Liquid nitrogen stored in tanks

[boink]

Hi firstly and this might make my question title wrong, but when liquid nitrogen is stored in a "gas" type tank is it still liquid? and therefore still cold?

I just wondered as in terminator when the mercury man becomes frozen then hit and he explodes into thousands of pieces, yet if metal became this brittle on cantact with liguid nitrogen surely the metal tanks would also be very fragile? or are the containers special?

[kempie]

I believe that it is commonplace for liquid nitrogen to be stored in double-walled vacuum containers (the Thermos flask principle).

The three main dangers when storage is compromised;
� the extreme cold (burns & frostbite)
� rapid rate of expansion (high pressure build-up & explosion)
� if the gas boils off it is non detectable by humans (asphyxiation).

An example of when it goes wrong...
http://www.hse.gov.uk/comah/sragtech/caseliqni tro92.htm

[jake-the-peg]

It is and as kempie says the dewars are effectively large thermos flasks.You certainly don't want to put your hand in it but it's not as dangerous as all that. For example you can put a mars bar in on the end of a piece of string and hit it with a hammer to share the pieces - happy days!

Liquid Helium however is a very different beast. This is only liquid a few degrees above absolute zero. If you were to pour it into a dewar it would solidify the air in the neck, making an air tight seal. It would then continue to boil within the flask until something gave way!

[brachiopod]

Note that nitrogen has to be kept at low temperature to remain liquid, hence the use of double-walled vacuum containers.

Compare this with carbon dioxide, which can be liquified by the application of pressure alone, without the need for lowering the temperature. (think of CO2 fire extinguishers, or the CO2 gas cylinders they use in pubs - these are all at room temperature, but are under high pressure).

In fact, liquid CO2 cannot exist at atmospheric pressure, no matter how much you lower the temperature, as it changes directly from a gas to a solid.

If you put liquid nitrogen into a gas cylinder like CO2, and allowed it to get warm, it would not remain a liquid, but would turn back into a gas within the cylinder, at a very high (and extremely dangerous) pressure.

[chakka35]

jake-the-peg is quite right to say that you would be stupid to put your hand in liquid nitrogen. But I once put my finger in some for a short time, on the grounds that the nitrogen which evaporates into a gas around the finger insulates it from the low temperature for a short while.
It's useful for getting rid of chewing-gum deposits - especially on carpets. Pour the liquid N2 onto the gum and then hit it with a hammer. It shatters into pieces which you can then sweep up. Fascinating stuff - the nitrogen not the chewing-gum.