Acid–base indicators – litmus, phenolphthalein, red cabbage, universal indicator
Integrated ScienceChemical ReactionsAges 13–14Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Chemical Reactions
Nine test tubes contain lemon juice, vinegar, HCl, distilled water, salt water, soapy water, baking soda, NaOH and limewater. Students dip litmus paper or add phenolphthalein, red cabbage juice or universal indicator and observe the color, place each solution on the pH scale and classify it as acidic, neutral or basic. A summary table records the color of every solution with all four indicators.
Lesson: Acids, bases and the pH scale – indicators
What it shows
Acid–base indicators are dyes whose color depends on pH, so they show whether a solution is acidic (pH below 7), neutral (pH 7) or basic (pH above 7). Litmus only sorts solutions into these three groups: red in acid, blue in base. Phenolphthalein is colorless below about pH 8.2 and pink above, so it detects bases but cannot tell an acid from water. Red cabbage juice, which contains anthocyanins, and universal indicator pass through a series of colors, letting students estimate pH and rank everyday solutions such as lemon juice, vinegar, baking soda and limewater.
How to use
Choose an indicator at the top, then tap each test tube or press Test all in turn, and read the colors in the summary table. Repeat with the other three indicators. Tick Show pH to place each solution on the pH scale, or tick Classify yourself so that students select a tested tube and press Acid, Neutral or Base before checking their answer. Indicator drops sets how deep the color of the liquid indicators appears.
Parameters you can change
- Indicator Litmus paper, Phenolphthalein, Red cabbage juice, Universal indicator
- Drops of liquid indicator 1–8 drops
- Show pH value
- Students classify acid/base themselves
Questions to explore
- Why can phenolphthalein not tell vinegar apart from distilled water?
- Using red cabbage juice, how would you arrange the solutions in order of increasing pH?
- Baking soda and limewater are both basic. Which indicator shows that limewater is the more strongly basic?
