Tales from the Prep Room: Argon Ice

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Andy attempts to freeze Argon.

The Ri's very own demo technician, Andrew Marmery experiments with the element Argon in the Ri Prep Room.

At room temperature Argon is an inert gas, requiring temperatures of below -189.3 degrees celcius to freeze into a solid. The temperature range at which Argon exists as a liquid is extremely narrow (three/ four degress). As it begins to melt under room temperature it passes into its gas state almost instaneously.

Tales from the Prep Room

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Andrew Marmery
London, UK
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Wrong needle, sorry.

Recently I've been a bit obsessed with argon the last couple of weeks because I wanted to find a gas that I could freeze. And I found - I settled upon argon because it's very easy to get hold of. You can use liquid nitrogen to liquefy and then freeze the argon.

This is my apparatus. First things first. Purge all the air from here with argon. It's a little bit denser than air, so it will just sit in here. I think the molar mass is about 40 compared to the air is about 29-ish. So yeah, another third as dense or something like that.

This flask is basically pretty full of argon. Just inflate the balloon a little.

Interesting facts about argon that I've uncovered over the last couple of weeks of my obsession. If you look at the periodic table, if you look at the molar masses of the elements, you find that argon is a bit of an anomaly in that it has a higher molar mass than potassium, which is the next element along. And generally, that doesn't really fit with the trend of increasing molar mass with atomic number.

The reason for this anomaly is that the molar mass of argon depends on the isotopic ratio, and the prevalence of all of different isotopes of argon on Earth. And interestingly, the ratio of those isotopes on Earth is very different to the ratio in the universe at large. Argon 36, which is the most common isotope in the universe, on Earth would be all the argon 36 has escaped from the atmosphere into space. Whereas, the Argon 40 is being continuously replaced by the radioactive decay of potassium 40 in the Earth's crust.

So actually this is all the argon that we have on Earth, and it's the third most abundant gas in the atmosphere. So it's an incredibly common element. All of it has come from the radioactive decay, basically, in the Earth's crust, which I think is very interesting.

I got a good 150 mils of or so of argon in there I reckon, which is a decent amount to be playing with. So the next stage, I'm going to freeze the argon in here. This is probably the point where I might start to need a glove.

Yeah, definitely freezing in there. It's looking better than the last batch I made as well. So fingers crossed, we'll let it freeze up a bit more and then we should have a nice little slug of solid argon.

I've frozen three lumps of argon in here. So we'll take one out and see what we can see. And then, I'll just pour this, it'll probably take a moment just to melt itself out of the tube and then just - there we go. That's very nice. So we'll see that this is melting before our eyes I think.

Argon freezes at about minus 190 degrees Centigrade, and boils about minus 187 or something like that. So it has a very narrow liquid phase. So within three degrees it goes from being a solid to being a gas. So this is the second I'm just pouring out. A bigger lump this time I think. Oh, there we go.

Because it has this very narrow liquid phase it just melts and boils away straightaway as it sort of disappears.

The vapour that you can see - this is just the moisture in the atmosphere that's condensing into the air that's being killed by the ice as it flows around it. But the liquid that you can see dripping off, that's the liquid argon that's melting from the ice, and immediately boiling as it lands on the surface of the cloth.

So there you go. Argon ice. A noble gas in solid form. I think that's pretty satisfying.

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