Water freezing and emitting heat?

[starwiz]

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.

 

Why?

 

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.

 

So what's correct? Can anyone give me something authoritative that explains this so I can finally put this argument to rest?

 

Thanks a lot...

-Starwiz

[-Demosthenes-]

When things get cold what they are doing is losing heat. So when water freezes the heat just transfers into the freezer, becuase it's so cold.

So when things get warm they gain heat, when they get cold they lose heat.

Am I right?

[chemistry]

Notice that the grapevine will decrease in temperature. Water covering the grapevine will be in direct contact with it and more heat will thus be transferred through this route then through transfer to air. This is why when you wish to cool a substance more effectively you place it in water rather then expose to air of the same temperature. Hope this helps.

 

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[starwiz]

Originally posted by -Demosthenes-

When things get cold what they are doing is losing heat. So when water freezes the heat just transfers into the freezer, becuase it's so cold.

So when things get warm they gain heat, when they get cold they lose heat.

Am I right?

That's what I thought.

 

Notice that the grapevine will decrease in temperature. Water covering the grapevine will be in direct contact with it and more heat will thus be transferred through this route then through transfer to air.

What you're saying is true, but it doesn't apply in this circumstance. Water has a very high specific heat. That means that it takes a long time for air outside to cool it to a freezing point. So once the water cools, the grape will cool more quickly than if it were in air of the same temperature, but until the water cools, the grape will stay warmer. Since the frost is (in theory) over in the morning, the ice isn't there forever, and so the idea is that the time it takes for the ice to freeze more than makes up for the increased cold-transfer between the ice and grape.

 

Still, my question stands...any other ideas?

[JaKiri]

starwiz said in post # :

What you're saying is true, but it doesn't apply in this circumstance. Water has a very high specific heat. That means that it takes a long time for air outside to cool it to a freezing point. So once the water cools, the grape will cool more quickly than if it were in air of the same temperature, but until the water cools, the grape will stay warmer.

 

You're totally wrong.

 

Specific heat capacity has nothing to do with amount of energy transfer at a given temperature.

[Cookie]

The whole idea is that grapes can handle temps down to about 31 degrees. Water will freeze at 32 degrees, so as long as there is a wet ice/air interface, the temp of the leaf will be held at 32 degrees. The grapes have to be continuously sprayed - obviously if the water freezes solid and there's no wet surface, then the temp can drop much lower than 32.

 

Cookie

[-Demosthenes-]

Maybe it insulates?

The heat given off the water when it's froaen will only be given to some thing that is colder than the water. So it wouldn't be very warm.

[starwiz]

Originally posted by -Demosthenes-

The heat given off the water when it's froaen will only be given to some thing that is colder than the water. So it wouldn't be very warm.

That's exactly what I told my chemistry teacher, and she doesn't agree with me. Hm...

[JaKiri]

Damn thing

[-Demosthenes-]

The cold air pulls it out. the heat is taken away, but it dosen't go into the grapes, but into the air. That's how things get cold. ??

[VendingMenace]

i am pretty sure that it is just an insulating effect of the water/ice on the outside of the vines. At least i think that is what i was told in my viticulture class

[starwiz]

I'm pretty sure you're right.

 

However, I was unable to find anything authoritative to back me up. Do you have something that I can wave in my teacher's face?

[Cap'n Refsmmat]

I had this same thing, but with oranges. Apparently, the water freezes, and then when it gets colder, the ice loses heat, heating the orange/grape. It doesn't have to continually be sprayed. I also heard ice is always at 32 degrees, and so is snow, so if you are in -50 weather you can bury yourself in snow and you will be insulated. They're a little conflicting though.

In conclusion, I believe that the ice melts from the heat of the orange, then gives off heat, heating the orange. Eventually, it freezes, and it repeats.

[starwiz]

Ice definately isn't always at 32 degrees. If it's -50 outside, it might take a while for the snow to get that cold, but I'm certain that it will, given enough time.

 

When ice gets colder, it releases heat. But as far as I understand, that heat goes to whatever's cooling it, by the definition of something's cooling something else.

[Cap'n Refsmmat]

No, water freezes at 32 degrees and stays there. The snow, being sublimated, might have different properties, but I'm not sure.

The heat goes to ANYTHING colder, like the orange/grape.

[starwiz]

If that's true, then how can we measure a specific heat for ice?

 

Look here if you don't believe me: http://dbhs.wvusd.k12.ca.us/Thermochem/Time-Temperature-Graph.html

 

They talk about starting ice off at -10 degrees. I promise, it's possible.

[Cap'n Refsmmat]

Maybe it's just snow. I dunno. I still think, as the ice cools, the heat from the ice goes to the grape.

[VendingMenace]

Maybe it's just snow. I dunno. I still think, as the ice cools, the heat from the ice goes to the grape.

 

Nope. Snow will cool too. Ice definately can loose heat

 

Also, the only way for heat from the water freezing to go into the grape is if the grape was colder than the water that was freezing. Otherwise heat would flow from the grape to the water :/

 

 

Sorry starwiz, i am afraid that i don't have anything aurthoritative to throw in your teacher's face. I will do somethinking about it though and see what i come up with

[chemistry]

What you're saying is true, but it doesn't apply in this circumstance. Water has a very high specific heat. That means that it takes a long time for air outside to cool it to a freezing point. So once the water cools, the grape will cool more quickly than if it were in air of the same temperature, but until the water cools, the grape will stay warmer. Since the frost is (in theory) over in the morning, the ice isn't there forever, and so the idea is that the time it takes for the ice to freeze more than makes up for the increased cold-transfer between the ice and grape.

 

Note that water may also act as an insulator. Particularly when it freezes. The temperature between the water and the grapevine is not too different at first, due to the specific heat capacity as you mentioned water takes a long time for it to have lost the all of the heat energy, especially to air.

 

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[Cap'n Refsmmat]

I know that my science teacher knows this. I'll ask him on Monday.

[starwiz]

Originally posted by chemistry

Note that water may also act as an insulator. Particularly when it freezes. The temperature between the water and the grapevine is not too different at first, due to the specific heat capacity as you mentioned water takes a long time for it to have lost the all of the heat energy, especially to air.

Exactly.