a property of the gas itself.

Granted, a perfectly uniform solid (a solid whose properties are identical in all directions; the word for this is "isotropic") could have constant shape and varying volume if the pressure around it were increased uniformly, and sufficiently (e.g., let it sink to a sufficient depth within the atmosphere of Jupiter). But then it wouldn't fit into your table, which assumes solids have fixed shapes and volumes.


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