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Author Topic: How to freeze stuff really cold.  (Read 2294 times)

Offline Salado

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How to freeze stuff really cold.
« on: 06/01/2013 14:39:13 »
How do you freeze for instance: Nitrogen to liquid?


 

Offline Salado

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Re: How to freeze stuff really cold.
« Reply #1 on: 06/01/2013 14:40:37 »
Also, is it possible to freeze down things to like -100's of celcius? I just wonder how you do all this.
 

Offline evan_au

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Re: How to freeze stuff really cold.
« Reply #2 on: 06/01/2013 15:26:49 »
To liquify Nitrogen:
  • If you compress a gas (like air), it gets hot. You can then let it cool down to room temperature to give you compressed air.
  • If you expand a gas (like compressed air), it gets cool.
  • If you do this enough times and keep the results properly insulated, you can get liquid nitrogen, oxygen and argon out of the air.
  • See http://en.wikipedia.org/wiki/Liquid_air#Preparation

To freeze things quickly
  • Dropping something in liquid nitrogen will quickly cool it down to the boiling point of Nitrogen: 77.36 K, -195.79 C, or -320.33 F. Liquid nitrogen is fairly cheap, and chemically inert.
  • To freeze things really cold, you need a refrigerant like liquid helium, which is rarer than air, much more expensive, and much harder to keep cold. The boiling point of Helium is 4.22 K, −268.93 C, or −452.07 F

Note: Always use gloves and eye protection when dealing with ultra-cold gases,  and work in a well-ventilated area.
 

Offline Salado

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Re: How to freeze stuff really cold.
« Reply #3 on: 06/01/2013 15:37:14 »
Many thanks, mate!
 

Offline CliffordK

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Re: How to freeze stuff really cold.
« Reply #4 on: 06/01/2013 21:05:57 »
Also, is it possible to freeze down things to like -100's of celcius? I just wonder how you do all this.
As Evan mentioned, you can get temperatures down to the boiling point of liquid helium 4He at 4.2K (-268C) or liquid helium 3He at 3.2K (-269C) relatively easily using compression/expansion cycles.  Perhaps a little lower if one uses a vacuum during the expansion cycle.   

The limit, of course, is absolute zero, 0K, or -273.15C.  Researchers have gotten mighty close, but it is technically impossible to get below that temperature.

Some substances such as propane can be liquefied in a single step, I believe by bringing the pressure up to about +130PSI at room temperature.

Then the temperature is allowed to equilibrate.

Then, if you quickly release the pressure, some of the propane will boil until the temperature of the liquid reaches its boiling point, -42 C, at which point it, the boiling will slow down, and it will maintain that temperature.

This is also why liquid nitrogen doesn't boil away all at once, and can actually stay in liquid form at normal atmospheric pressure for quite some time if kept in a well insulated container.

By the way, most household refrigerators work by repeating these compression/expansion cycles.
« Last Edit: 06/01/2013 21:15:18 by CliffordK »
 

Offline AndroidNeox

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Re: How to freeze stuff really cold.
« Reply #5 on: 13/01/2013 01:40:49 »
And, as was mentioned on the program recently, it's possible to cool something to the point where it (sort of) has a negative temperature. The definition of zero degrees Kelvin includes both the free energy and the entropy of the system. By cooling the sample to near zero (liquid Helium can get you pretty close) and then applying a magnetic field can force the atoms to orient themselves along the field... adding order and heat to the system. Then, the sample can be cooled back down to its former, near-zero temperature. By releasing the magnetic field you can leave the sample in a condition where it doesn't have enough free energy to return to its disordered state. To get the sample into the condition defined by zero Kelvin one would have to add energy. So, in a sense, it's even possible to cool something to less than zero degrees K.
 

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Re: How to freeze stuff really cold.
« Reply #5 on: 13/01/2013 01:40:49 »

 

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