eli5: Why are some molecules a liquid, others a gas and some are solids?

Question

For example, at room temperature H2O (water) is a liquid, CH4 (methane) is a gas, NaCl (salt) is a solid, why?

What are the properties of a molecule that determines its state?

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Answer 1

The size of the molecule/atom and the forces between them.

For example, it requires very high temperature to vapourise most metals, because most metal atoms bond very well with eachother. They’re not molecules, but I’m just illustrating the point.

A lot of molecules which are gaseous, like O2, N2, CO2 etc. don’t liquify or solidify until very low temperatures because they’re both light and they don’t attract eachother very much. It’s very easy for them to be knocked about by other molecules.

Then you have two cases:

Light but strongly interacting molecules. For example H2O is lighter than the above three gasses, but it interacts very strongly with itself and other molecules. It’s a dipole, that means it has a noticeable positively and negatively charged ends. And electrostatic forces are strong. So water should be flying freely, but can’t, because other water molecules hold onto it.

Other end are molecules which interact weaker, or maybe not at all, but are heavy. A lot of organic molecules fall into this category. Many interact weaker than water but are heavier than it, so they remain liquid in similar circumstances. Then some are so heavy that they can’t evaporate at all, and will burn if you apply enough heat hoping to force them.

The same goes for things that aren’t made from molecules, so interatomic bonding. Some metal bonds are relatively weak, like mercury or sodium. Some metallic bonds are extremely strong. There’s no such thing as a molecule of NaCl either, it’s a crystal of electrostatically attracted ions, and they’re bound fairly strongly, so their melting point is high. But these bonds allow for phase transitions because they’re in a way “flexible”. They’re non-directional and allow for “stretching”. You can have a liquid of atoms kept together with ionic and metallic bonds.

The one exception, really, are covalent bonds. They’re the strongest bonds and they are very stiff and ordered. Those are the bonds that make molecules. Something is either covalently bonded or it is not, there’s no equivalent of “liquid” here. You can have a liquid OF molecules, but if you try to break the molecule itself that’s no longer a phase transition, that’s a chemical reaction.