What does exactly means "Small molecules may appear as solid, liquid, and gaseous phases without losing their molecular integrity"?,

I can't image how just a molecule can be a gas, liquid or solid. Or in fact the previous phrase refers to the state of aggregations of those small molecules?

My source is this article:

B Wunderlich, A classification of molecules, phases, and transitions as recognized by thermal analysis, Thermochimica Acta, Volumes 340–341, 14 December 1999, Pages 37-52

If you could suggest me a reference that extend the previous one to more modern states of matter like Bose-Einstein condensates, i would really appreciate it.

up vote 1 down vote accepted

Yep, you're right, we can only talk about the phase of matter if we have a collection of molecules. It wouldn't make any sense to talk about a single molecule as being a solid, liquid, or gas because what matters is how the kinetic energy of the molecules (related to temperature) compares to the intermolecular bonding energy.

Solid: KE << BE

Liquid: KE < BE

Gas: KE >> BE

I think the context that this sentence appeared in adds further clarity, so I've included it below.

Here's the full paragraph along with Fig. 3:

Fig. 3 - Classification of molecules

The key distinction between the three classes of molecules is summarized in the bottom portion of Fig. 3. Small molecules may appear as solid, liquid, and gaseous phases without decomposition, while rigid macromolecules keep their bonding to nearest neighbors (their molecular integrity) only in the solid state. Due to internal rotation, flexible macromolecules can attain sufficient intramolecular disorder to melt (or dissolve) without breaking strong bonds. This property is at the root of many of the useful properties of polymers (plastic and rubber–elastic behavior in addition to high strength, light weight, and low melting temperatures). The three classes of molecules are thus very distinct in their phase behavior. No macromolecule can be evaporated thermally without decomposition. If one tries to place flexible macromolecules into the gas phase by evaporation of small solvent molecules from a dispersion of droplets of a solution with only one macromolecule per droplet, the macromolecules become microphase particles and collect at the bottom of the container [9].

I don't have access to the article, but it sounds like a contrast with large molecules which can't exist in liquid or vapor phases because the heat involved breaks them into smaller molecules. For instance, if you heat the liquid protein inside of an egg, it doesn't boil to form a vapor — it denatures to form a solid.

Similarly, lightweight molecules might be unstable in a plasma, where the valence electrons can be stripped away. This is why molecular absorption lines are found only in the very coolest of stars — and even then the lines are from species like CO or CO+ rather than "heavy" species like CO2.

This interpretation doesn't tell you anything about low-temperature condensates. Sorry.

I don't have access to the article, but judging by the abstract, it looks like the author is saying saying that collections of small molecules can form solid, liquid, or gas phases without the molecules falling apart. $H_2O$ is an example of a small molecule that can be a solid (ice), liquid (water), or a vapor (steam). A large molecule, such as DNA couldn't be in a gas phase because the molecule would fall apart at high temperatures.

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