Gay-Lussac’s Law of Combining Volumes illustrates a fundamental principle in gas reactions: Gases will react in volumes that are in simple ratios when measured at the same temperature and pressure. This law can be expressed mathematically as a ratio. For instance, in the reaction of hydrogen and oxygen to form water, 2 volumes of hydrogen (H₂) react with 1 volume of oxygen (O₂) to produce 2 volumes of water vapor (H₂O). Thus, understanding Gay-Lussac’s Law is essential for predicting how gases will behave during reactions, particularly in stoichiometric calculations, and links closely with Avogadro’s Law, which states that equal volumes of gases contain equal numbers of molecules. This interrelation is paramount in various applications including chemical engineering and environmental science.