Activity 1.11 Ncert Science class 10
Procedure: This activity asks us to heat copper powder in a china dish.
Observation: Surface of powder becomes black.
Explanation: All metals except gold and platinum are reactive elements. They react with various gases present in the atmosphere like oxygen, sulphur etc. These reactions take time. Heating metals hastens the reaction and we can easily observe the change during heating. Copper also reacts with atmospheric oxygen and forms a coating of its oxide on them. This oxide gives them a black appearance.
2Cu(s) + O2(g) ------------> CuO(s)
Some extra facts:
Copper is used in many forms as wires; as kitchen utensils; in an idol etc. Oxidation of copper with times makes them black. Most of the time these products also have a green appearance. This phenomenon occurs due to the reaction of oxide with sulphur present in the atmosphere.
Acidic substances like vinegar, lemon remove this layer. So we use them to clean our copper products.