In chemistry, knowing which substances are "acids," and which are "bases," is key. Setting molecular science aside, acids and bases are best thought of as opposites of each other, and the two kinds of substances can "neutralize" one another, rendering such dangerous substances as hydrochloric acid meek as a lamb. But beware! Both acids and bases can cause dangerous chemical burns if not neutralized, so knowing which is which is imperative!
So, how to test for acids and bases? 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. In this activity you and your child can make your own indicator from red cabbage, which contains a chemical that turns from its natural deep purple color to bright red in acids and greenish in bases.
What You Need:
- Red cabbage
- Rubbing alcohol
- White vinegar
- Household ammonia
- 3 clear plastic drinking cups,
- Plastic spoons
- 3 clean teaspoons for stirring
- Medicine dropper
- Safety goggles
(Although the following activity is safe, it is always good practice to have your child wear safety glasses whenever she conducts any type of activity that involves working with chemicals.)
What You Do:
- Place several red cabbage leaves in a container half-filled with rubbing alcohol. Allow to sit undisturbed until the clear alcohol has the same color as the cabbage.
- Have your child pour about 1/4 cup of cabbage juice into each of the clear plastic cups.
- Then add 1/2 teaspoon of vinegar to one cup and stir with a clean spoon, observing the color change. The vinegar and cabbage juice mixture will change from a deep purple to red, indicating that vinegar is an acid!
- Next add 1/2 teaspoon of ammonia to the second cup and stir with a clean spoon, observing the color change. (Work in a well ventilated area, and use only a small amount. Ammonia can be corrosive to the skin, eyes, and lungs). The ammonia and cabbage juice mixture will change from purple to green, indicating that it is a base!
The color changes are quite dramatic and should cause some oohs and ahhs from your child. Challenge your child to try and change the color of red cabbage juice in the acid (vinegar) back to the “neutral” purple color. Using a medicine dropper, add drops of ammonia and gradually stir using a clean spoon until the mixture turns from red back to purple.
As an extension to this activity, encourage your child to use the cabbage juice to test other common household products for the presence of acids and bases. You can make indicators from the juices of elderberries, blackberries, radish skins, or cherries!
By Mike Calhoun
Mike is a 20-year veteran science teacher, and runs an online business (www.scienceinabag.com). Over the years Mike has studied trends in science, education, and finance, conducting research, developing programs, and writing articles on these topics.
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