Question

why brass is cleaned with lemon?

Answer 1

We know that lemon juice contained citric acid and brass is a mixture of copper and zinc metal.

Copper and zinc both reacts with atmospheric oxygen and formed copper oxide layer on its surface as

2 Cu + $O_{2}$ → 2CuO  

When lemon applied on the surface of brass the layer of copper oxide (weak base) reacts with lemon juice (weak acid) as a result neutralization reaction takes place and copper oxide layer removed with citric acid of lemon and brass surface start shining.