Can we compress gas into a solid?

James - This is an interesting question. What it’s getting at is whether we can bypass the middle step matter usually takes when being converted from a gas to a solid. That middle step is, of course, the liquid phase.

Usually, when the temperature of a gas is reduced to the point where its molecules or atoms come together, they form a liquid. This is known as condensation. Similarly, when we reduce the temperature of a liquid to the point where its molecules and atoms come even closer together, they form a solid. This is freezing.

We do observe this bypassing of the middleman in the case of gaseous water vapour crystallising into ice - or frost - and this process is called deposition. But your question, Akula, specifies whether we can physically compress a gas into a solid, ignoring temperature as a factor. I’m going to need some help to get to the bottom of that one. Luckily, I’ve recruited science communicator and friend of the show, Dave Ansell, to help me provide an answer…

The short answer, Akula, is yes, but your material may not be the same when you reduce the pressure after compression. On a small scale a solid is just a load of atoms or molecules sticking together. If you compress a gas very quickly, applying a lot of pressure, there will always be some atoms leaving into the gas and some sticking back on. The higher the temperature, the more atoms will leave every second, and the higher the pressure the more will join. So when you increase the pressure more atoms will join than leave and you will build a solid, though there will always be some atoms left behind in the gas phase.

It gets a bit more complicated if you are at a pressure and temperature that a liquid can exist, but with most liquids if you squash them they will eventually freeze. Even water ice which can melt when you squash it, has some phases of ice that are denser than water that form under extreme pressures of over 10 000 atmospheres so it will work eventually.

Another complicating factor is that if you compress a gas or most liquids they will get hotter, which makes it harder to form a solid, but if you push hard enough they will eventually solidify.

So if you squash hard enough pretty much anything will form a solid but with a lot of materials if you apply these kind of temperatures and pressures to them they may well be altered chemically so they may not come out the same as they started.

So Akula, yes indeed a gas can transition into a solid without first becoming a liquid, although this is rare. Manufacturing to conditions under which we could compress gas into a solid is even more difficult, but could be possible. Thanks for sending that in.