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Testing for Acids and Bases

Homeschool Science Magic Using Red Cabbage

Jun 23, 2008

A knowledge of acids and bases is important in chemistry, in baking, and food preservation. The following activity gives directions for making a simple indicator.

How can you tell the difference between an acid and an alkali (also known as a base)? One way is to put different substances to the test!

You can help your child make a simple indicator that will cause acids to turn red and bases to turn blue. Making the indicator involves cooking, so supervision will be needed at first. However, since the indicator is food based your child can safely experiment on his or her own if you wish.

Make a PH Indicator

Supplies:

Red cabbage
A grater
Stainless steel pan
Water
Strainer
Jar for storing the liquid

Instructions:

Grate some cabbage into the pan.
Pour in just enough water to cover the cabbage.
Boil the cabbage until it becomes tender. The water will look blue or red. Allow it to cool.
Strain the water into a jar.

Test Your PH Indicator

Now that you have made an indicator, you are ready to experiment. Solids can be tested on a plate. Liquids can be tested in small, clear containers. (Clear plastic film canisters work well.)

Have your child add drops of cabbage water to small amounts of various foods such as apple juice, banana, cola, eggs, lemonade, milk--even water from cooked vegetables. If you don't have a dropper, try dipping a straw into your cabbage water, putting your finger snugly over the opposite end of the straw and lifting it out of the water. Now you have a dropper for transporting small amounts of cabbage water to each food to be tested.

Ask your child to state whether each food tested is an acid or base. Help them remember B is for "base" and B is for "blue." Bases will turn blue.

Acids will turn red.

(Note: it is also possible that some foods will not change color when tested. These foods are neutral.)

If your child is looking for a similar experiment with a "WOW" factor, have him or her try the following activity.

Pour a little cabbage water into a clear container.
Add just a little white vinegar. The liquid will instantly turn very red.
In a second clear container, mix some water with a little baking soda so that the water remains clear. If it turns cloudy add more water. You want the solution to look like ordinary water.
Pour the baking soda solution into the red water in the first glass. The liquid will instantly foam and the color will change again.
Explain that this change is the result of mixing an acid and a base.

Your child may wish to think up additional experiments. If so, the cabbage water will remain in usable condition for several days in the refrigerator. Just have your student label the jar so it's contents are not a mystery!

The copyright of the article Testing for Acids and Bases in Homeschooling is owned by Theresa Bledsoe. Permission to republish Testing for Acids and Bases in print or online must be granted by the author in writing.