A shiny brown coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed.
The element 'X' is Copper (Cu). The black coloured compound formed is Copper(II) oxide (CuO).
Explanation
The textbook context shows the reaction: 2Cu + O₂ → Heat → 2CuO. It explicitly mentions that when hydrogen gas is passed over heated CuO, 'the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.' This confirms that copper is brown in colour and CuO is black. When copper (a shiny brown element) is heated in air, it reacts with oxygen to form copper oxide, which appears as a black coating.
Solution Steps
Step 1: Identify from context that copper (Cu) is brown and forms CuO when heated in air.
Step 2: Note that CuO is described as 'black coating' in the reverse reaction.
Step 3: Conclude that Element X = Copper, Black compound = Copper(II) oxide.