When we think of an exotic state of matter we tend to think of the really weird things that matter can do in extreme circumstances - like how at very high emperatures we get the plasma that the sun is made of, or at extreme densities we get the nuclear matter of neutron stars.

Or in extreme cold we can have superconductors and superfluids.

But there’s one state of matter that’s not solid, liquid or gas, but is also not confined to extreme or rare environments.

In fact, there are planets in our solar system completely covered with this stuff, and you’ve often been the beneficiary of the powers of this state of matter without even knowing it.

I’m sure you know how states of matter work.

Heat a block of ice and it melts into liquid water.

Keep heating and it boils into gas - water vapor.

We’ve been over this stuff before.

If you watched our episode on states of matter you’ll know it’s a bit more complicated than this.

For example, it’s not just temperature that determines the state of matter, it's also pressure.

The reason solids melt and liquids boil is that rising heat energy allows the bonds between atoms and molecules to break.

But high pressure helps to keep particles together so that it takes more heat energy to break bonds.

That’s why water remains a liquid at 400 degrees celsius near hydrothermal vents on the ocean floor, while it boils at only 68 celsius on top of Mount Everest.

The relationship between temperature, pressure, and phase is mapped on a phase diagram.

Now, maps tend to be covered by nation states or states of the union, but this map has states of matter.

And these also have boundaries, the crossing of which means changing state - say, by increasing temperature through solid to liquid to gas.

At these melting and boiling boundaries the material is momentarily a mix of both states.

But there are some surprises here too.

For example, at low pressures there’s a boundary where increasing temperature takes you directly from solid to gas - we call that sublimation.

And there’s the so-called triple point, where all three states can exist simultaneously, From the triple point, if we increase both temperature and pressure in the right proportions we follow the phase transition boundary between liquid and gas.

If we’re careful, our fluid remains in an equilibrium state, simultaneously boiling and condensing.

But eventually we reach this spot, where the boundary appears to end.

It's almost like the map gives out - as though our explorer experimentalists haven’t traveled that far across the realms of pressure and temperature.

But that’s not the case.

This end point of the liquid-gas boundary is called the critical point, and it’s very real.

And beyond it is an entirely new state of matter - the here-be-dragons of the phase diagram.

This is a land with no boundaries - a sort of no-man's land where liquids can skirt around the phase boundary and become gasses without ever boiling.

In that region, matter becomes a sort of hybrid between liquid and gas called a supercritical fluid, sharing properties of both.

But this is a genuine, unique state of matter that can do things possible for neither liquid nor gases.

What does it even look like to transition between states without ever crossing a phase transition boundary?

Or to be a combination of liquid and gas?

Well, I’m going to show you.

But first, to understand what we’re seeing, we need to remind ourselves of the fundamental properties defining liquids and gasses.

To start with, both are fluids - they flow.

They can’t hold a rigid structure like solids can, rather they take on the shape of thxeir container.

A key property differentiating liquids and gases is their inclination to change their own volume or density.

Liquids, for example, are remarkably difficult to compress in volume by the application of external pressure.

That enables their use in things like hydraulic presses, where they can be used to exert enormous outward pressures.

We say that liquids are incompressible.

In some cases they're less compressible than their own solids.

And the volume of liquid doesn’t change even in the absence of external pressure.

The particles of a liquid are loosely bound to each other, which manifests as a surface tension and results in liquids having a distinct surface.

That pulls liquids into spherical blobs in freefall or in the absence of gravity, while under gravity they take the shape of the bottom of their container.

While pressure doesn’t influence liquid volume, temperature does.

Liquids expand with heat.

Gases on the other hand will always expand to occupy the entirety of their container, and are also relatively easy to compress by exerting pressure.

Gas particles zip around without significantly interacting with each other.

They keep moving until they hit a container wall.

Those impacts exert a pressure on the container.

Expand the container and the gas will expand, but it’ll take longer for each particle to travel between walls, so pressure drops; vice versa if you shrink the container.

Increase temperature and the gas particles move faster, hit harder, again increasing the pressure.

For a so-called ideal gas, which has no inter-particle forces, its behavior can be described by a simple relationship between pressure, temperature and volume given by the Ideal Gas Law: PV=NkT.

OK, that’s about enough blah pressure temperature blah blah.

We know what we need to know, to take a journey to that strange new land of the phase diagram.

Let’s see what it looks like for a liquid to pass into the supercritical phase.

We’re going to do that with the help of Nigel Braun of the NileRed and NileBlue YouTube channels.

I suggest you watch the entire video yourself, but with Nigel’s kind permission we’ll show you the highlights.

This is a pressure chamber containing dry ice - carbon dioxide in solid form.

That ice was presumably created below the 194.7 Kelvin freezing point of CO2 at atmospheric pressure.

Now at room temperature it begins to sublimate directly into a gas.

At atmospheric pressure, liquid CO2 can’t exist, but in the small volume of the chamber, sublimation causes pressure to rise, taking us into the realm of the phase diagram where liquid CO2 is possible.

That liquid fills the bottom of the chamber, while the top of the chamber is mostly filled with CO2 gas at a much lower density.

To reach supercriticality, we need to get to this part of the diagram, so both pressure and temperature need to increase.

Conveniently enough it’s possible to get there just by adding heat, and Nigel does that with a hairdryer.

As the temperature rises the pressure of the gas increases a proportional amount, but this temperature is also causing thermal expansion in the liquid, which reduces the volume of the gas, which further increases the pressure all around.

At first we might think this pressure has no effect because liquids are incompressible, but look at the phase diagram: At higher pressures the boiling point of CO2 increases, which means all this pressure makes it harder and harder for the liquid to evaporate Now we have a sort of feedback loop where the liquid wants to transition to a gas, but the more it tries the less it is allowed to do so.

