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Q:
I’ve been studying the phase diagram for water and haven’t been able to figure out why water remains frozen under a vacuum (<500mT) at room temperature in a lyophilizer. Samples are initially frozen prior to placing on the lyophilizer.
- Greg (age 13)
SPHS, CA
A:
The water won't stay frozen forever. It gradually sublimes (turns from
a solid into gas). If I remember my lyophilizers right from days in a
botany lab, the water doesn't really reach room temperature. It takes
heat (roughly the sum of the latent heats of the solid/gas and
gas/liquid transitions) to convert those water molecules to vapor. That
keeps the remaining water cold. That's the same effect that cools your
skin when water evaporates from it, only more so. So I believe the
water remains frozen because it remains cold.
Mike W.
(published on 10/22/2007)
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