- Red cabbage
- Clear drinking glasses
- White paper
- Apron or lab coat (cabbage juice can leave nasty stains!)
- Test chemicals: Vinegar, Baking soda, Lemon juice, Washing soda, Laundry detergent, Soda pop, Alka-Seltzer
Try soaking some filter paper in concentrated cabbage juice. Remove the paper from the cabbage juice and hang it up by a clothespin to dry. Cut the dried paper into thin strips. Dip the strips into various liquids to test their pH. The redder the strip turns, the more acidic the liquid is. The greener the strip turns, the more basic the liquid is.
How does it work?
Some substances are classified as either an acid or a base. Think of acids and bases as opposites - acids have a low pH and bases have a high pH. For reference, water (a neutral) has a pH of 7 on a scale of 0-14. Scientists can tell if a substance is an acid or a base by means of an indicator. An indicator is typically a chemical that changes color if it comes in contact with an acid or a base.
As you can see, the purple cabbage juice turns red when it is mixed with something acidic and turns green when it mixes with something basic. Red cabbage juice is considered to be an indicator because it shows us something about the chemical composition of other substances.
What is it about cabbage that causes this to happen? Red cabbage contains a water-soluble pigment called anthocyaninthat changes color when it is mixed with an acid or a base. The pigment turns red in acidic environments with a pH less than 7 and the pigment turns bluish-green in alkaline (basic) environments with a pH greater than 7.
Red cabbage is just one of many indicators that are available to scientists. Some indicators start out colorless and turn blue or pink, for example, when they mix with a base. If there is no color change at all, the substance that you are testing is probably neutral, just like water.
Science Fair Connection:
Red Cabbage Chemistry could be a great science fair project. Once you have tested the various chemicals and household substances listed above and have a clear understanding of acids and bases, you could make a few changes, run some new tests, and make some comparisons.
- Try testing a variety of beverages to see which ones are the most acidic. Some people think Starbucks coffee is very acidic. Could you use the red cabbage juice and the process described above to run some tests of different brands of coffee to see if those claims are true?
- What happens when you put pieces of acid-reducing medication in acidic liquids? Choose one acidic liquid and test different brands of acid reducers to see which ones are the most effective. Just make sure you use the same "dosage" of acid reducer and the same form of acid reducer (liquid vs tablet) and the same amount and type of acidic liquid so that your tests are fair and your conditions are standardized as much as possible.
- Is a liquid or a tablet acid reducer more effective? Use the same brand and the same "dosage" of medication and put it in the same amount and type of acidic liquid. The only variable you're testing is the difference between the liquid and the tablet medication. Do they both work? Does one work more quickly to reduce the acid?