Charlotte Water Distance Learning: Cabbage pH Indicator

pH Activity Overview:

Prep time: 30 minutes
Total time: 50 minutes
Age range: Elementary and Middle School aged learners (can be modified for older or younger learners)

Water is everywhere! Water is a part of our everyday life – it can take on many forms, and can be described in many different ways: temperature, the amount of salt, how clear it is, and more! Another common way to describe water is by measuring its protons, also known as the “power of Hydrogen,” or better known as pH.

What is pH?

pH is a scale from 0 to 14, which tells us how acidic or basic a liquid is. pH measures how many free protons, also known as positive hydrogen ions, or hydroxide ions, HO are in a substance. The lower the pH, the more free protons, or positive hydrogen ions, there are. The higher the pH, the more of another kind of molecule, negative hydroxide ions, there are. Solutions with a pH of 7, such as pure water, are known as Neutral, or have a perfect balance of hydrogen and hydroxide ions. Acidic substances (like soda, lemons, or batteries) have a low pH (below 7) while basic or alkaline substances (like egg whites, soap, or bleach) have a high pH (above 7).

Photo courtesy of Discovery Place

Water that comes from the faucets in our home should be between a pH of 8.0 and 9.2. Acidic liquids are caustic: they can corrode or eat away certain materials over time. The lower the pH, the more caustic and potentially dangerous an acid can be. Since the pipes that Charlotte Water use to transport our water are made of metal, they can be slowly eaten away by water that is even a little bit acidic. To keep the water at the right pH levels, Charlotte Water treats the water with a rock product called lime that increases the alkalinity of the water. It is important for the engineers and technicians at Charlotte Water to test the pH of its water all over the city to make sure that it stays in this range.

Now that we’ve learned about pH, let’s test our knowledge!

Here is what you will need:

  • Water
  • Red Cabbage (using the outside leaves or scraps is fine)
  • Knife
  • Cutting board
  • Household test chemicals (lemon juice, pickle juice, vinegar, baking soda in water, raw egg, etc.)
  • Clear or white cups or drinking glasses
  • 1 tbsp measuring spoon
  • Bowl
  • Pipette or eyedropper
    • Material Substitution Note: Placing a piece of tape at the end of a straw and poking a pinhole will work as a pipette

Directions:

  1. With an adult’s help, cut the red cabbage in small pieces. You only need enough cabbage to fill up a drinking glass.
  2. Boil water and pour into a bowl (Be careful! It’s very hot).
  3. Put cut up red cabbage into the bowl of boiling water. (Keep in mind that boiling cabbage in your home will make it smell like, well, boiled cabbage!)
  4. Let sit for 10 minutes.
  5. While waiting on the red cabbage water, gather the rest of your materials.
  6. Strain cabbage pieces out. The leftover cabbage pieces can be a delicious snack – just add some salt, pepper and olive oil – or they can be put into a compost or trash bin.
  7. Once cool, put 1 tablespoon of cabbage juice into a clear or white cup
  8. Add 5-6 drops (or as close to that as you can manage) of one of your test liquids (lemon juice, pickle juice, vinegar, soda, etc.) and stir
  9. Compare the color of the liquid to the pH scale to determine pH of your test liquid
  10. Repeat for as many liquids as you would like

Why can we use red cabbage to measure pH? That’s because it contains an indicator pigment molecule called flavin, which is a type of anthocyanin (a kind of molecule that changes color based on the pH of the liquid it is in). This pigment can also be found in apple skin, red onion skin, plums, poppies, blueberries, cornflowers, and grapes. Very acidic solutions will turn anthocyanin a red color, neutral solutions to a purplish color, and basic solutions make a greenish-yellow or yellow color.

For younger learners try this…

  • Use a blank piece of white paper as a canvas to paint with the cabbage juice and vinegar.
  • Use the leftover cabbage to make a delicious slaw to have with dinner.

For older learners try this…

  • Soak a coffee filter in the cabbage juice and allow to dry. Once dry, cut filter into strips and try testing other test chemicals (milk, soda, soap solution, etc.) by dipping the strip into the liquid for several seconds, pulling it out and watching for a change in color. Make a hypothesis before you test. Will the solution be neutral? Acidic? Alkaline?
  • Both older and younger kids could take the juice-soaked coffee filter and, using a cotton swab, rub the filter with vinegar or a mixture of baking soda in water. Try overlapping them to see what pHs and colors you can create.

Check out this video to see this experiment in action!

If you try this at home, let us know how it went! Share your photos on social media, and be sure to tag Charlotte Water and Discovery Place.

Happy learning!


The Charlotte Water Distance Learning activities are provided in partnership with Discovery Place, Inc.

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