Question

plastic cube is a release and in water will it sink or come to the surface of water

Answer 1

$\red{Buoyancy\:concept}$

Density of plastic is less than that of water so when it is put in water then it will come to surface of water means it will $\blue{float}$.

Answer 2

The plastic cube if released in water does not sink and rather floats on the surface. Plastic is water repellent object and it has high rates of buoyancy, when compared to other objects. As plastic is lighter than the water it tends to float on the surface.