Carnegie Mellon University

Product Tampering: Teacher's Note


  • dropper bottles for samples
  • testing tray
  • Litmus paper with dispenser color chart (Broad Range pH paper)
  • vinegar
  • pH 7 buffer solution
  • safety glasses


Before Possible Tampering - Fill bottle with household white vinegar.

After Possible Tampering - Fill with your choice depending on the outcome that you want. Possibilities are:

  1. white vinegar- no tampering,  pH is about 3
  2. tap water- tampering, pH is about 5
  3.  pH 7 buffer solution -obvious tampering as students will interpret as pure water

This experiment introduces the concept of pH and acids and bases. Advise the students to place only a few drops of the test liquids in the tray wells. Just the tip of the litmus paper should be placed in the solution. The pH = 3 color, orange, is not very distinctive, but the green pH= 7 color is very convincing that something has occurred.

Tap water should be tested beforehand to decide if it's pH is different enough from either the vinegar or buffer to be useful.  It should have a pH of about 5 due to dissolved carbon dioxide.

Litmus paper should be broad range (~1-12) test paper with a color matching chart on the dispenser. This paper is available in a roll or as a vial of test strips.  If using the roll type, encourage the students to take only two small pieces in order to conserve supplies.  The roll usually is sold wrapped in foil. Save the foil to rewrap the roll after use.  It serves to protect the paper from environmental reactions.  Store it away from the reagent bottles containing vinegar.

An inexpensive “test well” can be made from a contact lens holder, either the kind that lenses are disposed in or those that are used for storage or cleaning. Since only a few drops of sample is needed for testing, they are a perfect size.

This experiment can be followed by allowing the students to use the other end of the used litmus paper to test other examples of acids and bases such as lemon juice, baking soda (in water), orange juice, milk, tomato juice, soap, cleaning products, shampoos, lotions, or your drinking water.

Students who understand the concept of logarithms could calculate the H+ concentration from the equation

pH= -log[H+]