Resource 1: Making indicators from plants
Background information / subject knowledge for teacher
Extraction and testing of flower indicators
A lot of local flowers make good indicators to test for acids and alkalis. You can collect some flowers yourself or ask your students to bring some in. Hibiscus usually works very well as do red, violet, yellow or pink flowers. To extract the colour you can use ethanol, white spirit or petroleum ether. If you don’t have those then, for some flowers, hot water will work.
Apparatus per group
Flowers (collected by students or yourself); beaker, jam jar or tin can; 5 test-tubes + rack OR a white plate; mortar and pestle; teat pipette or drinking straw to add drops; candle spirit burner or Bunsen burner; tripod stand or improvised support for the beaker or can; glass rod or stick for stirring; 5 test solutions (e.g. wood ash solution, sodium hydroxide; lemon juice; hydrochloric acid; water; cleaning fluid, vinegar, bicarbonate of soda, toothpaste). For each of the substances, mix them with a few cubic centimetres of water.
- Pick some flowers from one type of plant.
- Tear or cut them into small pieces.
- Put them into a tin or beaker or mortar and pestle. Add about 10 ml of solvent.
- Grind the petals until the liquid stops getting darker and decant the liquid into a test tube. This is the indicator.
DO NOT HEAT the spirit as it is highly flammable. Keep it away from naked flames.
If you don’t have a suitable solvent:
Place the petals in a beaker or a tin can.
Warm the beaker. Stir until the water becomes a deep colour. This is the indicator.
Pour the solutions you are testing into five different test tubes.
Put a large drop of each solution you are testing onto a white plate. Make sure they are as far apart as possible.
- Add drops of the indicator to each solution you are testing.
- Note the colour the liquid goes in acids, alkalis and neutral solutions.
Notes for teachers
- See Risk assessment – Resource 2 .
- If the flower is large (e.g. hibiscus) one or two will be enough. More flowers will be needed if they are small.
- Ensure some groups do hibiscus or other local flowers that you know give good results. Bougainvillea does not dissolve and will need ethanol or another colourless spirit to extract the colour.
- You can filter or decant to separate the flower pieces from the solution, but this is not necessary.