Composition is what a material is made of. It is a property of the material, not a thing in its own right; the value of the composition property names the parts that, together, form the whole.
Chemistry is a compositional science. It is concerned with what something is made of and how to make things. To know a material's composition is to know its parts and their relationships, not just the parts in isolation. Take water: its composition is what we abbreviate as H₂O. The composition is not water; it is what we say about water when we name how it is put together.
Composition is a property; H₂O is its value. The same is true for any material. Salt has a composition; the value, in chemical shorthand, is NaCl. The composition is the property name; the formula is the value. To say a material has composition is incomplete until we say which composition. Without the value, the property is meaningless.
Composition is a relationship, not a thing. Materials are things; their composition is the relationship between the parts that make them up. Two materials with the same composition are, for chemistry's purposes, the same kind of material. Different composition, different material. This is how the fact that the world is composed of distinct identifiable things gives chemistry its categories.
Question
Two pieces of pure salt, one a crystal and one a powder. Are they the same material?
Reveal Merrill's answer →Hide answer
Yes. Composition is an essential feature; shape is accidental. The same composition gives the same kind of material, no matter how it is arranged in space. Both pieces are salt; neither has stopped being salt by being ground.