Red Cabbage pH paper
Summary
Some of the most important kinds of chemicals in science are acids and
bases. Although you may not realize it, you use acids and bases in your
home everyday. You can recognize an acid because it makes food and drinks
taste sour. If you have ever tasted something chalky or ever felt a liquid that
feels slippery like soap, those substances are probably bases.
One way to test if a substance is an acid or base is to test its pH. pH is the
scale used in science to rate acids and bases. The scale is numbered from 1
– 12. Acids have a pH between 1 and 6. Bases have a pH between 8 and 12.
If your pH is at 7, that means the substance is neutral, and it is neither an
acid nor a base.
Just like we use a temperature scale on a thermometer to find out if
something is hot or cold, we can use the pH scale to find out if something is
an acid or a base. An easy way to do this is to use pH paper, which is a piece
of paper with a chemical on it that will change colors if it is mixed with acids
or bases. Some fruits and vegetables contain this special type of chemical.
One example is red cabbage. In this activity you will learn how to make your
own pH paper using juice from red cabbage.
Once you make your red cabbage juice, you will want to know how to use it.
If the color of your red cabbage juice is a purple color, then you know that the
pH of your solution should be around 7, which means that your solution is
neutral. When you first make the red cabbage juice, the liquid should be this
color. When you begin to test the pH of other liquids, the juice will change
different colors if it is mixed with an acid or a base. Match the colors you get
while you are testing liquids to the chart below to see what pH you have.
pH scale for Red Cabbage Juice
pH 1-2 3-4 5-6 7-8 9-10 11-12
Color Pink Dark Viole Blue Blue- Greenish-
Red t Green Yellow
Acid/ Acid Acid Acid Neutral- Base Base
Base Base
Goals
Make red cabbage pH paper
Use pH paper to test the pH of some common products around the
house to determine if they are acidic or basic
Materials
One small head of red cabbage
Large glass measuring cup
Blender
Chopping knife
Strainer
Coffee filters
2 large glass bowls
Glass or plastic jar with lid
Eye dropper or turkey baster
Household products to test pH (feel free to choose your own. Here are
some examples to get you started)
o Orange juice
o Vinegar
o Milk
o Soap
o Baking Soda
o Stomach antacids
o Lemon/Lime juice
o Window Cleaner (see safety notes below)
o Bleach (see safety notes below)
o Powdered laundry detergent
o Aspirin tablet
Note: to test solids or powders with your pH paper, you will first need to
dissolve them in a small amount of water
Safety
This activity requires the use of sharp cutting utensils. Use caution when
handling knives or a blender. Ask an adult to help you if necessary.
Red cabbage juice may stain clothes and fabrics. Be sure to protect the area
you are working in with plastic or newspaper. Stains on hands and skin will
wash off with warm water and soap.
This activity involves testing some potentially toxic household chemicals.
Always use household chemicals with adult supervision. NEVER mix
household cleaning products together. This may cause a dangerous reaction.
Always test each substance separately. Do not eat or drink any of the
substances after you have tested them. Dispose of any liquids separately in
a sink with plenty of water.
Preparation
1. Begin by chopping the red cabbage into small pieces until you have
about 1 cup of the chopped cabbage.
2. Put the cabbage into the blender with about 1½ cups of water. Blend
the water and cabbage until finely chopped.
3. Place a strainer into a glass bowl and pour some of the cabbage
mixture into the strainer. Allow the liquid to separate from the
cabbage. The solid cabbage can be thrown away. Repeat until all of
the liquid is separated from the cabbage.
4. Store your red cabbage indicator in a labeled, sealed jar until you are
ready to use it.
Pre-Activity
Making your pH paper
1. Cut several coffee filters into 2 x 6 cm strips. Place the strips onto
several sheets of paper towel.
2. Using an eyedropper or turkey baster, place a few drops of the red
cabbage indicator onto the coffee filter strips in order to cover the
strips and allow the pieces to dry before moving onto the next part. Do
not drench the strips because they will take too long to dry.
Activity
1. Now you are ready to test different substances using your red cabbage
pH strips. First, choose a substance that you wish to test.
2. Using a clean dropper, add a couple drops of the substance to the red
cabbage pH paper. Do not dip the paper into the main container of
your substance. Also, do not add your substance to the main supply of
your red cabbage extract.
3. Record your results and throw away your pH strip.
4. Choose another substance to test and repeat steps 2 and 3.
Extension Activity
Obtain Litmus paper from an aquarium supply store. This is a different
type of paper that measures pH.
Test the same substances from the activity above with your litmus
paper. Compare the results. Of the two different types of paper, which
do you think works better and why?
Wrap-Up
Acid, bases and pH are an extremely important part of science. Acids and
bases affect almost everything around us. In nature, pH plays an important
role in survival. For example, if we changed the pH of our bodies by one
level, it could kill us. The same goes for most other animals and plants. The
environment is also affected by pH. Pollution can cause the pH of rain to be
lowered. This is what people call acid rain. Acid rain can ruin buildings and
stone, as well as affect plant and aquatic life. Manufacturers are also
concerned with acids and bases because they are used to make so many
things, including cleaning products and building materials.
Resources
Adapted from:
http://chemistry.about.com/library/weekly/aa012803a.htm
http://chemistry.about.com/od/acidsbases/
