The Case of the Terrible T. Rex (9 page)

•   1-quart jar with lid

•   pencil

•   scissors

•   tweezers

•   2 clean, dry plates

PROCEDURE

1.
   Collect and prepare four water samples for testing. Prepare each sample as follows (using the tape and marker, label the jars clearly with numbers 1 through 4, and replace the lids as you go):

a)
   Jar #1: fill with tap water to about ¼ inch from the top.

b)
   Jar #2: pour in 1 tablespoon of white vinegar, then fill with tap water to about ¼ inch from the top.

c)
   Jar #3: fill halfway with tap water, add 1 teaspoon of baking soda, replace lid, and shake until dissolved. Finish filling with tap water to about ¼ inch from the top.

d)
   Jar #4: take a sample from a local river, stream, or lake. (See “Good Science Tip” on page 69.) Fill the jar to ¼ inch from the top.

2.
   Ask an adult to dice about 3 cups of cabbage. Place the cabbage in a pitcher and ask an adult to pour approximately 1½ cups of boiling water over the cabbage, enough to cover it completely. Let it soak for 30 minutes. (Note: cabbage juice can stain, so wear a lab coat!)

3.
   Place one coffee filter into the coffee strainer. Over a sink, pour the cabbage juice through the coffee filter/strainer into the 1-quart jar. (Be careful that clumps of cabbage don’t fall—SPLAT!—into the strainer and stain your clothes!)

4.
   With the lid off the jar, let the juice cool completely. Once cool, replace the lid. Using a tape and marker, label the jar “Cabbage Juice,” and date it. (
NOTE:
keep it refrigerated and discard after twenty-four hours.)

5.
   Draw some fish on white coffee filters. Each fish should be about 2 inches long. You’ll need at least four fish, but make extra just in case. (Topnotch scientists are always prepared.) Cut out the fish with scissors.

6.
   Using the tweezers, dunk one fish completely into the cabbage juice, and then place the wet fish on a plate to dry. Repeat with each fish. Allow fish to dry thoroughly.

7.
   Using a pencil, now label your four (dry) fish “#1,” “#2,” “#3,” and “#4.”

8.
   Again using the tweezers, dip the #1 fish into the #1 water sample. Lay the wet fish on another clean plate. Dip the #2 fish into the #2 water sample, and so on.

9.
   Important: record your results while the fish are still wet.

How Does This Work? (Reading the Results)

Scientists created the pH scale as a measurement of whether a substance is an acid or a base. The pH scale is numbered from 1 to 14. Acids range from 1 to 6. Bases range from 8 to 14. A neutral is 7.

pH Scale

1   2   3   4   5   6   7   8   9   10   11   12   13   14

More Acidic      ←      Neutral      →      More Basic

The pH scale also answers the questions
How acidic
and
How basic
For instance, a substance that scores a 2 on the pH scale is more acidic than a substance that scores a 4 or a 5. Likewise, a substance that scores a 14 on the pH scale is more basic than a substance that scores an 8 or a 9.

Well, you say, this is all very fascinating, really, but … what’s that got to do with my poor little fish?

A most excellent question! You see, purple cabbage juice contains a purple pigment called
flavin
, which acts as a
pH indicator
. By dipping your paper fish into the cabbage juice, you created pH indicator fish! When mixed with acids, the pH indicator fish turn pink. When mixed with bases, they turn greenish yellow. But if a neutral liquid touches the indicator fish, the color doesn’t change!