Wednesday, Feb 01, 2006 at 15:45
Hi Guys,
When you have ice (fresh or salty water), it takes approximately 2 joules (J) of energy (heat) to raise it 1C per gram (specific heat of ice). But to change its state from ice to water, it takes 335 J per gram (latent heat). Now when its a liquid, it takes 4J per gram (specific heat of water).
Now what does this mean? Assume we start at -20C and end up at 5C, when we have 1 gram of ice (no salt) we have:
20C x 2J x 1g (specific heat ice) = 40J
335J x 1g (latent heat) = 335J
5C x4J x 1g (specific heat water) = 20J
total = 395 J
when using 20% salty water (melts at -16C) :
4C x 2J x 1g (specific heat ice) = 8J
335J x 1g (latent heat) = 335J
21C x4J x 1g (specific heat water) = 84J
total = 427J or ~8% more energy
Thus the salty ice gives you ~8% more cooling for nothing. But the other big advantage of salty ice is that it stays at -16C for ~80% of the time (as opposed to 0C for ice) while the ice is melting (melting ice is what takes a lot of heat energy).
I hope this makes sense and explains the advantages of salty ice!
Cheers
Captain
(Chemical Engineer)
PS. Some assumptions have been made in the above to simplify the explanation, but only alters the final values by a few%.
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