Acid-Base Experiments - Explore With Red Cabbage Juice In The Classroom

Did you know that red cabbage juice can be used as an acid-base indicator? It's true! The juice changes color depending on the pH level of the substance it is added to. This makes it a great science exploration and Halloween activity. It's a fun and educational way to learn about the properties of different substances and is easy to set up.


Do you want to get oohs and ahhs in the classroom?  Start out with this mysterious demonstration to get students to think about the scientific phenomenon they are observing.


Supplies:
4 or more tall clear containers
Red Cabbage Juice Extract (See directions below)
Water
Baking Soda
Epson Salt or other salts
Lemon Juice
Rubbing Alcohol

Other substances to try:  vinegar, baking powder, apple juice, and nail polish remover.  Always make sure the liquids or powders you choose can be mixed with water.  I try to use common kitchen items.

Please wear safety goggles.

What does it do?

Red cabbage juice is an acid/base indicator.  This means it will change color in the presence of an acid or a base.  It will turn red/pink in the presence of an acid and green in a base.  There are many colors in between.  The more acidic, the more pink.  The more basic, the more green.

How to Make Red Cabbage Juice Extract

Purchase a red cabbage at the store.  It will cost about $2.00.  Take off a bunch of the leaves and place them in a pot.  Cover the pot with liquid.  Bring water to a boil and boil for about 15 minutes or so.   You want to boil it until the color is pretty much out of the leaves.  The liquid should be a nice purple color.  Allow to cool, and then pour the liquid into a container.  One cabbage will make more than you need.  I will make a batch and freeze some for future use.  It freezes very well.




Set-up for the Mysterious Demonstration
 
Make sure they don't see you do this part.

1.  Place the tall containers on a table in front of where your class will sit.  Fill each with about 200ml of liquid.  You want to make sure there is enough room left for adding red cabbage juice.

2.  Place the following in each container.  This is a small amount because we want all of the liquids to look clear, so students think it is water.
  • Container 1:  plain water
  • Container 2:  Add a teaspoon of baking soda and stir until dissolved
  • Container 3:  Add a teaspoon of Epson salt or another salt.  Stir until dissolved.
  • Container 4:  Add a teaspoon of lemon juice.  Stir.
  • Container 5:  Add a teaspoon of rubbing alcohol.  Stir
Other ideas could be vinegar which is a strong acid, or baking powder, which is a base.  Anything that will dissolve so that it looks like water should be in all of the containers.  Test them first to see what color change they create.

3.  Make sure students will be able to see the containers well.  

4.  Tell the students to observe closely as you pour a small amount of red cabbage juice into each container.  Don't tell them what the liquid is.  Don't tell them anything else.  Add enough into each container until they change colors or are pretty full.  

5.  Once you are done pouring, tell the students to talk in their groups about what happened.  Then have a class discussion.  This can make an engaging CER prompt activity that they research for the reasoning section of CER.  Students are sure to be surprised. It is a great opener for an acid-base/chemical reaction unit.

Once you do the activity, students can test out different substances and try it out.  If they are going to test it out, we recommend older elementary, middle, or high school students perform the activity.  Always follow good lab practices.

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