What is the term for a number used to indicate the ratio of chemicals in a reaction?

The term that refers to a number used to indicate the ratio of chemicals in a reaction is "coefficient." In chemical equations, coefficients are placed in front of the compounds to specify the number of molecules or moles of each substance involved in the reaction. This is crucial for balancing chemical equations and understanding the stoichiometry of the reaction, as it ensures that the law of conservation of mass is upheld by showing the proportionate amounts of reactants and products.

For example, in the reaction 2H₂ + O₂ → 2H₂O, the coefficient "2" in front of H₂ indicates that two molecules of hydrogen gas react with one molecule of oxygen gas to produce two molecules of water. Coefficients therefore provide valuable insight into the relationships between different substances during a chemical reaction.