As new evidence presents itself, scientific theory is developed and refined over time.

Antoine Lavoisier

In 1777, the French chemist Antoine Lavoisier proposed that acidity was caused by the presence of oxygen in a compound. This was the basis on which oxygen gained its name – oxygen means ‘acid-maker’ in Greek.

Sir Humphry Davey

In 1810, following his investigation of hydrochloric acid, the British chemist Sir Humphry Davy concluded that it was the presence of hydrogen, and not oxygen, that made a compound acidic. However, Davy’s theory was complicated by the fact that there are non-acidic hydrogen-containing substances (like methane or ethanol, for example). To overcome this, Davy contested that hydrogen-containing substances are only acidic if one or more of their hydrogens can be ‘replaced’ by a reactive metal.

Whilst Davy's theory was valid for many acids and bases, it was identified that there are still substances that contain replaceable hydrogen but are not acids. For instance, methane, water and ammonia all contain replaceable hydrogen, but do not display acidic properties.

The Arrhenius Model

The first modern acid-base model was developed by Svante Arrhenius in 1884. The Arrhenius model defines an acid as a substance that dissociates in water to produce hydrogen ions and it defines a base as a substance that dissociates in water to produce hydroxide ions. However, this Arrhenius definition does not explain why non-aqueous substances can exist as acids and bases.

The Brønsted‐Lowry Model

To account for the acid-base behaviour of non-aqueous substances, Johannes Nicolaus Brønsted and Martin Lowry independently proposed what is known as the Brønsted‐Lowry model of acids and bases. Formulated in 1923, the model defines acids as proton donors and bases as proton acceptors. This acid-base model is the broadest and most accurate of those covered in the Year 12 Chemistry ATAR course.