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Name: Scott M.
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: Sunday, February 23, 2003


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?


Replies:
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


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


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


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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