Bronsted Lowry Theory of Acids and Bases

An acid is a substance that donates H⁺. A base is a substance that accepts a proton(H⁺)

For example:

H₃O⁺ + Cl^- à HCl + H₂O

H₃O⁺ is simply water with acid in it. The proton will be strongly attracted to water, and form an H₃O⁺ ion. In formulas such as pH = -log [H⁺], you can interchange H⁺ with H₃O⁺. Whether this ion actually exists has been the subject of recent research.

In any case, since H₃O⁺ becomes water, it donates or loses an H⁺. This makes H₃O⁺ an acid. Less obvious is that relative to H₃O⁺, Cl^- is acting as a base because in becoming HCl it accepts a proton.

Another example:

NH₃ + HCO₃^- = NH₄⁺ + CO₃^-2

In the forward reaction, NH₃, in becoming NH₄⁺, is accepting a proton, so NH₃ is acting as a base. HCO₃^- is losing a proton (compare to what it became: CO₃^-2), so in this case, HCO₃^- is acting as a Bronsted acid.

For the reverse reaction, NH₄⁺ is the acid, and CO₃^-2, the base. The adjective conjugate is often attached to the acid and base for the reverse reaction.