Question

why copper is cleaned by lemon .explain the answer

Answer 1

Copper oxide dissolves in a mixture of week acid and table salt and vinegar is an acid.You could also clean ur pennies with salt and lemon juice or orange juice becoz these juices are acida too

Answer 2

Oxygen present in air coats Cu and make it dull in appearance and hence we use acid which break down this oxide layer. As citric acid is a weak acid and liquid  hence it dissolve thin layer of oxide but it doesn't dissolve Cu.