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Investigating Natural Indicators for Acids and Bases


To investigate natural indicators for acids and bases.

HOW DOES IT WORK?                                                                

Time Allocation: 45 min

The class can be divided into groups and each group can prepare different indicators – red cabbage.

Learners chop up the plant source (red cabbage) into small pieces. Then it’s soaked overnight in isopropyl alcohol (propan-2-ol). If the extract is too diluted, it can be heated over a water bath. This can be done one day prior to the experimental procedure.

The extract can also be obtained by boiling the ground plant source for 4 – 5 minutes in water in a microwave oven.

Each group uses universal indicator to determine the pH of the following substances: distilled water, tap water, lemon juice, commercial vinegar, 0,1 mole∙dm-3 HCl(aq), soap, 20% ammonia solution and a saturated solution of bicarbonate of soda

  • Knife and cutting board
  • Tap water
  • Lemon juice
  • Vinegar
  • Liquid soap
  • Saturated solution of bicarbonate of soda
  • Red cabbage or Red Onion
  • 10ml measuring cylinder
  • 0,1 mole∙dm-3 HC (aq)
  • 20% ammonia solution
  • 1 x Beaker (1000 mℓ)
  • 1 x Beaker (600ml)
  • 6 x Test tubes
  • Safety goggles
  • Universal pH Indicator test paper
  • Filter paper
  • Funnel
  • Isopropyl (rubbing) alcohol
  1. Use the knife and cutting board to cut the red cabbage or red union as fine as possible in small parts
  2. Transfer the cabbage/onion to the 1000 mℓ beaker and cover with alcohol approximately 50ml – leave overnight.
  3. Next day – Filter the cabbage/onion with filter paper and funnel to extract plant material into a 600ml beaker.
  4. Pour the following respectively into Test tubes: Hydrochloric Acid (HCℓ(aq)), Ammonia (NH3(aq)), tap water, lemon juice, vinegar and Bicarbonate of Soda, place them in a test tube rack.
  5. Mark each test tube with A – Hydrochloric Acid; B – Ammonia; C – Tap Water; D – Lemon Juice; E – Vinegar and F – Bicarbonate of Soda.
  6. Transfer 10 ml of the plant extract into each test tube with a 10ml measuring cylinder.
  7. Observe the colour change and complete the table below.
  8. Use the universal testing paper test the PH of each substance.


  • Wear safety goggles when conducting the experiment.
  • Work in a well-ventilated room – alcohols have strong odours.
  • Do not heat the plant material in the alcohol solvent in an open flame – use a water bath, because alcohols are flammable.
  • Take care when handling HCℓ(aq).