^BLitmus indicator^b, a dye extracted from lichens, being compared in acid and alkaline solutions. Changes in colour (here from dark red to red or purple) 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 (pH 8-14) contain fewer free hydrogen ions than water, while acids (pH 1- 6) contain more. Water (pH 7) is considered neutral.