Concept preview

What you will be able to do

1. 01State the Arrhenius and Brønsted-Lowry definitions
2. 02Identify conjugate acid-base pairs
3. 03Recognise amphiprotic species

The first useful definitions of acids and bases came from Svante Arrhenius in the 1880s. He defined an acid as a substance that produces hydrogen ions, $\mathrm{H^+}$, in water, and a base as a substance that produces hydroxide ions, $\mathrm{OH^-}$. Hydrochloric acid fits, because $\mathrm{HCl \rightarrow H^+ + Cl^-}$, and so does sodium hydroxide, because $\mathrm{NaOH \rightarrow Na^+ + OH^-}$. The Arrhenius picture is correct as far as it goes, but it is limited: it only describes aqueous solutions, and it cannot explain why a substance like ammonia, which contains no $\mathrm{OH^-}$, is nonetheless basic.