A chemical substance is a portion of matter of constant composition and that is neither a metal or mineral. It is composed of molecular entities of the same type or of different types that is used in or produced by a chemical reaction involving changes to atoms or molecules. It is the physical output of an industrial process and has a commodity role.
A portion of matter is an aggregate of material entities that have a state of matter.
Editor note:
Portions of matter describe mass nouns like e.g. water, air and fuel. Mass nouns are uncountable, but portions of these can be quantified, like 1 glass of water, 1 m³ of air or 1 barrel of fuel. For further explanation please refer to: https://github.com/OpenEnergyPlatform/ontology/wiki/Explanation-on-mass-nouns
Ontology