Kinetic energy of expanding ice

[kublikhan]

Where does the Kinetic energy of the expansion of freezing ice come from? For example, say you had a gram of water inside a piston. The water freezes and the expansion of ice pushes the piston up. Since the heat of fusion of water is 80 calories per gram, let's say to initally melt that ice to water I had to apply 80 calories of heat to get the water to it's liquid form. Now when the ice is freezing, does it give off that same 80 calories of energy in the form of heat? Or is some of that energy transformed into mechanical energy to push the piston up?

 

[infinitetime]

Does the formation of ice involve expansion?

When water freezes, is it not giving off energy (exothermic), so it will get smaller?

 

[Andrew Mason]

Where does the Kinetic energy of the expansion of freezing ice come from? For example, say you had a gram of water inside a piston. The water freezes and the expansion of ice pushes the piston up. Since the heat of fusion of water is 80 calories per gram, let's say to initally melt that ice to water I had to apply 80 calories of heat to get the water to it's liquid form. Now when the ice is freezing, does it give off that same 80 calories of energy in the form of heat? Or is some of that energy transformed into mechanical energy to push the piston up?

Interesting question. It is a common conception that ice expands as it gets colder. But, from an energy point of view, this seems to conflict with the conservation of energy. The answer is: once ice is formed, it does not expand as it gets colder. It actually shrinks becoming more dense as it gets colder.

The maximum density of water is at 4 deg. C.

AM

 

[infinitetime]

false; i was wrong.

I thought about it...

When water is cooled below 4 degrees celsius, the volume stops decreasing and the density has reached a maximum. This is b/c the water molecules have been pulled as close together as possible; so the water begins to expand and its density decreases.

Below 4 degrees celsius, the water molecules are beginning to approacha a solid state, shich is highly organized; they begin to form an open arrangement which results from hydrogen bonding and is the most stable structure for the moleculs in or near solid state.

The forces involved in the expansion of freezing water are surprisingly great - strong enought eventually to break even boulders into small pieces.

So, basically the volume of water increases when it becomes a solid due to the open arrangement of the molecules - they move apart to form this arrangement.

I hope this helps!

 

[shyboy]

Actually in the case of ice the melting temperature under the pressure will be less, so the final ice should have smaller temperature than the initial water. Another point is that the system is not closed, so you can get additional heat from the walls.

 

[infinitetime]

Interesting question. It is a common conception that ice expands as it gets colder. But, from an energy point of view, this seems to conflict with the conservation of energy. The answer is: once ice is formed, it does not expand as it gets colder. It actually shrinks becoming more dense as it gets colder.

The maximum density of water is at 4 deg. C.

AM

Exactly about the 4 degrees, but not the shrinking!

 

[Andrew Mason]

Exactly about the 4 degrees, but not the shrinking!

I said once ice has formed it shrinks as it gets colder. Minimum density of ice is at 0 deg. C. The colder it gets, the denser it gets. Ice never reaches the density of water, however.

AM

 

[kublikhan]

Interesting question. It is a common conception that ice expands as it gets colder. But, from an energy point of view, this seems to conflict with the conservation of energy. The answer is: once ice is formed, it does not expand as it gets colder. It actually shrinks becoming more dense as it gets colder.

The maximum density of water is at 4 deg. C.

AM

What about when the liquid water freezes and expands into ice?
Does the exothermic energy needed to melt ice to water = the exothermic energy released when that same water freezes to ice? If so, how do you maintain conservation of energy when their is additional mechanical work being done when the water expands as it freezes into ice?

 

[Andrew Mason]

What about when the liquid water freezes and expands into ice?
Does the exothermic energy needed to melt ice to water = the exothermic energy released when that same water freezes to ice? If so, how do you maintain conservation of energy when their is additional mechanical work being done when the water expands as it freezes into ice?

The physics of water is a fascinating subject in itself.

When ice forms, it does work on its surroundings: W = \int Pdv. That energy must come from the latent heat of fusion - the heat that the water gives up when it freezes (without reducing its temperature).

Once ice forms it will expand slightly for a few degrees. I am not sure of the reason for this but it probably has to do with getting all of the molecules into the same crystalline structure. Once all molecules achieve that structure, further cooling makes it shrink. The reason it shrinks as it gets colder is because the water molecules have a lower amplitute of vibration as they lose thermal energy and, therefore, take up less space.

AM