Question

A tarnished copper vessel is rubbed with lemon to retain its shining give reason

Answer 1

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Copper metal slowly reacts with water, carbon dioxide and oxygen present in air to form basic copper carbonate which is green in colour.
Its layer slowly gets deposited on the surface of the metal. Now lemon juice contains citric acid while tartaric acid is present is tamarind.
Both these acids react with basic copper carbonate to form soluble salts such as Copper Acetate and Copper Tartarate.

Answer 2

copper get tarnished when exposed to air. it reacts with air (moisture) to form copper hydroxide and form a green layer due to corrosion and get tarnished. this is basically basic in nature.

lemon is acidic in nature and when rubbed against the tarnished copper vessel, it reacts with it and form salt. Salt can be washed away with water and we can get back the shiny copper vessel back.

that's why, a tarnished copper vessel begins to shine again when rubbed with lemon.