Reactant
A starting substance that is consumed during a chemical reaction.
Unit
mol
In depth
Reactants are written to the left of the arrow. Their identity, relative amounts and physical state determine what products form and how quickly. A reactant supplied deliberately in excess ensures that the more valuable or hazardous component is completely converted.
Examples in the real world
Methane and oxygen are the reactants when natural gas burns; zinc and hydrochloric acid when hydrogen is prepared.