A tarnished copper vessel begins to shine when rubbed with lemon .Why?

The citric acid in the lemon reacts with the top copper oxide layer (the cause of the tarnish), exposing the untarnished copper metal below.

The effect is even more noticable with stronger acids, an old darkened copper penny can be made to shine if dropped in home made hydrochloric acid (salt + vinegar).
The acid in the lemon reacts with the copperoxide exposing the clean copper.
Lemon contains citric acid as well as alkaline matter. When you rub copper vessel, the copper salts formed on the surface of vessel say oxides, sulphides will be removed. It will shine which is pure copper vessel.

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