Re: How would I flash freeze a ten foot cube of water?

David, thanks for your question.  Freezing a room full of water is an 
interesting thought.  I don't see any reason why you could not freeze water 
with liquid nitrogen.  I did a couple of quick "back of the envelope" 
calculations to see how reasonable it would be.  You said in the question 
that the room was 10 feet cubed.  That means there is 1000 cubic feet of 
water.  By making a couple of assumptions, I calculated the amount of 
liquid nitrogen necessary to freeze your room of water to be about 6000 
pounds (3 tons).  That is a very large canister of liquid nitrogen.  A 
concern with your hero using that much liquid nitrogen is that after all of 
the nitrogen evaporates, there may not be enough oxygen left for him to 
breath.  

Freezing this much water will require the removal of a tremendous amount of 
heat, which makes it somewhat difficult.  Another idea for cooling and 
possibly freezing the water would be to use the same principles that are 
used for the instant cold packs that athletic trainers use to treat 
injuries.  In these packs, a simple endothermic chemical reaction occurs 
that removes heat from the liquid.  (Endothermic reactions require heat, 
while exothermic reactions give off heat during the reaction.)  Several 
chemicals that have been reported to work for these applications are sodium 
acetate, ammonium chloride, sodium nitrate, sodium thiosulfate, potassium 
iodide, calcium chloride, and ammonium nitrate.  All of these are supposed 
to work with water.  (I found these in US patent #3977202, issued to 
Johnson & Johnson, New Brunswick, NJ.)  Some of the chemicals are pretty 
common:  calcium chloride is used as ice melt and ammonium chloride is a 
fertilizer.  

I don't know how rigorous you want to be with this topic, but I should warn 
you that there are potential mixing problems that will arise during the 
reaction that would stop the reaction.  If enough salt is added to freeze 
water, the water closest to the salt will freeze, effectively creating a 
protective layer around the salt and stopping any further reaction.  If the 
system was well mixed, this problem could be reduced.  

I hope I have helped with your novel.  It may simply require a little 
literary license to freeze the room of water.