Question

why do different solids have different density? why It consider an important physical constant of pure substance

Answer 1

Mass is relative to density.All solids doesn't have same mass.Hence density of different solids have different density

Answer 2

Density is defined as mass per unit volume and solid is havin' mass as well as it is occupying space that is volume.
Pure substances means a sample of matter which is made of only one kind of matter.
Note it is a matter and matter is havin' density whether it is solid or liquid or gas.
Hope it helps you.