What is a reactant?

• A. A substance that speeds up a reaction.
• B. A starting substance in a chemical reaction.
• C. A substance formed as a result of a chemical reaction.
• D. The energy change during a reaction.

Answer: (B)

A reactant is the starting substance in a chemical reaction—the material that undergoes change as the reaction proceeds. It is present before the reaction begins and is consumed to form the products. For example, in the formation of water from hydrogen and oxygen, hydrogen gas and oxygen gas are the reactants, and water is the product. The other ideas describe different things: a substance that speeds up a reaction is a catalyst, a substance formed as a result of the reaction is a product, and the energy change describes how much energy is released or absorbed, not what you start with. So the best description is that a reactant is a starting substance in a chemical reaction.