Acid-Base Indicators


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This activity introduces the idea of adding an extra substance to a reaction to assist in clarifying what is happening in the reaction.

You will need:

  • a quarter of a red cabbage OR coloured plant material (red or purple flowers generally work well) OR fresh beetroot juice
  • a small saucepan
  • a saucer
  • a piece of filter paper or white construction paper, about 1 ↔ 10 cm.
  • 10 ml of vinegar (2 teaspoons)
  • 1 teaspoon of bicarbonate of soda dissolved in water.
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    What to do:

    Cut up the plant material into small pieces and place it in the saucepan with a small amount of water. Boil gently until the water has the colour of the plant material. (Add more water if the plant material is getting too dry.) You have now extracted the plant dye. (Make sure the colour in the water is strong.) Pour the water and dye into a saucer to cool and then dip one section of your filter paper into the dye. You have now made indicator paper.

    Place a small drop of vinegar onto one side of your dyed paper and a drop of bicarbonate of soda dissolved in water onto the other.

    Note and record any differences in the effects of the two substances on the colour of the dye.

    (If your dye is truly an indicator then the acid and the base should give a different colour reaction.)

    Optional: You can also test other household chemicals to see whether they have acidic or basic properties. Or you can try other dyes to see whether they can act as indicators (e.g. tea, tumeric).

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