A metal cylinder is melted and the whole mass is cast in the shape of the cube . what happens to its density ?

Dear Student,

Density is an intrinsic property of matter.
It is defined as the mass per unit volume of a substance.
In the given case, when the metal cylinder is melted, the mass of the the metal remains the same, as no new metal is added.
However, the volume of the shape changes from the volume of a cylinder to the volume of the cube. 
Since the mass is constant but the volume changes, the density will change.

Hope it helps.