Red Cabbage Indicator

that really changes colour.

 

 

 

Category - Chemicals & Reactions

Key Idea - Acids & Bases

 

Purpose - Students can create their own indicator and use it to test if various substances are acidic or basic.
Nature -  Class Practical
Materials -  2 X 250 ml beaker, red cabbage, stirring rod, Bunsen burner, tripod, gauze mat, water, matches, knife
Method - 
  1. Chop enough red cabbage to three quarters fill a 250ml beaker, and fill with water.
  2. Set this to boil for 15 minutes.
  3. Decant the extract into the other beaker and allow to cool.  It should be a purple colour.
  4. Test the colour or the extract in a known acid and a known base.  You can now use this to test other unknown substances.
Safety - Care needs to be taken when handling boiling water.
Explanation -

Indicators change colours depending on the pH of the solution they are placed in.  

Natural red pigments from plants tend to be indicators, being red in acidic solutions and green in basic solutions.
Notes - You can also try this with other red plant parts such as red rose petals, but the cabbage seems to be the most effective.  Being organic, this solution is best stored in the refrigerator in the short term, and cannot be kept for more than a few days.