A reactant is a starting substance in a chemical reaction; it is written to the left of the reaction arrow and is consumed as the reaction proceeds. Reactants are converted into products through the rearrangement of atoms. For example, in the burning of methane, methane and oxygen are the reactants.

反应物是化学反应的起始物质;它写在反应箭头左侧,并随反应进行而被消耗。反应物通过原子的重排转化为生成物。例如,在甲烷燃烧中,甲烷和氧是反应物。