A chemical species is an acid or base, depending on how it reacts in aqueous solution.
According to the Bronsted-Lowry definition, an acid (HA) is a species that donates a proton in aqueous solution. A base (B) is a species that accepts a proton in aqueous solution. Compare this with Arrhenius definition and Lewis definition.
The ability to accept or donate a proton depends on the chemical structure of the molecule.
The protons (H+) donated by an acid combine with the water molecules to form hydronium ions (H3O+). A base receives protons from water molecules. The water molecules then become hydroxide ions (OH-).
These two different reactions occur because water is amphoteric - it can either donate or receive protons.
The pH scale represents how acidic or basic a substance is.
It is important to note that acids and bases, especially strong ones, can corrode metals and burn human skin.