Exothermic Water Freezing
Most recent answer: 10/22/2007
- John (age 17)
california
Well, it’s a little easier going the other way. Melting ice is endothermic -- you can see this by putting a thermometer in a glass of warm water, adding an ice cube, and watching the temperature go down as the ice melts. The melting process needs heat to proceed and takes it from the warm water.
Going the other way: your refrigerator makes ice by cooling the water off. The heat gets transferred to the environment by air flowing past the coils in the coolant loop after the compressor, usually located in the back of your refrigerator. The heat dissipated in these coils is the sum of quite a few sources -- heat leakage from the outside into the refrigerator, and energy expended by the compressor.
You can also freeze pure water by putting it into a vacuum. Water will boil when the vapor pressure exceeds the ambient pressure, and in a vacuum, this is the case at all temperatures for which the water is liquid. Boiling is endothermic (it takes energy to boil water), and so the water cools off as it boils. Eventually, what’s left will freeze.
You can also freeze water by putting dry ice or liquid nitrogen into it. The dry ice will vaporize and the liquid nitrogen will boil, both indicating that they are receiving thermal energy given up by the freezing water.
Tom
(published on 10/22/2007)
Follow-Up #1: endothermic and exothermic
- Ray Burns
Tucson, AZ, USA
You're quite right. With a little care, however, experiments like these can show whether freezing is exothermic. Let me try to explain how. The idea is that you can figure out what temperature things would end up at if the reaction was neither endothermic nor exothermic. Then see what temperature it actually ends up at. The difference shows whether the reaction released heat or absorbed it.
For instance, take one part ice at 0°C and 9 parts water at 10°C. If the melting were neither endothermic nor exothermic, when these equilibrate their temperature will be 9°C. (Here I've assumed correctly that the heat capacity of water is nearly independent of temperature.) Any deviation either way tells you whether the melting took up heat or gave off heat. Of course, melting takes up heat. So that means that the final temperature will be below 9°C. I believe the final temperature here will be about 2°C.
If melting takes up heat, then the opposite process, freezing, better give off heat. You could check that directly with a similar experiment starting with ice well below 0°C and some water at 0°C. The final temperature will not be the weighted average of the temperatures, but something higher because of the heat given off in freezing.
Mike W.
Lee H
(published on 10/22/2007)
Follow-Up #2: heat flow in the cold
- Angelo (age 15)
Philippines
I'm not sure I follow all your questions, but the description of what will happen is not very complicated.
If hot water is placed in a -370°F (why that temperature?) environment, heat will flow out of the water into the environment until they reach the same temperature. If the environment is big enough compared to the water, that will be close to -370°F, because the amount of heat leaving the water won't be large enough to heat the environment much.
As the water cools, it will reach 32°F, at which point it will start to freeze. Heat still flows out, but it comes from the energy lost as the water freezes. You could say that's an exothermic process. Once it's all frozen, the temperature will start dropping again.
One thing is for sure- there is no process whereby the heat just 'disappears'. Energy is conserved.
Mike W.
(published on 05/16/2013)