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Grades 8–10 Intermediate About 15 min Version 1 Awaiting review Premium

How acidic is it? The pH scale

Some indicators only tell you whether a substance is an acid or a base. Universal indicator is a mixture of several indicators, so it shows a whole range of colours. Each colour matches a number on the pH scale, which runs from 0 to 14. A pH below 7 is acidic, exactly 7 is neutral, and above 7 is basic (alkaline). The further from 7, the stronger the acid or base.

What you will learn

  • Use universal indicator and a colour chart to find the pH of a solution.
  • Classify solutions as acidic, neutral or basic from their pH.
  • Compare how strongly acidic or basic different household liquids are.

Before you start

  • Know that indicators change colour in acids and bases. Try "Natural indicators: acids and bases" first.

Materials

  • Universal indicator
  • Lemon juice, vinegar, milk, distilled water, baking soda solution, soap solution and limewater

Equipment

  • 6 test tubes in a rack
  • pH colour chart

Procedure

  1. Add a few drops of universal indicator to a test tube.
  2. Add one household liquid to the tube and stir.
  3. Match the colour to the pH chart: red (pH 1–2), orange (3–4), yellow (5–6), green (7), blue-green (8), blue (9–10), purple (11–14).
  4. Test as many liquids as you can and record each pH.

How this simulation is simplified

  • Each liquid is shown at a typical pH value. Real samples vary slightly.
  • The colour is read instantly after stirring, as with a pH meter reading.