Final answer:
The law of conservation of matter states that mass is conserved in a chemical reaction, but in a nuclear reaction, mass is converted into energy.
Step-by-step explanation:
The law of conservation of matter (seen in an earlier chapter) states that there is no detectable change in the total amount of matter during a chemical change. In a chemical reaction, mass is conserved, meaning that the total mass of the reactants is equal to the total mass of the products. However, in nuclear reactions, mass is not strictly conserved because some of the mass is converted into energy. This is described by Einstein's famous equation E = mc².