Acid, bases, and alkalis

A particular chemical species can be defined as an acid, a base, or an alkali, depending on how it behaves in water.

Acids have been defined in three ways.

• An acid increases the amount of hydrogen ions when dissolved in water. (The Arrhenius definition)
• An acid is a species that donates a proton or hydrogen ion (H+). (The Brønsted-Lowry definition)
• An acid is a species that accepts an electron pair. (The Lewis definition)

Also, bases have three definitions that are basically the opposite of an acid.

• A base increases the amount of hydroxide ions when dissolved in water. (The Arrhenius definition)
• A base is a species that accepts a proton or hydrogen ion (H+). (The Brønsted-Lowry definition)
• A base is a species that donates an electron pair. (The Lewis definition)

An alkali is a base that dissolves in water. For example, both copper oxide and sodium hydroxide are bases, but only sodium hydroxide is an alkali because it dissolves in water.

Measuring acidity

The acidity of a solution depends on the number of hydrogen ions in the solution. Since there are millions of hydrogen ions in solution, we measure acidity on a scale called pH. pH is a logarithmic scale of the hydrogen ion concentration ([H+]).

pH is calculated with this equation: pH = - log[H+].

The values on the pH are usually between 0 to 14. An acidic solution contains more hydrogen ions than hydroxide ions, and the pH is less than 7. A basic contains less hydrogen ions than hydroxide ions, and the pH is more than 7. A neutral solution has an equal number of hydrogen and hydroxide ions, and the pH equals 7.