Question

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

Answer 1

it depends on the density of the plastic and whether or not it is an air filled cube or a solid block

Answer 2

It depends on the plastic. Most are more dense than water, but polythene and polypropylene (and a few others) are less dense.

Most commercial plastics are not the pure compound but have other materials incorporated including fillers (material which is cheaper than the plastic) but also those for particular physical or chemical properties.

Some, notably polystyrene and polyurethane are often used in the form of solid foam , with lots of gas bubbles in them, making them much lighter. The cube may of course be hollow.