Acids and Bases: Red Cabbage pH indicator

DWBlogCabbage1
Posted by on October 2, 2011

The school year has started back up, and that means it might be time to start thinking about upcoming science fair projects.

Don’t even think about building a Baking Soda/Vinegar Volcano.I’m not kidding.  Step. Away. From. The. Volcano.

However, if you/your child is interested in acid and base reactions, this is a good first step towards a science fair project that won’t make your teacher want to shut herself in the broom closet and sob quietly for the rest of the day. It is NOT a science fair project by itself * but it can add an extra dimension to an Acid/Base experiment.

Purpose:  To create a homemade pH indicator, that will let us know if the material we want to work with is an acid or base.

Materials:

  • Knife/Blender
  • Microwavable Bowl or large pot
  • Water
  • Strainer or coffee filter
  • large jar
  • at least 1/2 red cabbage

 

 


Method:

  1. Chop your cabbage up. I suggest cutting pretty small pieces, but not so small that you need a coffee filter to filter out the solids after you finish.  It’s best to chop them large enough that you can use a fine colander or s
    trainer to filter out the pieces. You can certainly ignore that suggestion and use the coffee filter, but my way is easier (and faster). 
  2. If you want to use the microwave, put the cabbage in a large, microwaveable bowl. Add enough water to almost cover cabbage, cover the bowl, and microwave for 3-5 minutes. Don’t leave it unattended; if the cabbage water begins to boil over, turn the microwave OFF and consider the cooking portion finished. Let cool. See how my overly-excited boy is enjoying himself?
  1. If you want to use the stove, put your cabbage into a large pot. Add enough water to almost cover the cabbage. Bring the water to a boil, and boil for no more than 3-5 minutes. Don’t leave it unattended; if the cabbage water begins to boil over, turn the stove OFF and consider the cooking portion finished. Let cool.
  2. A third option involves pouring boiling water over your cabbage (till almost covered), and covering. Let sit until cooled.
  3. A fourth option, which I’ve seen used in schools, is to put all of the cabbage in a large ziplock bag, add hot tap water, seal it and leave in the classroom sink or a tub, overnight. In the morning, cut a corner and let drain into a jar.
  4. Using the coffee filter, and or strainer, drain the (cooled!) water from the cabbage into your clean jar.
  5. If your homemade indicator solution appears to be too dilute, you can boil it down until it’s as concentrated as you’d like. It won’t hurt the indicator.
Your water will be a deep purple/blue, depending on the pH of your water. It should be fairly close to pH=7 (Neutral).
Recall: pH=1-7 is acidic.  pH = 7-14 is basic.
Now, for testing (Boo had a lot more fun testing items, than he did creating the pH indicator solution):
  1. Pour 1-4 T of the substance you want to test into a clear or white bowl/small glass/test tube. ‘Cuz we should all have test tubes lying around the house (see mine?)
  2. Add a few drops of your homemade indicator solution to the substance you want to test.
  3. Swirl gently to mix (especially important if you test something like Clorox Bleach. Be careful and use common sense when testing any substance!) Add a few more drops if you’d like, to see what will happen.
  4. Very generally, the indicator will turn shades of pink in the presence of acids. The indicator should turn blue, then green, and finally yellow in the presence of bases.

 

One Response to Acids and Bases: Red Cabbage pH indicator

  1. Rachel

    Hey there, I went looking for you from the January boards. :) This looks like a fun experiment, thanks for sharing it! We may definitely need to give this a try at our house!

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