Answer :

The unknown element X is copper (Cu). When copper is heated in air, it forms a black coloured compound called copper oxide (CuO). The reaction for above chemical change is given below:

The color of the heated copper powder becomes black when air is passed over it because of its reaction with oxygen i.e Air. When it reacts with oxygen it forms copper oxide which is black in color. 

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A metal X forms a waLakhmir Singh & Manjit Kaur - Chemistry