What best describes a chemical reaction?

A chemical reaction is defined as a process that transforms substances into new substances with different properties. This transformation occurs when reactants undergo chemical change, resulting in products that have distinct characteristics compared to the original materials. For instance, during a chemical reaction, bonds between atoms are broken and new bonds are formed, leading to changes in composition and structure.

When considering the other options, mixing substances may or may not lead to a chemical reaction; for example, mixing salt and sand does not create new substances. The change of state refers to physical processes such as melting or boiling, which does not involve any alteration of the chemical composition. Similarly, a physical alteration of a compound involves changes in form or appearance but does not result in the formation of new substances with different chemical properties, which is a hallmark of a chemical reaction. Thus, the correct description of a chemical reaction involves the fundamental transformation of substances into new entities with unique properties.