Final answer:
Substances can have the same volume but different masses due to differences in density. Avogadro's law explains that equal volumes of all gases will contain the same number of particles under the same conditions.
Step-by-step explanation:
The reason that two different substances can have the same volume but different masses boils down to their density. To begin with, the concept of density is crucial when discussing mass and volume. Density is determined by dividing the mass by the volume (density = mass/volume), so if two objects have the same volume but different masses, they must have different densities.
Looking at Avogadro's law, we understand that equal volumes of all gases at the same temperature and pressure will contain the same number of particles. This means that, regardless of the gas's molecular weight, the volume occupied by a gas depends on the number of molecules and not their size.