The teaching strategy is both multifacetted and multileveled. At the lowest level, litmus paper testing of different household products might be an effective means of introducing the student to acid and bases. However, for purposes of this discussion, it will be presumed that the class is very advanced and the teacher may decide how far and how fast to proceed. Acid-base indicators are substances that change color as a function of pH, usually over a range of 1 to 2 pH units. These indicators are weak acids or weak bases which disassociate and change colors when equilibrium shifts per Le Chatelier's principle.
Indicators are available that shift color for every value of the pH scale.
The earliest indicators were extracted from plants. To beaker 1, add 100 mL of 0.1 M HCl. This solution has a pH of 1. Measure 10 mL of this solution and add it to beaker 2 with 90 mL distilled water. The resulting solution has a pH of 2.