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Is Water H20?

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SCI Cambridge & Great Eastern Region, RSC Mid-Anglia Section & CU ChemSoc

Every schoolchild knows that water is H2O , but it was a terribly difficult thing for scientists to learn originally. The story begins with the Chemical Revolution of the late 18th century, in which Lavoisier’s notion that water was a compound of oxygen and hydrogen flew in the face of the traditional wisdom that it was an element. Opposition to Lavoisier persisted in various corners for much longer than usually recognised. Cavendish, whose experiments had taught Lavoisier how to make water from hydrogen and oxygen, thought that hydrogen and oxygen were merely water with an excess or a deficit of ‘phlogiston’. Priestley, who had made oxygen before Lavoisier, adopted Cavendish’s view of water and defended it to his death in 1804. Even the electrolysis of water in 1800 failed to produce a complete consensus, and in fact a significant condundrum was raised by the macroscopic distance between the locations of oxygen-production and hydrogen-production in electrolysis.

Disputes about the mechanism of electrolysis continued throughout the 19th century, and deciding that water was a compound was by no means the end of the story. When Dalton published his atomic theory in 1808, he gave the formula of water as HO, with atomic weights of H and O as (roughly) 1:8. Avogadro’s H2O formula was published shortly after this, but it was initially rejected by the majority of chemists. It took half a century of debates, and in the end, insights from organic chemistry before consensus, was reached on the modern set of atomic weights and molecular formulae including H2O . This story of the changing ontology of water is not only fascinating in itself, but illustrates many important points about the nature of scientific knowledge and its development.

Prof Chang’s research interests include history and philosophy of chemistry and physics from the 18th century onward; philosophy of scientific practice; other topics in the philosophy of science, including measurement, realism, evidence, pluralism and pragmatism. His book, Is Water H2O ? Evidence, Pluralism and Realism, will be available shortly from Boston Studies in the Philosophy of Science (Springer).

This talk is part of the Department of Chemistry series.

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