Science, Tech, Math › Science Law of Combining Volumes Definition Share Flipboard Email Print JoZtar / Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated January 15, 2020 In chemistry, the law of combining volumes is a relation stating that the relative volumes of gases in a chemical reaction are present in the ratio of small integers (assuming all gases are at the same temperature and pressure). The law of combining volumes is also known as Gay-Lussac's law, as Gay-Lussac described how the pressure of enclosed gas is directly proportional to its temperature circa 1808. Gay-Lussac found two volumes of hydrogen and two volumes of oxygen reacted to yield two volumes of water. Amedeo Avogadro stated the hypothesis in terms of molecules, although his hypothesis was not accepted until 1860. Avogadro's statement of the same reaction would be two molecules of hydrogen plus one molecule of oxygen react to yield two molecules of water. Examples In the reaction 2 H2(g) + O2(g) → 2 H2O(g) 2 volumes of H2 react with 1 volume of O2 to produce 2 volumes of H2O. Sources Crosland, M.P. (1961). "The Origins of Gay-Lussac's Law of Combining Volumes of Gases." Annals of Science 17 (1): 1. doi:10.1080/00033796100202521Gay-Lussac (1809). "Mémoire sur la combinaison des substances gazeuses, les unes avec les autres." (Memoir on the combination of gaseous substances with each other) Mémoires de la Société d'Arcueil 2: 207–234.