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Colourful Chemistry

Cook up a storm in the kitchen... just don’t eat it! Investigate the chemical nature of the stuff in your kitchen and learn about indicators, acids and bases.

Australian Curriculum links

  • Science > Science as a Human Endeavour > Year 2 > ACSHE035
  • Science > Physical Sciences > Year 5 > ACSSU080
  • Science > Chemical Sciences > Year 9 > ACSSU179

You'll need

  • tumeric
  • disposable plastic cups
  • methylated spirits
  • unbleached toilet paper
  • disposable plastic plates
  • bicarbonate of soda
  • vinegar

Try this

Safety: Turmeric can stain clothing and surfaces if not cleaned immediately. Methylated spirits is a poisonous and potentially hazardous chemical. Perform this activity with adult supervision in a well-ventilated area.

  1. Place two heaped teaspoons of turmeric into the bottom of a plastic cup.
  2. Cover the turmeric with about 3 cm of methylated spirits and stir.
  3. Place a sheet of toilet paper on the bottom of a disposable plastic plate.
  4. Once all the turmeric has settled to the bottom of the cup, pour only the yellow fluid onto the toilet paper until the toilet paper is yellow.
  5. Leave the turmeric-dyed toilet paper to dry.
  6. Cut the turmeric-dyed toilet paper into strips.
  7. Place two teaspoons of bicarbonate of soda into a clean cup and mix with two teaspoons of water.
  8. Dip a strip of the turmeric-dyed toilet paper into the bicarbonate of soda solution. Observe what happens.
  9. Dip the same strip of turmeric-dyed toilet paper into 2 teaspoons of vinegar. Observe what happens.

Further investigation

Your kitchen probably contains lots of substances with a range of pH levels. Why not test some substances in your house using the turmeric-dyed toilet paper to see if you can work out which are acidic and which are alkaline? Try milk, window cleaner, soap, lemon juice, soft drink, coffee or anything else that is safe to handle.

What's happening?

Turmeric (Curcuma longa) is a plant whose parts are often used as a spice in cooking. Turmeric is an example of a natural pH indicator, which means that it can be used to determine a substance’s pH.

The pH of a substance is its level of acidity or alkalinity. To be more precise, pH is a measure of the concentration of hydrogen ions (H+) relative to hydroxide ions (OH-) in a substance. The range of pH values is represented on a logarithmic scale from 1 to 14. If a solution contains more hydrogen ions than hydroxide ions it is considered acidic and will have a pH between 1.0 and 6.9. However, if there are more hydroxide ions than hydrogen ions, the solution will be alkaline and will have pH between 7.1 and 14. If the amounts of hydrogen ions and hydroxide ions are equal - such as in pure water - the solution is considered neutral and has a pH of 7.0.

Indicators, such as turmeric, work by changing their colour with changes in pH. There are many different pH indicators and each indicator changes colour at a particular pH level. Often an individual indicator will only undergo one or two colour changes. This usually means that using a single pH indicator will tell you the general pH range of an unknown solution. Usually several different indicators must be used and the results compared in order to accurately determine the pH.

The turmeric indicator changes colour between roughly a pH of 7.4 and 8.6. If turmeric is exposed to neutral or acidic substances (those with a pH of less than 7.4) it will retain its yellow colouration. However, if turmeric is exposed to more alkaline substances (those with a pH greater than 8.6) it becomes a dark pink/red.

Bicarbonate of soda solution is alkaline so the turmeric-dyed toilet paper turned from yellow to dark pink/red when it was dipped in the bicarbonate of soda solution. Vinegar is acidic so the turmeric-dyed toilet paper turned from dark pink/red to yellow when it was dipped in the vinegar.

Real world links

Turmeric has long been used for medical treatment in Asia. In India, turmeric is often used as an antiseptic for burns and cuts. People in some Asian countries take turmeric as a dietary supplement to help with stomach problems. Western medicine is also beginning to realise its therapeutic properties. Curcumin, the active ingredient in turmeric, is being investigated for its potential to treat various cancers, Alzheimer’ disease, arthritis and cystic fibrosis.