Barrons SAT Subject Test Chemistry, 13th Edition

Figure 1. An Exothermic Reaction

Figure 2. An Endothermic Reaction

TIP

Know how to read these graphic representations.

Conservation of Mass

When ordinary chemical changes occur, the mass of the reactants equals the mass
of the products. This can be stated another way: In a chemical change, matter can
neither be created nor destroyed, but only changed from one form to another. This
is referred to as the Law of Conservation of Matter (Lavoisier—1785). This
law is extended by the Einstein mass-energy relationship, which states that matter
and energy are interchangeable.

ENERGY

Definition of Energy

The concept of energy plays an important role in all of the sciences. In chemistry,