Not a fact anymore

Every acid contains oxygen.

What we know now

Many acids contain oxygen, but some do not. Hydrochloric acid, hydrobromic acid, and hydroiodic acid are familiar counterexamples.

Why it changed

Lavoisier named oxygen from the belief that it was the acid-forming principle. Humphry Davy's work on chlorine and hydrogen chloride showed that a strongly acidic substance could contain no oxygen.

Status: Overturned
Category: Chemistry
Accepted for: ≈35 years
Accepted approximately: 1770s–early 19th century
Changed approximately: 1810s

Lavoisier’s theory successfully organized many known acids, most of which did contain oxygen.

Its failure helped shift chemistry away from defining substances by one presumed ingredient and toward reaction behavior and molecular structure.

Evidence

Sources and what they establish

Previous belief

OxygenRoyal Society of Chemistry
Explains that Lavoisier named oxygen 'acid-forming' because he thought it was the basis of all acids.

Historical context

ChlorineRoyal Society of Chemistry
Describes Davy's recognition of chlorine as an element and the clarification of hydrochloric acid chemistry.

Current evidence

AcidInternational Union of Pure and Applied Chemistry
Provides modern acid definitions that do not require oxygen.

Related entries

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Around 1810

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