Law of Combining Volumes Definition

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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.


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.


  • Crosland, M.P. (1961). "The Origins of Gay-Lussac's Law of Combining Volumes of Gases." Annals of Science 17 (1): 1. doi:10.1080/00033796100202521
  • Gay-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.
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Helmenstine, Anne Marie, Ph.D. "Law of Combining Volumes Definition." ThoughtCo, Aug. 29, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 29). Law of Combining Volumes Definition. Retrieved from Helmenstine, Anne Marie, Ph.D. "Law of Combining Volumes Definition." ThoughtCo. (accessed January 23, 2021).