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Hank: 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 actually 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 thing's 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, which is the 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 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, something that's 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 stay really cold liquids 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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