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! Thanks for watching this episode of SciShow,
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There is no spoon.