Differentiate between density and relative density of a substance?

With the use of Archimedes' principle, state how you will find relative density of a solid denser than water and insoluble in it. How will you modify your experiment if the solid is soluble in water?

A body weighs W₁ gf in air and when immersed in a liquid, it weighs W₂ gf, while it weighs W₃ gf on immersing it in water. Find: (i) volume of the body (ii) upthrust due to liquid (iii) relative density of the solid and (iv) relative density of the liquid.

The density of copper is 8.83 g cm^-3. Express it in kg m^-3.