What is meant by a substance in chemistry?

A pure substance has a definite and uniform chemical composition and cannot be separated into simpler components by physical methods.

Two types:

1. Elements one kind of atom (e.g., gold, oxygen)
2. Compounds two or more elements in fixed ratio (e.g., water, salt)

A pure substance has a uniform, fixed chemical composition and cannot be separated by physical means it is either an element or a compound.