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Each Tuesday we generally celebrate plants with "Seed of the Week" posts. Today we'll look at chemicals found in plants for Chemistry Week.

Plants contain an encyclopedia of different chemicals, which they use for growth, communication, and defense. For this lesson we are going to look at three different classes of plant chemicals that can be used to study the pH of acids and bases.

A Quick Introduction to Acids and Bases

People have known for centuries that acids:

  • Taste sour (like lemons)
  • Dissolve or corrode metals
  • Turn blue litmus paper red

On the other hand, bases:

  • Taste bitter (like caffeine in coffee)
  • Feel slippery
  • Turn red litmus paper blue

Note:  Scientists know:  never taste, drink or eat anything from a science experiment!

Chemists have created a scale to measure how acidic or basic a substance is, called the pH scale. Although no one knows for sure how the name pH came to be, it is acceptable to think of pH as the “power” of hydronium ions, or "how many hydronium ions are present." (Hydrodium ions are technically H3O+ ions, but are often written as H +.) It is a logarithmic scale, which means that lower numbers, like 1 or 2, indicate a larger number of hydronium ions, and that a substance is more acidic.

Gather at least some of the following possible acids or bases to test:

  • Lemon and/or orange juice
  • Apple juice
  • Soda (pop)
  • Vinegar
  • Baking soda (mix with water to moisten)
  • Dish and/or laundry determent
  • Shampoo
  • Milk of Magnesia
  • Coffee
  • Salt
  • Cream of Tartar (in water to moisten)

Add any other household materials you might be curious about, but only under adult supervision.

Note: Bleach doesn't work well because it interferes with the dye molecules. Also, adding vinegar to baking soda is fine, but do not mix strong acids and bases.

Gather small clear containers such a plastic cups or clean baby food jars to carry out the experiments in. Label each container when you add a substance to be tested so you remember which is which.

Plant-Based pH Indicators

Certain molecules and substances change color when exposed to specific acidic or basic conditions, and thus can be used as pH indicators. Examples of  pH indicators from plants include: 

lichen-and-moss

1. Litmus is a complex of dyes extracted from certain lichens. 

Litmus paper is a classic for exploring acids and bases. All you need to do is dip the litmus paper strip into the liquid to be tested. If the blue litmus paper turns red, it is an acid. If the red (pink) litmus paper turns blue, it is a base. If neither changes, the substance you are testing is neutral.

Litmus paper is available online or in science supply stores. Pool supply and aquarium supply stores may also carry it.

2. Anthocyanins- derived from red or purple fruits and berries, such as:  red cabbage!

red cabbage indicator

I know, I know, red cabbage indicator is all over the Internet and has been probably over-exposed, but that is because it is easy and works for both acids and bases. If you haven’t done this, it really is fun. Even if you have done it before, pull it out again and try something new with it. Using red cabbage as a pH indicator works well for an activity with mixed-age groups because each age may achieve a different level of understanding. If you are really tired of red cabbage, try the juicing beets (the root part), or berries instead.

All you need is red cabbage from the grocery store and a blender.

Some recipes call for boiling the cabbage but that is smelly and unnecessary. Simply grind up the fresh red cabbage leaves 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.) You can also strain the slurry through a strainer to remove the solids.

Pour about 1/3 cup of the red cabbage juice into testing containers such as clear glasses or plastic cups. Mix in about a Tablespoon of one of the testing compounds into one of the containers and label it. Does the color of the liquid change? Have fun admiring the wild colors you can make.

Try another material in the next container.

Questions to try:

  • Does the color change more if you add more test material?
  • Can you figure out which colors indicate acids and which indicate bases?
  • Does the temperature of the liquid matter? Hot or cold?
  • How does the color change when you add baking soda to vinegar with the cabbage juice already in it? Can you add enough vinegar to turn the color back? How much does it take?
  • What happens when you dilute the test mixture with water?

cabbage-juic-acid-and-base
What is happening? The pigment molecules in the red cabbage juice change shape, and thus color, when in the presence of acids or bases. Lemon juice, vinegar and soda are acids; detergents and soaps are bases. In the above photograph, from left to right are:  red cabbage juice in water, dish detergent, baking soda in water and lemon juice.

Edit:  Sarah of Share It! Science blog just stopped by with a link to a post about using the anthocyanins in poinsettia bracts (the red ones) as a pH indicator.

3. The third pH indicator is curcumin, which is found in turmeric.

Gather:

  • Paper towel
  • Isopropyl (rubbing) alcohol
  • Paper plate or waxed paper
  • Turmeric (sold in the spice aisle of most groceries)

Watch out, turmeric will stain like crazy! Wear old clothes and do this outdoors, if possible.

Cut paper toweling into strips. Mix a few teaspoons turmeric with isopropyl or rubbing alcohol in a small container. Dip the thin strips of paper towel into the solution. Pull out and allow the paper towel strips to dry on a disposable or stain-resistant surface, such as a paper plate or sheet of wax paper. The alcohol should evaporate fairly quickly.

tumeric-strips-close

One the strip are dry, dip them into test substances placed in small containers, as discussed above.

tumeric-paper-basic

 Note: Turmeric is bright golden yellow below pH 7.4, orange above pH 7.4 and bright red above pH 8.6.

Extra turmeric strips can be stored in a plastic bag for later use (up to months).

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Check our Chemistry Week Table of Contents for more children's chemistry activities.

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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!