# A simple way to make liquid oxygen at home

I have an idea to make liquid oxygen at home, but without any calculations, I am not sure whether it will work at all. The idea is based on the fact that gases tend to liquefy easily in higher pressures.

So here is what I plan to do:

1. Get a soda bottle, and install a bicycle valve on the cap.

2. Pressurize the bottle using a bicycle pump as much as possible.

3. Put the bottle in the freezer.

4. Take out the bottle from the freezer, and pump in more air because keeping the bottle in the fridge would have lowered its pressure.

5. Repeat a few cycles until no more air can be pumped in, without increasing the pressure excessively.

6. Finally, release the valve. Some liquid gases should have formed in the bottle.

• There is no way this will work. Just look up the pressure and temperatures required to liquefy air. The cooling you get in the freezer is insignificant. And the pressure you get with a bicycle pump is also insignificant. You could try to liquefy other gases like chlorine.
– nasu
Dec 13, 2020 at 19:45
• Commercial production of liquid air requires multiple cycles of compression, heat removal, and expansion. en.wikipedia.org/wiki/Hampson%E2%80%93Linde_cycle But, your title said liquid oxygen. If you want to separate oxygen from the nitrogen and from the traces of other gasses present in air, then you will need additional process steps it. Dec 13, 2020 at 21:41
• @DavidWhite yes that is what I was looking for, please put your comment as an answer, thanks a lot. Dec 14, 2020 at 7:11
• Why must both the critical temperature and pressure be exceeded to achieve the supercritical phase? See link below: chemistry.stackexchange.com/questions/37088/… Dec 14, 2020 at 7:22

## 3 Answers

The critical temperature of oxygen is -181.5 deg F. If your freezer can't produce a temperature somewhat below this, it will be impossible to liquify oxygen, regardless of how much pressure is involved.

• sorry i just realised that your reasoning is a little bit faulty, you are not considering the cooling that would occur when I release the pressure inside the bottle (cooling due joule-thompson effect) or correct me if I am wrong Dec 14, 2020 at 16:09
• You are using a batch process, which means that you open your bottle one time to achieve cooling. The Joule-Thompson effect will not be able to cool the contents of the bottle by 150-200 deg F in "one shot". Industrial processes do indeed use the J-T effect, but they highly compress air first, cool it to ambient temperature with a heat exchanger, THEN expand it to obtain low enough temperatures to liquify it, in a continuous process that uses recycle loops as necessary to achieve cryogenic temperatures. Dec 14, 2020 at 16:36

the fact that gases tend to liquefy easier in higher pressures. (correction mine)

There is such a thing like critical temperature. You may compress oxygen however you like and it will not become liquid unless it is cold enough (like, ~154K). You have to cool it WAY lower in order to make it liquid in the pressure range of the bicycle tyre pump.

This is not to say you cannot liquefy some other gas, if you pretty much want to. Pure carbon dioxide or propane can both be made liquid at household freeser temperatures and bicycle pump pressures.

This is not to say you cannot liquefy air in a somewhat advanced garage workshop. It will not be pure oxygen, but you can further try to rectify it into nitrogen and oxygen (still pretty much available in the garage workshop frame). Well, you'll need to use some more sofisticated equipment than the tyre pump and you'll have to learn a lot more.

And then, liquid oxygen is QUITE dangerous (as in: causing frostbite, making other materials break, explode or self-ignite, making everything around much more flammable and its vapors - pure gaseous oxygen - are toxic themselves).

What exactly you need it for?

I was a cryogenic technician in the marine corp. I assure you, you will not make liquid oxygen in a bottle in the freezer. You must compress air,remove the moisture,cool the air repeatedly through a regenerator, remove impurities,through a side bleed gel trap,swirl it through a phase separator,then filter through a fractional distillation process. All inside a generating plant with columns and matrix material , pall rings, and a product sub coolers. ….so there. That’s a very condensed version of how it’s done.