If you follow our sister channel SciShow Space, you might remember something that we pointed
out recently:
Even the coolest places on the Sun’s surface are hot enough to melt every compound that
humans have ever found, created, or even predicted.
And it turns out that there's a whole field of scientific research devoted to finding
the compounds with the highest melting and boiling points -- meaning they’ll stay solid
or liquid even if it's super hot.
Scientists are also pretty sure that they've already found the substance with both the
lowest possible melting and boiling points, plain old helium, which really likes to be
a gas.
But before we get into why that is, let's talk about why things melt and boil in the
first place.
A things’ melting point, where it goes from a solid to a liquid, is mostly determined
by how much its particles, the atoms or molecules that make up the substance, attract one another.
That's because solids are made up of particles that are held together very rigidly, but in
liquids they're much more loosely attached.
And at higher temperatures, particles have the energy to move around more.
So, our mystery substance will have a higher melting point when the attraction between
its particles is stronger; they need the extra energy of a higher temperature to move around
enough to turn into a flowy liquid.
And when the attraction’s weaker, the melting point is lower.
The same thing is true for boiling points, when something goes from a liquid to a gas.
If you keep heating the atoms or molecules until they have enough energy to totally escape
each other's attraction, they start rocketing around as a gas.
But pressure's important too, because it forces the particles closer together -- to the point
that they bond and form a liquid or solid again.
That's why we generally compare melting and boiling points at a constant pressure -- usually
standard atmospheric pressure, or about a hundred thousand pascals.
And helium has the lowest ones because its atoms attract each other extremely weakly,
so it doesn't take much energy to separate them.
In fact, helium doesn't even /have/ a melting point at standard pressure.
It can never be a solid -- not even as you approach absolute zero temperature, where
the atoms wouldn't moving around at all.
You have to put helium under 25 atmospheres worth of pressure in order to get it to be
solid, and even then you have to cool it down to just 1 Kelvin: 1 degree Celsius above absolute
zero.
But it does have a boiling point at standard pressure. Helium’s boiling point is about
4 Kelvin, the lowest of anything that we’ve ever found.
And there’s no reason to think that record will be broken any time soon.
Helium is one of the only elements that we’ve never seen bonded to anything else, both because
of how weakly it attracts other atoms and because it's what's known as a noble gas,
it has just the right number of electrons that none of them are available for bonding.
It has the lowest boiling point of any element, which is easy to test. And any full-fledged
molecule, because it’s a group of atoms already attracted to each other, is going
to attract other molecules more than helium does, giving it a higher boiling point.
So helium’s records seem to be safe -- and are also incredibly useful.
See, things that tend to stay liquid when are really good at ... keeping things cold.
Some of the world's fastest trains, for example, float over the tracks, but they need to be
really cold -- supercooled, in fact -- for that to work.
Engineers do that by flowing a ridiculously cold liquid like helium around the tracks.
And lots of complicated equipment, like particle accelerators, needs to be kept very cold to
work.
But hot liquids and solids can be just as useful, which is why researchers are hunting
for the compounds with the highest melting and boiling points ever.
We already know that tungsten has the highest melting point of any pure element, melting
at just under 3700 Kelvin at atmospheric pressure.
But we’ve found compounds that stay solid /way/ above that.
In 1930, chemists discovered that an alloy called tantalum hafnium carbide has a melting
point of about 4200 Kelvin.
That’s over 500 Kelvin hotter than tungsten, the hottest we know of -- at least, when it
comes to compounds we've actually /made/.
Because in July 2015, a team of researchers published a paper in the journal Physical
Review B predicting an even /higher/ melting point for a compound made of nitrogen, hafnium,
and carbon, 4400 Kelvin.
The authors found it by simulating how molecules attract one another and just trying a bunch
of different compounds until they found the one that would melt at the highest temperature.
It’s much easier to do that sort of thing in the computer than in the real world, where
you have to, you know, make the actual compounds.
They haven't actually tested their prediction by making the compound yet, though.
As for boiling point, a compound called tungsten carbide holds the current record.
It boils at 6273 Kelvin, which IS actually higher than the temperature of the surface
of the Sun!
That's why we can't say that the Sun's surface is hot enough to boil everything we know of.
Not only do these compounds tend to stay solid at super high temperatures -- they're also
very hard to break.
So compounds with high melting and boiling points are often used in places that need
to be solid and unbreakable even when it's hot -- like inside engines and as heat shields
for spacecraft.
These kinds of compounds with really high melting points were just right for the job.
And unlike helium’s record, compounds with higher melting or boiling points might still
be waiting to be discovered.
We just need to keep looking.
And burning stuff, just making it real hot!
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