Rose litmus paper

Activity

Discover how some plants can be used to create an indicator paper to show pH levels. Test to see which everyday solutions are acid or alkali.

  • Estimated time: 30 minutes
  • Location: Indoors
  • School term: All year round
  • Level of experience: No experience needed
  • Subject(s): Science, Art&DT

Learning objectives

  • Investigate acid and alkali solutions
  • Through scientific enquiry into properties of materials, look for changes, patterns, similarities and differences in order to draw simple conclusions and answer questions

Preparation

Introduce the concept of acid and alkali by discussing/describing harmless, everyday kitchen substances. 

Equipment

  • Fresh rose petals
  • A4 paper
  • Scissors
  • Bicarbonate of soda
  • Lemon juice
  • White vinegar
  • Bar of handsoap
  • Cotton buds

Background information

In a plant anthocyanins serve as key antioxidants and contribute to the colour of flowers.

Anthocyanins are pH dependent. When the acidity changes, the colour changes.

Anthocyanin pigments can be found in red or purplish fruits and vegetables, including purple cabbage, blackberries, blueberries, cherries, raspberries and purple grapes.

The amount of anthocyanin can affect the colour change.

Step by step

Many flowers, fruits and vegetables contain chemical substances that change colour in solutions of different pH such as hibiscus, turmeric and red cabbage.

  1. Rub fresh red or purple rose petals over two A4 pieces of plain paper.
  2. Cut one of the A4 sheets into small test strips.
  3. Cut the other rose sheet into two pieces. Paint one of the pieces all over with a solution of bicarbonate of soda and allow to dry, then cut into small test strips.
  4. Paint the remaining piece of your second sheet with lemon juice and allow to dry. Then, cut into small test strips.
  5. Using a cotton bud and test strip, dot on samples of acidic and alkaline solutions.
  6. Record the results.
  7. Try the same test with other plants and see which make good pH indicators. 

Hints & tips

  • Plant pigments can act as acid-base indicators. Do all red and purple plants work?
  • Keep a check list to show what happens when they come in contact with acid or alkali.
  • More advanced learners could be encouraged to be systematic in their testing.
  • Check out the exciting Red cabbage indicator lesson plan.