CBSE Class 10 Science 2020 · Paper 31/1/1 · Question 15

On heating, the surface of the brown copper powder turns black, because copper combines with oxygen of the air: $2\text{Cu}+{\text{O}}_2\stackrel{\text{heat}}{\to }2\text{CuO}$ Product: copper(II) oxide, black in colour. When hydrogen is passed over the heated copper(II) oxide, the black coating turns brown again, because hydrogen removes the oxygen: $\text{CuO}+{\text{H}}_2\stackrel{\text{heat}}{\to }\text{Cu}+{\text{H}}_2\text{O}$ Products: copper, brown in colour, and water, colourless (given off as vapour).