Ask A Scientist Chemistry Archive

name         Scott M.
status       other
age          30s

Question -   If dry ice sublimes at -78 Celsius, and it's then
dropped into a glass of water, why doesn't the water freeze?  If I had enough
dry ice, could I freeze the water?
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Scott,

Yes. Try it and see. If the piece of dry ice is large enough to enable the
water to cool to its freezing point, liquid water will freeze into a crust
of ice on parts of the dry ice surface.

Regards,
ProfHoff 564
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Yes, dry ice will freeze water.  If you drop a piece of dry ice in water,
you will see that after a while it gets a coating of ice around it.  The ice
coat will form sort of a chimney at the top end where the escaping carbon
dioxide gas bubbles out.

The reason that putting one chunk of dry ice into a glass of water doesn't
freeze the whole glass of water is that more heat is released by freezing a
whole glass of water than is absorbed by vaporizing the chunk of dry ice.
If you put a glass of water into a box of dry ice, I guarantee that the water
will freeze.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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 > IT DOES, AND YOU CAN, but the process is more complex. First, the
evaporating CO2 produces strong convections that keep the water away from
the surface of the dry ice. The vapor also "insulates" the dry ice from the
surrounding water. Second, the CO2 vapor also keeps bringing in warmer water
and sweeps away colder water. This is made more complicated by the maximum
in the density of water at about 4 C. Third, when ice forms on part of the
dry ice surface, the ice acts as an insulator slowing the flow of heat from
the warmer water to the colder surface of the dry ice. Fourth, even when
encapsulated with ice the evaporating CO2 vapor keeps breaking the surface
ice.

If you have a large enough piece of dry ice, a shell of ice does form and
grows slowly. You can see this happening because the vapor cloud produced by
the evaporating cold CO2 tends to get smaller with time.

Vince Calder
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When you drop the dry ice into the water the first thing that happens is a 
layer of water ice forms around the dry ice.  Because the dry ice is 
subliming violently this layer of water ice will be somewhat porous and 
allow carbon dioxide to escape, taking the cold along with it -- and 
forming a nice fog around the glass.  The water ice insulates the dry ice 
from the heat of the liquid water so that the rate of sublimation of the 
dry ice slows.  You loose a lot of the heat capacity of the dry ice to the 
vapors that come off and rise rapidly to the surface of the water.

Given a sufficient quantity of dry ice you could freeze a glass of water 
by dropping the dry ice into the water.  But you would be more successful 
if you put the dry ice and the water into an insulated container.

Greg Bradburn
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