Question

Why can I not use mass or volume alone to identify substances

Answer 1

Knowing the volume or the mass alone does not give us much information about the physical characteristics of certain substances. Volume refers to how much space a substance is occupying. Mass refers to the amount of atoms in a substance. I can have a sand that occupies a certain volume and a liquid that occupies the same volume, these two substances are different. Same goes to mass. There could be the the same number of atoms in a sand and liquid. For example, let's say we have 10 g of sand and liquid, it is not the same because each of them occupies space differently. That means knowing the mass or volume only is not enough. That's why density is important because it allows substances to stand out. Density is the ratio between a mass and a volume, not all substances have the same ratio of mass and volume.