In this unit I learned that an exothermic reaction is a chemical reaction that more energy is given off than taken in. Another thing I learned is that that the substances in a chemical reaction that are being changed are called reactants. Something else that I learned is that the new substances formed during a chemical reaction are called products. The one thing that I liked best about this unit was the chemistry in a bag lab. What I liked least in the unit was the study guides. What is an endothermic reaction?