Activity 2.7 NCERT Class 10 Science, Chapter 2 Acids, Bases, and Salts
Activity 2.7 asks us to add dilute hydrochloric acid to copper oxide and check the solution of the colour.
The colour of the solution turn green.
Copper oxide is black; it reacts with hydrochloric acid to form copper chloride. Copper chloride is brown but it absorbs water and turns the solution into a green colour.
CuO(s) + HCl(aq) ------------> CuCl2(aq) + H2O(l)
Copper is an ornamental metal. Many small idols of the Gods are made of copper. This turn into black and green by reacting with atmospheric air. People use the acidic nature of lime to wash this colour.