Chemical substance and Reactant

Chemical substance noun - Material produced by or used in a reaction involving changes in atoms or molecules.

Reactant and chemical substance are semantically related. In some cases you can replace term "Reactant" with "Chemical substance", this terms are similar.

Reactant noun - A chemical substance that is present at the start of a chemical reaction.

Chemical substance and reactant are semantically related. Sometimes you can use "Chemical substance" instead a noun "Reactant".