Question

Why are elements and compounds called pure substances ?

Answer 1

Elements are made of only one kind of atom. The particles can be a single atom or a molecule made of only one kind of atom. There is no physical change that can separate elements into more than one kind of substance. This makes an element a pure substance.

Compounds are made of only one kind of molecule. The molecule is made up of two or more kinds of atoms. There is no physical change that can separate the compounds into more than one kind of substance. This makes a compound a pure substance.