What determines if a reaction is classified as heterogeneous?

A reaction is classified as heterogeneous when it involves reactants in more than one phase. This means that at least one of the reactants must exist in a different physical state from the others, which can include combinations of solids, liquids, and gases. For example, a chemical reaction where a solid reactant reacts with a liquid or gas is considered heterogeneous.

In contrast, a reaction involving only a single phase, such as all gaseous or all aqueous species, would be categorized as homogeneous. The presence of multiple products does not inherently indicate heterogeneity, as products can also exist in a single phase. Similarly, the speed of a reaction pertains to its kinetics and does not contribute to its classification as heterogeneous or homogeneous. Therefore, the correct classification refers to the phases of the reactants involved in the reaction.