Letter 22058

Copper [Canada] 

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I'm in grade 8 and I have a science lab 2 do! where is what i have 2 do... Do some research on one of the solids that you calculated the density of( I picked copper).Write a short report on how the substance is used. Include an explanation of how its DENSITY influences its practical uses. Is the density of the solid ever changed during an industrial process? If so, explain how and whi this occurs. I got some reasearch on copper but the think is I can't find if the density ever changes during an industrial process. plz help!

Keshia F.
- Toronto, Ontario, Canada

If your question had been posted by a high school or college student, I wouldn't answer it, I would only give hints about the answer. But an 8th-grader will probably not have learned enough chemistry to understand the hints :-)

The density of copper metal does not change. Each atom of copper can be pictured as a tiny ping pong ball with a specific weight and diameter. If you packed a cardboard box with ping pong balls, a specific number would fit, and that specific number would weigh a specific amount. In any given volume of copper, a specific number of copper atoms will fit. Since the weight remains proportional to the volume, the density is fixed.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

Ted's explanation is correct but does not take into account a significant temperature change. If you take the copper block (box in his example) from 68F to say 212F, the atoms become more active and "spread out" a little bit. The box weighs the same, but it grew slightly, changing the density. At the library, look up the thermal coefficient of expansion for copper. It is not that much, until you have a large temperature change on a large part. Copper wire conducts less electricity as it heats up, but I do not think that it has anything significant to do with density. One use of density would be inserting bearings in another part by freezing them in liquid nitrogen or a dry ice/ alcohol mixture. They will very nearly drop in the hole and will be unmoveable as soon as it warms up. Granted, most bearings are a copper alloy and few are pure copper.

James Watts
- FL

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