The Brønsted–Lowry definition of acids and bases explains them in terms of proton transfer. According to this concept:
- An acid is a substance that can donate a proton (a hydrogen ion).
- A base is a substance that can accept a proton.
This definition is broader than the Arrhenius theory because it is not restricted to water-based solutions. It can be applied to reactions in gases and in solvents other than water.
An important idea in this theory is the concept of conjugate acid–base pairs. When an acid donates a proton, it turns into its conjugate base. Similarly, when a base accepts a proton, it turns into its conjugate acid.
In simple words: acids give away protons, bases take in protons. This approach helps explain many acid–base reactions that the Arrhenius definition cannot describe.