The weight per unit volume is density and is constant for a certain type of material. Identical boxes filled with pennies, syrup or water are all easily identified because of their different weights. Because different volumes of these substances can be found that weigh the same, it is not weight but the weight of a set volume that identifys a substance. This is density. Whenever something is placed in a solution of lower density, it floats.
Wood floats in water, helium floats in air, oil floats on water, a penny floats on mercury.
Whenever something is placed in a solution of higher density it sinks. Copper sinks in water, carbon dioxide sinks in air and syrup sinks in water.