Why was Antoine Lavoisier’s belief that oxygen caused acidity not completely correct?

Antoine Lavoisier’s belief that oxygen was the essential cause of acidity was not completely correct because it failed to account for acids that do not contain oxygen.12

Lavoisier had hypothesized that oxygen was the "generator of acids"—a concept reflected in the etymology of the element's name—after observing that many known acids were formed by combining nonmetals with oxygen. However, this theory was challenged by later experimental evidence. Specifically, the discovery that hydrochloric acid contains no oxygen demonstrated that the element was not a universal requirement for acidity.32

Subsequent research by chemists such as Sir Humphry Davy revealed that hydrogen is the key element in acids. Further developments in the 19th century, including the work of Justus von Liebig and the later electrolytic dissociation theory, shifted the scientific understanding of acidity toward the role of hydrogen ions in solution.2