Question

why copper lose shining after some time.

Answer 1

This is because copper is a highly reactive metal which forms copper(II) oxide when in contact with air. 
Copper vessels lose their shine when exposed to air because of corrosion the corroded copper will be green in colour.

Answer 2

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A copper vessel reacts with oxygen present in air when exposed to it , they forms copper oxide which , in turn , is a black coating over the vessel and makes the vessel to lose its shine .

2Cu + O2 = 2CuO

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