Question

the plastic cube is released in water will it sink or come to the surface of water

Answer 1

it will come in surface of water

Answer 2

Assuming you have pure water, its specific gravity will be about 1. Find out whic type of plastic your cube is made from and then look at the chart below. If your plastic is less than 1, it should float, if it's more, it will sink. One thing to watch out for though is additives which can sometimes push the specific gravity above or below and make a plastic sink or float.

Structure is also important. Polystyrene should sink. But when it's expanded (got bubbles in) it will obviously float.

PET isn't here which is annoying because it's highly prevelant. That usually has a specific gravity of more than 1.2.

HOPE THIS WILL HELP YOU