The ancient world viewed water as an irreducible part of the cosmos, recognizing its pervasive influence on life and nature. For millennia, it was not viewed as a substance that could be broken down. This perspective was fundamentally challenged during the Scientific Revolution, leading to one of the most profound discoveries in chemistry. The discovery that water is the compound H₂O marks a pivotal shift from philosophical speculation to quantitative, modern science.
Water as a Fundamental Element
For nearly two thousand years, the understanding of matter was dominated by the philosophy of Aristotle. His system posited that all terrestrial matter consisted of four fundamental elements: earth, air, fire, and water. Water was characterized by the qualities of being cold and wet, and was considered a simple, unchangeable, and primary substance.
This belief system persisted throughout the Middle Ages and into the 18th century, profoundly influencing early chemical thought. Early chemists assumed water was an elemental building block of the universe. This long-standing philosophical framework made the eventual experimental proof of water’s composite nature in the 1780s revolutionary.
The 18th-Century Composition Experiments
The true nature of water began to reveal itself in the 1780s through meticulous experiments involving gases. In 1781, the English philosopher Henry Cavendish systematically sparked a mixture of “inflammable air” (hydrogen) and “dephlogisticated air” (oxygen). This experiment resulted in a measurable quantity of pure water forming on the walls of the sealed container.
Cavendish’s quantitative methods showed that a precise two-to-one ratio of the two gases completely converted to water. However, adhering to the prevalent phlogiston theory, Cavendish initially misinterpreted his results, believing he had merely condensed water from the gases.
The groundbreaking interpretation was provided by the French chemist Antoine Lavoisier. Lavoisier correctly recognized that Cavendish’s experiment demonstrated the synthesis of water from two simpler elements. In 1783, Lavoisier performed follow-up experiments, including the decomposition of water by passing steam over red-hot iron, which yielded hydrogen and oxygen.
Lavoisier concluded that water was a compound made of hydrogen—a name he coined meaning “water-former”—and oxygen. This evidence, confirmed by both synthesis and analysis, definitively established in the mid-1780s that water was a compound of two elements in a fixed proportion.
Establishing the Chemical Formula H₂O
Although the composition of water was known by the mid-1780s, the modern chemical formula H₂O was not immediately adopted. The precise symbolic representation required the development of a coherent atomic theory and standardized nomenclature.
John Dalton’s atomic theory, published in the early 1800s, proposed that atoms combine in simple, whole-number ratios to form compounds. Dalton’s initial formula for water was HO, as he incorrectly assumed elements combined in the simplest possible one-to-one ratio.
The realization that the formula was H₂O—reflecting two atoms of hydrogen for every one atom of oxygen—came from later experiments involving combining volumes of gases, notably by Joseph-Louis Gay-Lussac and Amedeo Avogadro.
The current system of using letters and subscripts was formalized by the Swedish chemist Jöns Jacob Berzelius in the 1810s. Berzelius assigned letters as symbols and used numbers to indicate the proportions of atoms in a compound. This system allowed chemists to represent water as H₂O, though universal agreement on the exact formula took several decades.