Acid-base indicators being compared in an acidic (along top) and alkaline (along bottom) solution. The indicators and the colour changes from acid to alkali are, from left to right: phenolphthalein (colourless-pink), methyl orange (red-yellow), and bromocresol green (yellow-blue). Alkalis and acids are the two sides 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.