Reagents are chemicals used in chemical reactions and laboratory tests. Synonyms for reagents include reactants, chemicals, substances, compounds, agents, ingredients, elements, solutions, and additives. These terms all refer to substances that are used to produce a chemical reaction or change in a reaction. Reactants are substances that are present at the start of a reaction, while chemicals and substances are broader terms that can refer to any substance used in a reaction. Compounds are substances made up of two or more elements, while agents and ingredients are terms that refer to substances used in a specific recipe or formula. Elements are substances that cannot be broken down into simpler substances, while solutions are mixtures of two or more substances. Finally, additives are substances added to a product to improve its performance or appearance.