As a basic rule, objects that are denser than water density will sink and the ones that have less density will float. In reality things are not so simple, because there are more physical forces and phenomenas that takes effect. For example if you throw a book in the water at first it will float and when the pages soak with water it will sink. If you place a CD horisontaly in the water it will float, but if put verticaly with the thin edge it will immediately sink. Also objects which hold air, like bottles and canteens are tricky to say exactly if they floats or sinks. Constructions like ships, which have specific forms to displace water also floats, although they are denser and heavier. The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume.
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