(I know it's not astronomy, but this is the best science board I know of...)
We were having a discussion in chemistry class today, and I wanted to see what you guys thought about it:
Temperatures lower than twenty-eight degrees F damage grape vines. Grape growers often spray water on their plants before a night when the temperature is expected to drop below freezing to keep the grape plants from being damaged, and the technique works.
We've agreed that the water on the outside of the grape will freeze before the grapes themselves will. I hold that because of this, the grape will maintain a temperature higher than freezing for a longer period of time: the cold takes longer to get to it since it has to freeze the water outside the grape first.
I'm told that I'm not totally correct. In order for ice to melt, it must absorb a heat of fusion. Thus, when ice forms, heat must be released. While some of this heat travels to the cold air outside, some of that heat, I'm told, travels inside to warm the grape.
I have a lot of difficulty buying that. As I understand, when the water freezes, heat has to be removed from it; the difference between removing and giving off heat is important. When something radiates heat, that heat travels in all directions, so in this case, if the water radiated heat when it froze, it'd certainly warm the grape. However, since the heat is just removed, the heat is transferred only to the cold air outside. By the definition of "cooling something off," the heat must be transfered, not radiated, and so if the air "cools off" the water, then shouldn't the heat only go to the cold air? And isn't that part of the second law of thermodynamics, anyway?
So what's correct? More importantly, can anyone give me something authoritative that explains whatever's correct so I can finally put this argument to rest? We've been going back and forth, and without some authoritative source, I don't think we'll get anywhere.
Thanks a lot...