• Question: Could you freeze air?

    Asked by BethBeebeedaniel to Francesca, Laura, Matthew, Andrew, Rebecca on 16 Nov 2014.
    • Photo: Matthew Camilleri

      Matthew Camilleri answered on 16 Nov 2014:

      Freezing means that you cool the atoms to a temperature where they form a solid. This can be done, although the temperature would have to be very low, together with high pressures. This is very difficult to achieve, and normally, gases in the air, like nitrogen are transported as liquids, since this would be cheaper to make.

    • Photo: Laura Schofield

      Laura Schofield answered on 17 Nov 2014:

      In theory, yes. The air is made up of 5 major compounds: nitrogen, oxygen, argon, carbon dioxide and water. They can all be frozen and water (below 0°C) and CO2 (below -78°C) can be quite easily achieved. Oxygen (below -218.8°C) and argon (below -189.4°C) can be achieved too but to get frozen nitrogen you have to cool it to below -210°C while it is under vacuum. If it wasn’t under vacuum the temperature would be too low to reach to freeze the nitrogen.

    • Photo: Andrew McKinley

      Andrew McKinley answered on 17 Nov 2014:

      I’m sure you could freeze air. Nitrogen will freeze at -210 ºC, and oxygen will freeze at -218 ºC. Assuming the mixture is perfect (i.e. chemically perfect – meaning that the molecules do not interact with each other), it should be quite possible to freeze air.

      Helium is a liquid at -269 ºC, so putting air through liquid helium will freeze it. However, helium is a precious resource – and we’re running out! Helium comes from natural gas – it can’t be rescued from the air, and as soon as it is released it will escape our atmosphere in seconds. Helium is also vital for running our NMR machines and our MRI scanners in hospitals. Soon we’re going to have to make a choice: pretty helium balloons, or healthcare and research…

      For information, helium costs around £300 per litre for liquified helium. Compare this to liquid nitrogen, which costs about £1.50 per litre (or, to put it another way, the same price as milk!)

    • Photo: Francesca Palombo

      Francesca Palombo answered on 17 Nov 2014:

      Of course you can – the problem is whether it would be affordable…during a flight you will have noticed the crystals formed on the internal surface of the double glazed window: I think of it as frozen air although it is actually ice formed from condensation of water vapour from the air.

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