Question

how will knowing the density of a substance help you determine whether an object made from that material will float in water

Answer 1

If the material is not as dense as water, then it will most likely float. If it is denser, it will most likely sink.

Answer 2

The density of water is about 1kg/m³, therefore is the substance has a density of less than 1kg/m³ then it will most likely float however if it is more than 1kg/m³ then it will most likely sink.