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"Let us strive to make each moment beautiful."
Saint Francis DeSales
painting by Katie Bergenholtz

CSI: More Labs: Determining pH, Iodine and Vinegar Demonstrations and Fingerprint Analysis

This week we analyzed the cola and the ice cube tray for poison, determined what the white powder found at the crime scene was and examined the fingerprints on the glass.

Class Preparation:
  • You will need to have fingerprints for the students to compare with the cup. Use only the thumb and of course make sure that the same person who left the fingerprint on the cup is the same as the one you are using for Mr. Body. Have fingerprints for comparison for the suspects as well.
  • You will also need to take with you the cup that was taken from the crime scene in which you developed fingerprints.
  • You  need to make up some cabbage juice pH indicator. I found the easiest way is with a blender. You will also need a can of cola and three containers, preferably very small. An eyedropper is very helpful.
  • You will need to bring with you water from the ice cube tray (the water you have added 1 Tab. baking soda to 1 cup of water.)
  • In addition to the mysterious white powder that was collected from the crime scene, you will need to take with you a package of corn starch, a package of baking soda and some egg carton trays cut into six sections. Small plastic spoons and an eyedropper are very useful as well.

Class Activities:
  • Analyze the cola for poison. Obviously, I didn't put actual poison in the cola (and I told the students this) but it does afford us an opportunity to do some pH demonstrations and it mimics similar techniques used to determine what a substance is in actual labs. I put about 1/8 teaspoon of baking soda in the cola to change its pH. For this demonstration, have three small containers available (I used vials). Have your students put some cabbage juice pH indicator in each of them. I used an eyedropper for this. At this time, I talked about what pH was, related it to gardening, and showed them that cola was acidic by the fact that phosphoric acid is one of its ingredients, and that for the purposes of this scenario, our possible "poison" was alkaline. I then talked about how cabbage juice can indicate the pH of a substance, determining whether the substance is acidic or alkaline, by its color change. I then had a student add some of regular cola to one of the vials and the students were thrilled to see the color change despite the fact that the cola was colored brown. They then added some cola from the crime scene and were delighted to see that it changed to a different color. This proved that "poison" was added to the cola! What does this clue tell you?
  • Analyze the water in the ice tray for poison. Do this in the same way as you did the cola, using plain water and the water from the ice cube tray (in which you have already added baking soda to.) The colors will be even more dramatic without the cola brown to mute them. Your students will find out that the poison was in the ice, and therefore got into the cola probably from the ice. What does this clue tell you?
  • Analyze the mysterious white powder to determine whether it is cornstarch or baking soda. To your six-section containers add cornstarch to the two cells of the first column, and baking soda to the two cells of the second column. Now have your students test the first row by adding three or four drops of iodine to the powders. If the iodine turns black, that means there is the presence of a starch. You can see that it doesn't change color in the baking soda. Now have your students test the second row by adding a few drops of vinegar to the cells. The vinegar will fizz in the presence of baking soda, and will do nothing to the cornstarch but get it wet. Now have your students test the mystery powder. Is it cornstarch or baking soda? What does this clue tell you?



  • Compare the fingerprints that have shown up on the cup with those in the case file. You might need to help your students by guiding them to look for loops, whorls and arches. 
  • source
    What does it mean that only Mr. Body's fingerprint was found on the glass?

Next week we will be analyzing all of the clues to help us determine what happened to Mr. Body.

4 comments:

  1. This series is just getting better and better! Glad to hear you didn't use real poison ;)

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  2. Your CSI lab is so cool! I keep pinning your sessions for the future!

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  3. I like the liquid rather than pH paper for the tests.

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