A shiny brown coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed.

Element 'X ' is copper. On heating in air it forms Copper (II) oxide, which is black in colour. 2Cu (s) + O 2 (g) 2CuO (s)

What do you mean by a precipitation reaction? Explain by giving examples.