I think the important thing here is that liquid nitrogen in the lab
tends to be exactly at its boiling point, since the temperature is
maintained by continuously boiling off some of the N2.
This means the only mechanism for heat absorption is through vaporization,
depending on the latent heat of vaporization rather than the heat
capacity. So as soon as any heat is removed from the object, some gas
is formed, and the gas layer insulates.
Propane is chilled with LN2 to near its freezing point, so it can absorb
quite a lot of heat before any vapor is formed. If you spill some on your
hand you will have a nasty burn immediately, whereas you can usually get
away unscathed with splashing LN2 on your hand briefly.
Petr Leiman wrote:
yes you are right, but I assumed if people see a cloud of condensed
fog over their LN2 bath they should remove that by
a) filling up the bowl completely e.g. some LN2 drips out of the bowl
b) blow the fog away before you dip
I think the original poster meant the relatively low heat conduction of
liquid N2, which causes boiling around the crystal immediately after
plunging.
The best way to freeze things is to put a small container of liquid
ethane or propane into a liquid N2 bowl, and plunge into the
ethane/propane (this methods was suggested earlier).
Petr
