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Q & A: weighing frozen and thawed fish

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Most recent answer: 10/22/2007
Q:
Will an object (for example, a fish) weigh the same when it’s frozen compared to when it’s defrosted? Thanks for your help!
- Cynthia (age 14)
NHS
A:
The answer is yes to a very very good approximation, but you have to be very careful to keep the total amount of material that's being weighed constant.

Usually, when a frozen fish is thawed on the countertop (at least in my house) water vapor condenses on it, first making white frost, and then it melts, leaving a puddle of water around the fish. The frozen fish itself may have little ice crystals which will melt and flow away. You have to make sure that all the little bits of water are weighed before and after the thawing, and that no extra bits are added, and then you should get pretty much the same weight. Leaving the fish in a sealed container from which you can wipe the condensation should help keep the total amount of matter constant.

There is a very tiny relativistic correction to this -- thawing a fish increases its total energy, and since E=mc^2, raising E a tiny bit increases m a tiny bit. But I'll bet this is unmeasurable, given that even with the most carefully controlled weighing, a few atoms will sneak on and off, or the scale won't measure to the precision needed.

Tom

(published on 10/22/2007)

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