Exothermic Water Freezing

Most recent answer: 10/22/2007

Q:
I wanted to know if you can give a physical example of how freezing water is exothermic because it is hard to see how it releases energy when you cant see gases release or anything of the such happen when water freezes. I just want an example of how it can be witnessed with the naked eye how water is exothermic when it freezes.
- John (age 17)
california
A:

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

Q:
Well, not exactly. These experiments need an additional element to be convincing. The results of each experiment described above would be the same whether water freezing were exothermic, isothermic, or endothermic! For example, consider the liquid nitrogen experiment: You put liquid nitrogen in water and it boils continuously as the water is chilled and then freezes. Does this prove that freezing is exothermic? No. If freezing was isothermic or even endothermic, the liquid nitrogen would also boil continuously. (It will continue to boil until the ice is at 77ºC or until the liquid nitrogen is exhausted, whichever comes first.) If we could measure the rate at which the nitrogen boils accurately enough, we could detect the freezing point by its effect on the boiling rate of the nitrogen: The boiling rate will not slow as quickly during the freezing period because the temperature is not decreasing during the time the water is freezing. It will slow, of course, because there is less and less nitrogen to boil off as time goes on. But there is a much easier solution: Add a thermometer to the water and separate it from the cooling agent with a thermally-conductive barrier. You can then see that as the cooling agent (nitrogen, dry ice, etc) removes energy from the water, the temperature lowers to 0ºC, stays there for a while as the water freezes, then starts going down again. During the time the thermometer pauses at 0ºC, the freezing water continues to put out energy, as evidenced by the dry ice or liquid nitrogen vaporizing continuously. You can use two nested pots or other containers to prevent the water from mixing with the liquid nitrogen. Gently stir the water during the experiment to keep its temperature the same throughout.
- Ray Burns
Tucson, AZ, USA
A:

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

Q:
Sir, I would like to give my opinions,how about putting a hot water into a temperature about -370 fahrenheit,does the exothermic occur?because in my opinion,exothermic do not occur rather than the endthermic because first it is the absence of heat only,second if the hot water would be placed in a -370F where would the heat go? I would probably say that the heat stays on the water and disappeared because there is no place for the heat for that temperature.can you give me sir an explanation for that?
- Angelo (age 15)
Philippines
A:

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)