Red Cabbage Chemistry How to make your own indicator using cabbage juice. email to friend
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1. Peel off six big cabbage leaves and put them in a blender filled half full with water. Liquify!
2. Pour the purplish cabbage liquid through a strainer to filter out all of the big chunks of cabbage. Doesn’t cabbage juice smell great? Save the liquid for the experiments to follow.
3. Set out three glasses, side by side against a white piece of paper. Fill each glass half full with cabbage juice.
4. Since you know that vinegar is an example of an acid, add a little vinegar to the first glass of cabbage juice. Stir with a spoon and notice the color change to red, which indicates that vinegar is classified as an acid.
5. In the second glass add a teaspoon of washing soda or laundry detergent. Notice how the liquid turns green, indicating that this chemical is a base. Keep these two glasses of red and green liquid for future reference.
6. Try adding each of the other “test chemicals" to a small amount of cabbage juice and note the color change to determine if the chemical is an acid or a base.

Use your cabbage juice indicator to test the acid or base properties of other common substances. You might want to try orange juice, lemonade, milk, salt, ammonia, or soap.

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 anthocyanin that 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.