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Lab Tips in Acids and Bases

pH Indicator: Red Cabbage Juice

Materials Needed:

  • A head of red cabbage
  • Tap water
  • Porous white paper or paper cards
  • Vinegar
  • Baking soda
  • Other house hold items that are acidic or basic—get creative

Safety Info: In this lab you may be working with some nasty kitchen and household products. While it wouldn't hurt to wear goggles and gloves, getting scalded with boiling water or burned with acid certainly would. Use caution throughout any chemistry experiment even when it's taking place in your own home.

Preparation: Cut the cabbage into thin strips and put them in a pot with enough water to just cover the cabbage. Boil the cabbage for 30 minutes and then let cool. Strain and collect the cabbage juice. (Feel free to use the boiled red cabbage in your favorite red cabbage recipe or one of here.)

Procedure: The cabbage juice can be used as your own homemade pH indicator. To start, use an eyedropper or spoon to put a spot of the cabbage juice on a white background surface (preferably paper or something you're not afraid of staining). Now sprinkle a little vinegar or baking soda into the cabbage juice. You should see a color change indicating that the pH of the juice has changed. You can make your own pH test strips by cutting up squares of paper and letting them soak in the cabbage juice. After about 30 minutes remove the paper and let dry. Cut up the squares into test strips. Now your ready to test the pH of solutions of your choosing. To get you started, try testing lemon juice, Soda, diluted shampoo, aspirin (one tablet crushed into ~20 mL water), and ammonia-containing cleaning products (be careful!)

To Think About: Red cabbage juice contains a molecule, called anthocyanin, which changes its color depending on the pH of the solution it's in. Under acidic conditions anthocyanin is pinkish. As the pH is raised to basic conditions, the color changes to green and then yellow. What substances can you classify as acidic and basic using the red cabbage juice pH indicator?

Results: You should find that acidic substances like lemon juice and vinegar turn your red cabbage juice pH strips to a more pinkish hue. Basic substances, like baking soda, should turn the strips green or even yellow if the solution is basic enough.

A brief video demonstration of this lab, with intense background music to pump you up, can be found here.

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