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Colors with Acids and Bases

Thinking of the lovely spring colors, the pastel blues, lavenders, yellows and pinks? Why not try a few really fun experiments creating and changing colors using chemistry?

The first experiment requires some fresh red, blue or purple flowers (purple petunias work really well); a jar big enough to hold a few flowers with a lid; some twist ties, string or yarn to suspend the flowers; household ammonia; and for the optional last part, vinegar (any kind). The ammonia is pretty strong, so this will require help from an adult.

Have an adult pour about 1/2-inch ammonia into the bottom of the jar. Wrap a twist tie or yarn around the stems of the flowers, enough that will fit comfortably into the jar. Suspend the flowers upside-down into the jar so that they are close to, but not touching the ammonia, by wrapping the yarn or twist tie over the lip of the jar. Put the lid on the jar as much as you can. Wait about 15 minutes and you should see the flowers start to change color.

This part of the experiment is based on one by Robert Krampf in his excellent Experiment of the Week series, although I couldn't find it in his archives. Edit: Robert Krampf has revamped his site, and so this link is no longer valid. Check out the fun stuff he has to offer at the Happy Scientist.

Now comes my addition: once the flowers have changed color nicely in the ammonia (which is a base) then try to change the color back by dipping the flowers in a bowl of vinegar (an acid). Using purple petunias, I was able to turn them a bright teal blue in the ammonia, and then back to purple in the vinegar. You can actually dip them into the vinegar, because it won't bleach. It you dip the flowers into the ammonia, however, it may bleach or discolor them. That is why they need to be suspended in the fumes instead.

The second experiment is the classic use of red cabbage as a pH indicator. If you haven't done this, it really is fun. All you need is red cabbage from the grocery store, a blender (ask for an adult's help), glasses or plastic cups and items to mix with the red cabbage solution, such as lemon juice, soda, vinegar, baking soda, dish detergent and laundry detergent.

Some recipes call for boiling the cabbage (smelly!), but I just ground up the fresh red cabbage in small batches with just enough water to allow the blender to work properly. Pour the batches together in a pitcher (which can be placed in the refrigerator for use later in the day if necessary.) Pour about 1/3 cup of the red cabbage juice into testing containers such as clear glasses or plastic cups. Then mix in about a Tablespoon of one of the testing compounds. Does the color change? Try another material in the next glass. Does the color change more if you add more test material? What happens if you mix two materials, like vinegar and laundry detergent? Have fun admiring the wild colors you can make.

What is happening? The pigment molecules in the red cabbage change shape, and thus color, when in the presence of acids versus bases. Lemon juice, vinegar and soda are acids; detergents and soaps are bases.

For the grand finale, create more wild colors with markers. Fold up a diaper wipe that contains alcohol and tie in bunches with rubber bands. Color with Sharpie-type markers. Allow it to set a few minutes and then unfold to reveal a rainbow of colors in a cool tie-dye pattern. Experiment with more wipes to see what you can create. Note: allow to dry suspended on a line so the dye doesn't move onto other surfaces.

If you don't have diaper wipes, then try white fabric. Tie with rubber bands, if desired and then color with markers. Drop or dribble on some rubbing alcohol and the colors should separate and move through the fabric. Once again, keep on a line or suspended until dry. Once you have the technique refined, you might want to create your own tie-dye T-shirts or socks.

Spring colors rock!

1 thought on “Colors with Acids and Bases

  1. Shirley Duke

    I used the cabbage experiment in middle school science and they enjoyed it so much. I first heard of it at a science conference. Great to see it printed out here. Science teachers--take note! It's fun.

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