Water: A Bronsted-Lowry Acid and Base

Water is amphoteric because it can act as both an acid and a base. In acidic solutions, water accepts protons from acids, functioning as a base. In basic solutions, water donates protons to bases, functioning as an acid. This dual capability allows water to participate in acid-base reactions regardless of the solution composition.

Water autoionization occurs when one water molecule donates a proton to another water molecule. The donating molecule acts as an acid, while the accepting molecule acts as a base. This produces a hydronium ion and a hydroxide ion. Although the process occurs to only a slight extent, it establishes the ion-product constant for water, KW, which equals 1 × 10−14 at 25 °C.

The ion-product constant for water, KW, equals the concentration of hydronium ions multiplied by the concentration of hydroxide ions, with a value of 1 × 10−14 at 25 °C. KW is important because it allows calculation of either ion concentration when the other is known, and it determines whether a solution is acidic, basic, or neutral based on relative ion concentrations.

A solution is acidic when hydronium ion concentration exceeds hydroxide ion concentration. A solution is basic when hydroxide ion concentration exceeds hydronium ion concentration. A solution is neutral when both concentrations are equal. Using KW and the known concentration of one ion, you can calculate the other and classify the solution accordingly.

In pure water at 25 °C, the concentrations of hydronium ions and hydroxide ions are equal, each at 1 × 10−7 M. This equality results from water's autoionization, which produces one hydronium ion for every hydroxide ion formed. These equal concentrations make pure water neutral.

Use the ion-product constant expression: KW = [H3O+][OH−]. Rearrange to solve for hydroxide ion concentration: [OH−] = KW / [H3O+]. For example, if hydronium concentration is 5 × 10−7 M, then [OH−] = (1 × 10−14) / (5 × 10−7) = 2 × 10−8 M, indicating an acidic solution.

Yes, water autoionization is endothermic, so it increases with temperature. At 25 °C, KW equals 1 × 10−14, but at 100 °C, KW increases to approximately 5.6 × 10−13, roughly 50 times larger. This temperature dependence means hydronium and hydroxide ion concentrations increase at higher temperatures.