According to the definition used by environmental chemists, an acid is a substance that increases the hydrogen ion (H⁺) concentration in a solution and a base is a substance that removes hydrogen ions (H⁺) from a solution. In water, removal of hydrogen ions results in an increase in the hydroxide ion (OH^-) concentration. Water with a pH of 7.0 is neutral, while lower pH values are acidic and higher pH values are basic.