Science: Need Something To Do With Your Leftover Cabbage?

Science: Need Something To Do With Your Leftover Cabbage?

You may have heard that citrus juices are "acids" or that ammonia is a "base."

The terms acid and base refer to the concentration of hydrogen ions (pH) in the substance; acids have a high concentration of hydrogen ions, while bases have a low concentration of hydrogen ions.

But how can you test whether a substance is an acid or base?

In this experiment, which you can try at home, you will make your own pH indicator from red cabbage and use it to measure some liquids to see whether they are acids or bases


You Will Need:

    •   A head of red cabbage
    •   A stove
    •   A pot
    •   Distilled water
    •   A funnel or coffee filter basket
    •   Coffee filters
    •   A set of eyedropper bottles
    •   A medicine cup with milliliter markings
    •   Several small paper cups
    •   Test substances

Substances to test might include lemon juice, a clear sports drink, lemon-lime soda, window cleaner and vinegar.


Performing the Experiment

To prepare the indicator, shred some red cabbage leaves, place them in small volume of water (1/2 cup) in the pot, bring the pot to a boil on the stove, and boil for 5-10 minutes (Note - You should wear some form of eye protection and use pot holders when handling boiling solutions).

Pour the water with the boiled leaves through the coffee filter to remove the pieces of cabbage, collect the filtered juice in a clean glass, and let it cool; it should appear purple.

You can then pour the juice into the eyedropper bottles for use and storage.


What does this all mean?

This experiment will tell you whether a substance is an acid or base, but not the exact value of pH; the pH scale ranges from acid (0-6), through neutral (7) to base (8-14).

If you want to calibrate your cabbage juice pH indicator, you will have to test your substances with another quantitative indicator (e.g. litmus paper) and compare those results to the colors of the cabbage juice pH indicator in those solutions.

This is what a group of current Year 8 students did to produce a relatively accurate Universal Indicator solution.

Interestingly, Hydrangea flowers do something similar. The flowers of a given Hydrangea will be a different colour depending on whether the soil is acid or alkali.

In acid soil they are blue and in alkaline soil they are red, which might not be what you expect.

The key thing to remember is that colour changes seen in these examples are totally dependent on the indicator used.


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