High School Biology- Chemistry SMILE Meeting
24 September 2002
Notes Prepared by Porter Johnson

Pat Riley [Lincoln Park HS] Milk of Magnesia
Pat led us in a discussion of the properties of Milk of Magnesia Mg(OH)2, [MOM for short] and its use in treating upset stomach, heartburn, and the like.  Then she began an experiment, based upon the laboratory exercise Upset Tummy? MOM to the Rescue! A colorful Antacid Demonstration [Flynn Scientific: http://www.flinnsci.com/]. She proceeded as follows:

  1. She put 20 ml of Mg(OH)2 into a beaker and added 200-300 ml of ice to slow down any reactions, and diluted to 800 ml with water. She then turned on a stirring motor.
  2. Next she added 4-5 ml of a universal pH indicator [http://www.purchon.com/chemistry/ph.htm], which turned purple.
  3. She handed out color cards to use to translate the "indicator color" into pH. [pH is a quantitative measure of hydrogen ion concentration, on a negative logarithmic scale; see http://www.purchon.com/chemistry/ph.htm.]  The color of the solution indicated that the pH was around 10, showing that the Mg(OH)2 is rather basic.
  4. She then added 2-3 ml of Hydrochloric Acid (HCl), and the stirred solution turned orange, indicating a pH of about 5. Over the next minute or so the colors continued to change sequentially to yellow (pH =6) to green (pH = 7-8) to blue (pH = 9) to purple (pH = 10). We concluded that the Mg(OH)2 was gradually neutralizing the HCl.  By the same mechanism, HCl in our stomachs is neutralized by Mg(OH)2.
  5. She added more HCl to the beaker, and the same cycle occurred.
Discussion:
  1. Mg(OH)2 is 5 times more soluble in hot tap water than cold tap water. In water ions are formed through the reaction
    Mg(OH)2 « Mg+2(aq) + 2 OH-(aq)
    The equilibrium constant for the reaction, determined from the concentrations, is much greater in hot than cold water.  In our experiment most of the Mg(OH) does not dissolve, and thus our MOM solution looks milky.
  2. The hydroxyl ions [from soluble Mg(OH)2] and the hydrogen ions [from HCl] combine to completion in the neutralization reaction
    OH-(aq) + H+(aq) ® H2O(l)
    The color indicator thus makes a gradual transition toward the "red" end of the visible spectrum.
  3. This reaction is a direct consequence of Le Chatelier's Principle:
    When stress (an excess of hydrogen ions) is applied to an equilibrium state, the reaction proceeds in the manner (direction) that will relieve the stress.
  4. The experiment can be continued many times, until all the Mg(OH)2 has dissolved and excess OH-(aq) is neutralized.
A terrific miniteach, Pat!

Marva Anyanwu [Worley-Green Elementary School, K-8] Fingerprints [handout]
One handout described the basic fingerprint patterns:  whorl, arches, loop, and composite, as shown on the website http://www.odec.ca/projects/2004/fren4j0/public_html/fingerprint_patterns.htm, whereas the other involved a study of the characteristics of fingerprints and skin, similar to the website http://safety-identification-products.com/fingerprint-information.html. Marva challenged us to explain what fingerprints have in common with tire treads.

Very thought-provoking, Marva!

Carl Martikean [Wallace HS, Gary] Teaching of Science from a Humanistic Perspective
Carl
raised the following questions:

Comment by Porter Johnson:  similar issues are discussed in the book Guns, Germs, and Steel: The Fates of Human Societies by Jared Diamond [W W Norton 1999] ISBN 0-393-33755-2, as well as another book by the same author,Third Chimpanzee: The Evolution and Future of the Human Animal.[Harper 1992] ISBN 0-06-098403-1.
Interesting ideas, Carl! See you next time!

Notes taken by Ben Stark.