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The Arrhenius Definition

According to Arrhenius, an acid is a substance that increases H+ ions in an aqueous solution. A base is a substance that increases OH– ions in an aqueous solution. The Arhhenius definitions explain the reactions involving acids that contain acidic hydrogens, and bases that contain basic hydroxyl groups (e.g., metal hydroxides). But the definitions are not able to explain the behavior of acids and bases of other types, such as in nonaqueous solutions.

The Bronsted-Lowry Definition

During the early twentieth century, scientists J.N. Bronsted and T.M. Lowry put forth their theories. According to them, an acid is a proton donor, and a base is a proton acceptor.

Consider the ionization reaction of hydrogen iodide. Since hydrogen iodide is a strong acid, it completely dissociates. In this ionization reaction, note that the water molecule acts as the base.

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In this ionization reaction, obviously HI is the acid. HI transfers or donates its proton to water. Thus, water is the base in this reaction because it accepts the proton. The conjugate base of HI is I–.

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