Which of the following statements about chemical reactions is true?

In the context of chemical reactions, the statement that energy changes are commonly involved is accurate. Most chemical reactions either absorb or release energy in the form of heat, light, or sound. This energy change is a key aspect of chemical processes and plays a critical role in determining the reaction's favorability and mechanism.

For instance, exothermic reactions release energy, often felt as heat, while endothermic reactions absorb energy from their surroundings, leading to a decrease in temperature. Understanding these energy changes helps in predicting whether a reaction will occur spontaneously and how it can be controlled or harnessed.

The other statements do not hold true across the broad spectrum of chemical reactions regardless of their specific context. Products can be solids, liquids, or gases, depending on the reaction conditions and substances involved. Reactants can also be in different states—liquid and gas, for example. Finally, the law of conservation of mass states that the mass of the products in a chemical reaction is equal to the mass of the reactants; thus, products cannot have less mass than reactants.