University of Virginia
Physics Department

Flower Indicators

A Physical Science Activity

Adapted from an activity in Holt Physical Science by Mapi Cuevas and William Lamb (1995).

2003 Virginia SOLs



Students will:


Motivation for Learning

Demonstration-Discrepant Event

Demonstrate the use of an indicator by adding 250 ml of alcohol into a flask.  Then add 5-6 drops of bromothymol blue indicator solution.  The solution should be blue in color.  If it is not, then add a few drops of dilute NaOH.  Tell the students that you are going to change the color of the solution in the flask by reciting a special incantation.  Repeat the incantation several times.  After each incantation stopper the flask and swirl gently.  The solution should change from blue to green to yellow.  This occurs as the CO2 from your breath dissolves to form carbonic acid.

Background Information

Indicators are substances that change color in an acid or a base.  Some indicators show only two color changes, one for acids and one for bases.  Other indicators show a range of color changes depending on the pH.  A good indicator should change colors over a narrow pH range.  Some pH test papers are made from mixtures of indicators in order to get color change over the entire pH scale.

Teacher Preparation

Prepare the acid and base solutions and put each in a bottle with a medicine dropper. The following flowers work well: red roses, orange hibiscus, purple violets, red carnations, etc.


Student Activity

To print out the Student Copy only, click here.

Materials (for each lab group)



  1. Prepare a table like the one shown below.
  2. Using a mortar and pestle, crush the petals of one of the flowers.
  3. Add 8 ml of alcohol to the mortar and continue to crush the flower petals.
  4. Use the medicine dropper to transfer one ml of the alcohol and crushed petal solution into a single well.  Continue adding the solution to six separate wells lined up vertically.
  5. Thoroughly rinse the mortar and pestle with tap water.
  6. Repeat steps 1-5 with each type of flower.
  7. Use a dropper to add one ml of  HCl to each of the wells in line #1.
  8. Use a dropper to add one ml of  vinegar to each of the wells in line #2.
  9. Use a dropper to add one ml of distilled water to each of the wells in line #3.
  10. Use a dropper to add one ml of ammonium hydroxide to each of the wells in line #4.
  11. Use a dropper to add one ml of sodium hydroxide to each of the wells in line #5.
  12. Use a dropper to add one ml of unknown acid or base to each of the wells in line #6.
  13. Draw a diagram showing the color change after adding the acid and base solutions.
  14. Determine relative pH of the unknown solution.




  1. Search the Internet or the library to find out more about photo chemicals and the photography process.
  2. Using what you know, write a paragraph explaining how acids and bases are not only used in science laboratories.


Students with Special Needs

For students with processing difficulties it may be helpful to read one step at a time and have all groups complete that step before going further.  Then the teacher could be sure that the students are doing what they should and getting proper results.  Another strategy would be to have students physically check off the procedure steps as they complete them.

Click here for further information on laboratories with students with special needs.




To print a copy of the assessment only, click here.

  1. What is the relative pH of the unknown solution?

  2. Which flowers made the best indicators?  Explain why.

  3. Could two flowers of different colors be used to indicate the same pH?  Explain your answer.