2ne1

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Q. Supercooled water at -10C can spontaneously warm to 0C in a perfectly insulated container. This is possible because

a. supercooled materials do not obey energy conservation laws
b. supercooled water needs less energy for change of temperature than ordinary water
c. a small amount of energy is given to the surroundings
d. some of the water freezes and this provides energy
e. ice is unusual in that it is less dense than liquid water

From a very reliable source....I didn't even know what supercooled water was before encountering this problem. Still, I don't get it. Anyone?
 

UndergradGuy7

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Well I didn't get the question that easily, but...

a is wrong because there is no reason they wouldn't obey the laws. Everything does

c is wrong because the problem tells you the container is insulated.

e is a true statement ice is less dense than water, but this has nothing to do with what the question is asking.

So this leaves b and d. The answer is d. In thermodynamics whenever you go from one phase to another like solid to liquid you have to give extra energy. For example, going from a liquid at 20C to a gas at 120C. Assume the boiling point of the liquid is 100C. To warm the liquid from 20C to 100C requires some energy. At 100C, the boiling point, a phase transition of liquid to gas exists. To change phases you need to add more energy. It takes quite a bit of energy to go from one phase to another. See here http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html or google another source.

So you have to add a lot of energy to go from liquid to gas. Going the opposite way from gas to liquid gives off energy. This will warm the container. Like in this example going from liquid to solid would give off energy.

The ice started to form probably because the system was disrupted and caused a piece of ice to form. This caused more ice to form. In a supercooled liquid if you add something to it, it will disrupt the system and cause it to form ice. This gave off energy as explained above.

This is more of a general chemistry question than bio as the topic says.
 
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2ne1

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The answer was d. Thanks for the clear explanation!:D