rdp@teddy.UUCP (08/12/85)
[]
There was, recently, quite a buit of flame going back and forth in
net.cooks over the old argumenty as to which freezes faster, hot or
cold water.
This discussion had a lot of nonsense like "my grandmother said", or
"you people who think this way are nothing but stupid fools" or "it's
so obviously counter-intuitive it's silly".
I thought I might post my contribution in this group as
well as it seems to have generated lots of favorable replies.
Well, after seeing all the flames about which freezes faster, cold or
hot water, I think I will throw in my two cents worth, backed up by a
carefully orchestrated experiment.
When I was in high school, the assertion was made that hot water freezes
faster than cold. As this was completely counter-intuitive, we decided
to perfrom an experiment to prove this incorrect.
We took two otherwise identical ice-cube trays made out of aluminum,
filled one with cold tap water (about 48 F) and the other with hot
tap water (about 125 F) and put them next to each other in a freezer.
Fully expecting the cold to freeze faster, we were amazed that the
hot DID freeze sooner!
WOW!
From there, we decided to win the Nobel prize in why hot freezes faster
than cold.
We tried different kinds of trays, different configuration, made sure
we use distilled water for all experiments, etc.
We did discover the following:
o When the trays are isolated from the walls or floor of the
freezer (by a block of styrofoam) the hot water seems to
freeze even faster (relatively speaking) than if it was in
direct contact (That shot our theory that the hot tray melts
into the ice, making better thermal contact, which was a
crock anyway since we were using a frost-free unit).
o When the atmosphere was very very humid, the difference was
almost nil, sometimes with the cold beating the hot to
freezing.
o Tall and narrow containers favor the cold freezing first,
flat and wide containers favor the hot freezing first.
Many experiments were done (taking nearly 6 months of time!) and it
wasn't until the end of things that a remarkable (to us) discovery
was made. Somebody decided to weigh the ice cubes that resulted.
In ALL cases where the hot water froze sooner, the ice cubes weighed
FAR less, sometime as little as a third the weight of those formed
from cold water! What happened?
Well, the hot water sat there, and got rid of the vast majority of
excess heat, not through convection or radiation or conduction,
but by EVAPORATION! So the temperature very quickly dropped to the
same as the water in the cold tray, BUT, by that point, there was
much less water left to freeze in the hot tray, so all things being
equal, the lesser amount of water froze faster. This was confirmed
by trying to freeze hot and cold water in otherwise identical sealed
containers. As expected, the hot water took forever to freeze.
Yes, it is really true, in the environment of many freezers, hot
water can freeze faster than cold, in normal, shallow ice cube trays,
and sometimes, the hotter the water, the faster it freezes (due to
greater loss of material) BUT the freezer is going to work a bit harder
(because when that vapor refreezes on the cooling coils, it's going to
have to release all that energy it absorbed evaporating)
However, rest assured that hot water DOES boil sooner than cold water.
So much for obvious counter-intuition.
Dick Pierce