We remain stuck following the transition boundary, upwards in temperature and pressure, trapped in our state by thermodynamics.

But we’re approaching our strange new region of the phase diagram, where the rules are going to change Let's talk about density for a moment.

Since the gas is being compressed but its mass is not changing by much, its density must increase.

Meanwhile the liquid is expanding in volume, so its density drops.

Eventually the density of the gas and the liquid become the same.

At that point, microscopic droplets of remaining liquid are free to flow and swirl through the gas phase to diffuse.

We’re now at the critical point.

And then all the CO2 goes supercritical, and appears to vanish - it becomes perfectly transparent.

From its visual appearance the supercritical CO2 could easily be a gas.

And really, it is a lot like a gas.

It fills its container and is compressible, and doesn’t have the surface tension of a liquid.

The viscosity of supercritical fluids is extremely low, so they flows and diffuse more like a gas than a liquid.

But it has the density of a liquid, and that leads to behaviors not seen in gases, which I’ll come back to.

The high density of a supercritical fluid means that its particles do interact with each other, unlike an ideal gas.

That means the supercritical equation of state is much more complex than the ideal gas law.

To demonstrate the hybrid nature of this stuff, Nigel put some silica beads into the chamber.

On shaking the chamber the beads move only slightly, like they’re under a liquid.

But now with only a gas in the chamber they rattle around as expected.

Finally, let’s reverse this.

When ice is applied to the chamber, temperature drops and CO2 liquid rapidly condenses from the supercritical phase.

On the other hand, if the chamber is opened then pressure drops, and the supercritical fluid transforms straight into a gas.

Perhaps the most useful property of the supercritical fluid is its ability to dissolve stuff.

Due to its high density compared to a gas, it has more atoms or molecules to bond to the dissolved substance.

But it still moves like a gas, so it can flow and diffuse into places that liquids can’t access.

One very familiar application of this super-solvent is decaffeinated coffee.

If you put coffee beans into a bath of supercritical CO2, the fluid will diffuse into the beans like a gas, and bonding with the naturally soluble caffeine and then diffusing out again.

Then the CO2 can be brought back to gas form, leaving a decaffeinated bean and a fine powder of caffeine molecules - I guess for the energy drink that you need after your decaffeinated coffee didn’t work.

Here's another one.

The lowest density solids in the universe, Aerogels are made the same way.

A gel is a molecular scaffold full of water.

If you try to dry the gel it shrinks.

That’s because when water evaporates from the surface, capillary action draws water from the interior.

That causes an inward tension on the scaffold, and so it collapses.

But if you put the gel in a supercritical CO2 bath, that fluid will replace the water and then you can turn the CO2 back into a low-density gas phase, leaving the gel lattice intact and full of air.

These aerogels have a ton of applications, but by far the most interesting is to send it out into outerspace to catch cosmic dust, as in NASA’s Stardust mission.

What else.

Ever had something drycleaned?

Again, supercritical CO2 is used to dissolve stuff - in this case whatever’s making your clothes filthy - but without the risk of getting your precious, I dunno, hand-dyed silk pyjamas wet with actual water.

There are countless applications of supercritical fluids, even beyond the many applications as a solvent.

For example they’re increasingly important in materials science for their ability to deposit dissolved elements for growing nano-scale particles and layers.

They’re great at moving heat energy around due to their high density and corresponding heat capacity, so we are seeing more and more application as the working fluids in power plants and heat pumps, and as refrigerants.

Industrial applications often use supercritical carbon dioxide, but supercritical hydrocarbons also see a lot of use, and supercritical water is important also.

It's good for all those things you want water for - electrolysis, hydrolysis, oxidation, but when you want to avoid actual wetness, or you need it to flow and diffuse like a gas.

OK, good for us - we discovered supercritical fluids and you can now sip your decaf latte in your nicely drycleaned silk pyjamas.

But Nature knew about this state of matter long before we did.

Unlike other exotic states, supercritical fluids can be found on Earth.

Or, rather, in the Earth.

Water circulating deep below ground in geothermally active areas can reach the high temperatures and pressures needed for a supercritical state, and even, albeit rarely, emerge in that state in hydrothermal vents on the ocean floor.

It’s rare on Earth, but if you lived on Venus you wouldn’t think of the supercritical fluid as a fringe state of matter at all.

The Venusian atmosphere is almost entirely carbon dioxide, creating a greenhouse effect that heats the surface to more than 700 K. Combined with the extreme atmospheric pressure, this puts CO2 within a few kilometers of the surface well beyond the supercritical point.

We often think of Venus as a rocky planet, but it’s also fair to think of it as an ocean world - an ocean of supercritical fluid that I don’t recommend you try swimming in.

Jupiter, Saturn, and probably the other gas giants are also supercritical ocean worlds in a way.

They have solid cores of rock and ice, in the case of Jupter and Saturn surrounded by solid metallic hydrogen, and then a thick layer of supercritical fluid - mostly hydrogen.

All of that is buried beneath extremely thick atmospheres of gas-phase mostly-hydrogen.

But the supercritical layer is an integral part of the structure of the biggest planets in the solar system.

So, let’s review all of the states of matter.

Solids liquids gases plasmas, bose-einstein condensates and the resulting superconductors and superfluids; nuclear matter of various types; photonic matter; various spin-based states from ferromagnets to quantum spin liquids to time crystals; come to think of it, let’s not review all the states of matter, because I’m starting to think there may be no end to the weird ways that matter can be.

Many of these are total edge cases, but some, like supercritical fluids, are incredibly and increasingly useful.

And while this stuff is rare on this mostly wet and gassy rock, supercritical fluids are surprisingly abundant in other parts of space time.