In our work on concepts in Physical Science, we now come to the point where we need to combine two quite different ideas. On the one hand, there is the concept from Unit 4 that chemical reactions involve chemicals. On the other hand, there is the concept from Unit 1 that any change must involve energy. Since chemical reactions involve change for the chemicals involved there must also be energy involved in a chemical reaction. We say that whenever a chemical change occurs, it is accompanied by an energy transformation.