Reactants (or 'substrates') are two or more elements/ compounds that chemically interact to form a new substance. They are the starting materials for a reaction and are always found at the left side of a chemical equations.A reactant is one of the inputs in a reaction, that get changed. The word reactant is used in chemistry. A substance that has a reaction and undergoes a change is considered reactant.hey are the starting materials for a reaction and are always found at the left side of a chemical equations. A reactant is one of the inputs in a reaction, that get changed. The word reactant is used in chemistry.