Question

a ternished copper metal regains its shine when rubed with lemon

Answer 1

The citric acid present in lemon reacts with oxide layer of copper which has the property of base and it gives it a shiny appearance..

hope it helped you

Answer 2

it is so because while left openly in air it's uppermost layer reacts with air and forms oxide but when it is rubbed with sand paper or lemon this oxide layer gets removed and from behind a shining copper metal comes

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