What it shows
The red pigment in hibiscus (gudhal) petals is an anthocyanin that turns pink red in acids and greenish or dark in bases. By testing kitchen liquids you can build a colour chart that places each liquid roughly on a scale from strong acid to strong base.
The science
Anthocyanins are coloured molecules that gain or lose hydrogen ions depending on how acidic the liquid is. In acid they hold an extra hydrogen ion and look red; as the liquid becomes basic they lose it and shift to purple, then green and yellow. The colour therefore acts like a rough pH meter, with acids at the red end and bases at the green end.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 5 to 6 fresh red hibiscus flowers (or dried petals from a tea shop)
- A cup of warm water (adult pours it)
- Six small transparent glasses or steel cups
- Lemon juice, vinegar, plain water, milk, baking soda water, washing soda or soap water
- A spoon and a dropper
- White paper for the chart and crayons or pencils
Safety
For this project: An adult must pour the warm water. Do not taste any of the test liquids once the petal extract is added, and wash hands after handling washing soda.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Tear the petals into small pieces and soak them in half a cup of warm water for ten minutes, crushing with a spoon until the water is deep red.
Strain the extract through a cloth into a clean cup; this is your indicator.
Pour two tablespoons of each test liquid into its own glass and label the glasses.
Add ten drops of indicator to each glass and swirl gently.
Line up the glasses against a white wall and arrange them from the pinkest to the greenest.
Colour a strip on the chart paper for each glass, in order, and write the liquid name underneath.
Add a line under the chart marking the acid end on the left, neutral in the middle and the base end on the right.
Test a mystery liquid (ask a family member to choose) and match its colour to your chart to guess what it is.
What you should see
Lemon juice and vinegar give bright pink red, water and milk stay purplish, baking soda gives blue violet and washing soda gives green. The chart lets you classify any new kitchen liquid by colour.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which end of your chart is acidic?
The pink red end, where lemon juice and vinegar sit.
2Why is the petal colour useful?
It changes colour with acidity, so it works as a natural indicator.
3What colour did plain water give and why?
A purplish colour, because water is neutral, neither acid nor base.
4Why use warm, not boiling, water?
Warm water pulls the colour out quickly without cooking the pigment and dulling it.
Take it further
Repeat with red cabbage, beetroot and rose petals, and compare which plant gives the clearest range of colours.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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