Cabbage Juice Indicator - Experiment 22

Chemists use indicators to test whether a substance is an acid or a base. Indicators work by turning a distinctive color in the presence of an acid or a base. You can make your own indicator from red cabbage. You can also make indicators from the juice of elderberries, blackberries, radish skins, apple skins, or cherries.

Materials Substitutions
hot plate
1 head red cabbage
food processer knife and cutting board
1000 mL beaker large size saucepan
500 mL beaker large jar
4-5 250 mL beakers 4-5 small jars
sieve tea strainer or collander
substances to test **
distilled water
rubbing alcohol

** recommended materials: baking soda, bathroom cleaner (e.g. Formula 409™), washing soda, vinegar, lemon juice, milk, cream of tartar, orange juice, milk of magnesia, lime, soft drinks, or ammonia

Procedure

  1. *Chop red cabbage up finely. Boil a pint of water in a saucepan.
  2. *Add the red cabbage carefully to the boiling water and take the saucepan off the heat. Let it stand for 30 minutes or until it is completely cool.
  3. *Strain the liquid into a jar and throw away the used cabbage. The liquid should be a dark reddish-purple color. Add rubbing alcohol, or refrigerate, to reduce the spoilage of the indicator. Use a 1 : 5 ratio of alcohol to water.
  4. The color will change as you add acids or alkalis. To test a substance, pour a small amount of your substance into a small jar. Then add a drop or two of the cabbage juice indicator. A change in color indicates its acidity or basicity.

* See Teacher’s Notes

Colors of Red Cabbage Juice and Different pH values

color      red         rose     purple     blue          green       yellow

pH       1   2    3   4    5    6    7    8   9  10   11   12   13   14

        ACID                     neutral                           BASE

Data and Observations

Substance Color Approximate pH Acid or Base?
lemon juice
lime
washing soda
ammonia
cream of tartar
muriatic acid
Formula 409™
baking soda
vinegar
Sprite™

Extensions
Soak some filter paper in the cabbage juice indicator. Allow the paper to dry, then cut it into strips. Conduct an “at home” pH test of other household items. Tape your strips to a piece of notebook paper and bring them back to class. Compile your results. What can you say about household cleaners? Where are most household acids found?

Teacher’s Notes

  1. Lemons, vinegar, cream of tartar (potassium acid tartrate), orange juice, and sour milk will be acidic solutions.
  2. Pure distilled water is the only substance listed that should be neutral.
  3. Tap water may be slightly acidic—owing to dissolved carbon dioxide. Baking soda is a weak base.
  4. The strong bases will be bathroom cleaners, ammonia, washing soda, milk of magnesia, and lime.
  5. The indicator can be frozen in ice trays and saved for use. The indicator mixed with alcohol will last for months! The strips can be refrigerated and will also last for months.

*An alternate source of cabbage juice is to purchase a can or jar of cabbage, drain off the juice, and discard the cabbage.

Disposal
All solutions can be poured down the sink. Solid bits of cabbage should be put into a solid waste container (and emptied at the end of the school day—owing to their odiferous nature.)