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