Finn

Worth adding what you get instead of a liquid above the critical temperature, because it is genuinely interesting rather than a consolation prize.

Push past both the critical temperature and the critical pressure and the substance becomes a supercritical fluid. It is not a gas and not a liquid — it fills its container like a gas, dissolves things like a liquid, and there is no boundary or meniscus anywhere because there are no longer two distinct phases to separate.

The practical importance is real. Supercritical carbon dioxide is used industrially as a solvent, most famously for removing caffeine from coffee beans, precisely because it penetrates like a gas and dissolves like a liquid.

So the phase diagram does not end at the critical point. It stops having a line there, which is a different thing.

The cylinder observation is what makes this concrete for me — I had noticed that propane gauges are useless and never connected it to anything.

There is also a hard computational reason, which is the one I find most satisfying.

A reaction is not two molecules meeting once. It is an enormous number of molecules colliding in every orientation, at a distribution of energies, with solvent molecules involved, exploring a landscape of possible arrangements. The number of dimensions in that landscape scales with the number of atoms, so it explodes almost immediately.

Physics gets clean answers where it can isolate a small number of degrees of freedom. A thrown ball is essentially one object with a handful of relevant variables. Chemistry rarely gets that luxury — even a modest organic molecule has dozens of atoms, and the interesting question is about the transition state, which is a saddle point on a surface you cannot draw.

This is why computational chemistry is a large field that gives genuinely useful answers and still cannot simply be asked "what will this produce".

One more coincidence in the same family, since we are collecting them: the prefix "pro" in propane has nothing to do with the Greek "pro". It is a truncation of propionic acid, whose name means "first fat" — it was the shortest acid still showing fat-like behaviour.

So the first three alkanes are named after wood, air and fat respectively. It reads like a joke and it is simply what happened.

Worth separating two different strains that get muddled here, because your model kit felt fine for a reason.

Angle strain is what you get from forcing bond angles away from where they want to be. Cyclobutane already has this — its corners are near ninety degrees where carbon prefers about 109 — and it copes, at an energy cost.

The linearity requirement of cumulated double bonds is much more severe. You would need to bend something that wants to be 180 degrees down to 90. That is not a strained molecule, that is a different bonding arrangement entirely.

Model kits are built to be forgiving because otherwise nobody could assemble anything. The plastic joints flex, so they will happily let you build molecules that would cost hundreds of kilojoules per mole. Building it proves the connectivity is imaginable, nothing more.

The cleanest way in is to look at the smaller constant hiding inside it.

R is just Boltzmann's constant multiplied by Avogadro's number. Boltzmann's constant is the fundamental one, and what it does is convert temperature into energy. That is its whole job. It says how many joules of molecular kinetic energy correspond to one kelvin, for a single particle.

R is the same statement scaled up to a mole instead of a particle. So:

• Boltzmann's constant: energy per kelvin, per molecule.
• R: energy per kelvin, per mole.

That reframing answers your "property of gases or of the universe" question directly. It is neither a property of gases nor an accident of units — it is the exchange rate between our temperature scale and energy, and it looks like an arbitrary number only because the kelvin was defined from water long before anyone knew what temperature was made of.

That is exactly the right takeaway — once you see it as a conversion factor, the fact that it turns up wherever temperature meets energy stops being surprising and starts being expected.

There is a further wrinkle worth knowing: di- and bi- do not mean the same thing, despite both suggesting two, and confusing them causes real errors.

In the older naming that survives on shop shelves, bicarbonate does not mean two carbonates. That prefix carries a historical meaning about acid salts. Meanwhile dichloride does mean two chlorines.

So the prefixes in chemical names are not a single consistent counting system. They are several systems from different eras layered on top of each other, and the vowel question you asked is one of the more harmless places where the layers show.

There are also non-metals sitting in that band worth knowing about — several common inorganic salts melt in the 500 to 700 region, and some hydrated ones lower.

Whether any is available at home depends entirely on what your home contains. But if you have access to a lab supplier rather than only a hardware shop, the gap stops looking like a gap. It is an artefact of restricting yourself to materials that turn up in everyday objects, and everyday objects have no reason to sample that temperature range.

The answer is yes, but only slightly, and the reason your searches kept flipping the question is itself the explanation.

Salts and metals are both good solvents for their own kind of thing, and they are poor solvents for each other. Molten salt is ionic, so it dissolves ionic compounds readily and dissolves metals to a small and interesting extent. Liquid metal is metallic, so it dissolves other metals readily — that is what an alloy is — and dissolves salts only slightly.

So the two systems are not symmetric in interest. Metal-in-salt is technologically important, so it is measured and written about at length. Salt-in-metal is a small number that mostly matters as a contaminant, so it is measured in narrow contexts and reported in places that do not surface in a general search.

The short version of what would actually happen: mostly it sits there, with a small amount going into solution and a large amount forming a separate layer, usually floating because salts are typically less dense than the metals you named.