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Name: Lanny Status: educator Grade: 9-12 Location: ND Country: USA Date: Winter 2011-2012

Question:
A student asked how can the energy increase when ice sublimates in a freezer, where does the energy come from for this to happen when freezer has little or no energy?

Replies:
Good question!

A fan typically blows air about inside a freezer to circulate cool air. This lowering of air pressure near the surface of the ice tends to facilitate sublimation. Thermodynamically, the fan does work on the surface of the ice cube, which is a form of energy.

In addition, the freezer is periodically opened, which lets cold air out and warm air in. That provides air circulation and also provides thermal energy.

Discovery Online has a very nice description of sublimation within a freezer. See http://www.discovery.com/area/skinnyon/skinnyon970509/skinny1.html .

best, dr. topper


Lanny,

The student is working under at least two misconceptions: (1) that there is no energy available in the freezer, and (2) that if a substance is under conditions in which sublimation can take place, that no gaseous substance becomes solid at the same time that some solid substance is becoming gaseous.

Ask your student to imagine the conditions and temperature of a freezer of a poorly built refrigerator. Then ask the student to imagine that the same freezer could somehow be made colder (maybe with a better cooling system, a more efficient compressor, etc). What has to happen in order for a freezer to get colder? [Guide the student to the answer that more energy has to be taken away.] This then should suggest that there is energy (to be taken away) in the first place; even in the cold temperatures of a freezer, for as long as it can be made colder still, there is energy in the system.

The second misconception is a bit harder to get to because it is easy to imagine that phase changes go in one direction only. However, the student should have some experience with boiling water in a pot that has a glass lid (if not, this might be a demonstration that can be shown to a student). The idea here is that while the liquid is boiling, we can also see it condense on the glass lid of the pot. But if we look closely, the liquid on the glass lid is also evaporating, liquid does not just build up on the lid, some of it slide back into the pot, but some actually evaporate (since the temperature is certainly high enough for this to happen). ...if this is not convincing for the student, try to find some animations that show phase changes at the atomic/molecular level. Any good animation will show that phase changes are dynamic and go both ways.

Greg (Roberto Gregorius) Canisius College


Lanny,

This is a great question. One thing we have to remember is that at any temperature, even “low” temperatures such as in the freezer, particles will have a distribution of energies (the Maxwell-Boltzmann distribution). This means that, even at a low temperature, there will be some very energetic particles which have enough energy to overcome intermolecular forces and enter the gas phase.

The sublimation you describe can be equated to evaporation of a liquid. We know that liquids will evaporate, even at temperatures below the liquid’s boiling point (otherwise puddles on tarmac would never dry up). This is because some of the particles at the surface have enough energy (because of the M-B distribution) to break any intermolecular forces and enter the gas phase. The same thing happens with solids, even at temperatures below the solid’s melting or sublimation point. Ice in the freezer at -10oC should, by all rights, stay a solid forever. However, we know that ice cubes shrink over time and a frost develops on the colder element at the back of the freezer. This can be explained in the same manner as the evaporation of a liquid.

Best wishes, Tom Collins


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