Phenolphthalein indicator being compared in acid (left) and alkaline (right) solutions. Changes in colour (here pink to colourless) are a common way to test the pH of a solution. Alkali and acid are the two parts of the pH scale, indicating the hydrogen ion concentration in a solution relative to that found in water. Alkalis contain less free hydrogen ions than water and acids contain more of these hydrogen ions. Water is considered neutral. Phenolphthalein (C20.H14.O4) is colourless, but the ion that forms with decreasing acidity is pink. The colour change occurs around pH 9, and is used to indicate acid neutralisation of an alkali.