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.
began an experiment, based upon the laboratory exercise Upset
Tummy? MOM to
the Rescue! A colorful Antacid Demonstration [Flynn Scientific:
proceeded as follows:
- 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.
- Next she added 4-5 ml of a universal pH indicator [http://www.purchon.com/chemistry/ph.htm],
which turned purple.
- 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.
- 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)
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.
- She added more HCl to the beaker, and the same cycle
A terrific miniteach, Pat!
- 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)2
does not dissolve, and thus our MOM solution looks milky.
- 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.
- This reaction is a direct consequence of Le Chatelier's
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.
- In our case the stress of excess H+
ions causes the removal of OH- ions, and
is shifted to the right side the reaction, in order to replace the
OH- ions that are lost.
- The net result is that more of the insoluble Mg(OH)2
dissolved in the solution.
- The color of the universal indicator goes back toward the
violet (purple) as OH- ions are restored.
- The experiment can be continued many times, until all the Mg(OH)2
has dissolved and excess OH-(aq) is neutralized.
Marva Anyanwu [Worley-Green Elementary School, K-8]
One handout described the basic fingerprint patterns: whorl,
loop, and composite, as shown on the website
whereas the other involved a study of the characteristics of
skin, similar to the website
Marva challenged us to explain what fingerprints have in
common with tire treads.
Very thought-provoking, Marva!
- We began by sharing our knowledge on fingerprints. Marva
asked how fingerprints
can be altered. We concluded that they can be burned off and rubbed off
for short periods, but that deep cuts are
not effective because the fingerprints are regenerated.
Fingerprints have ridges and grooves to provide traction, just like
tire treads. We also discussed the dermis -- the outer layer of skin
cells that continually regenerate fingerprints as the dead cells slough
- We made fingerprints by inking fingers and then pressing them
onto a piece of
paper, as described in the second handout and website. We all
our left thumb, so that we could make person-to-person comparisons in a
controlled way. We also categorized our own left thumb
one of the four basic patterns. We found that it is not very easy to
- Gary Guzdziol described a project in his school, in which
submitted his/her print as that of "perpetrator", and had
gather prints from their teachers to identify the "prime
suspect". Carl Martikean described a project involving a
"mock crime", which students would solve by gathering clues such as
fingerprints and other material evidence.
- We discussed the uniqueness of fingerprints, as well as foot/toe
other physical features. Footprints of newborn babies are taken
establish their identity and avoid mix-ups
- We then discussed these questions:
- Why do your hands have so many creases on them? [for folding
- Explain how the calluses on a person's hand can be used to
guess the person's occupation?
- Pat suggested a tie-in to genetics, to determine whether
fingerprints that are in the same class as their parents.
- Carl suggested that the Evidence Laboratory of the
Chicago Police Department
[now located next door at 35th Street and State Street] would be an
resource, and that somebody from there might be asked to present to a
Carl Martikean [Wallace HS, Gary] Teaching of Science
Carl raised the following questions:
Comment by Porter Johnson: similar issues are discussed in
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
- Should animals with human gene sequences spliced into their own
DNA be considered human?
Why or why not?
- Why are chimpanzees immune to the HIV virus? Is it
possible that there was an
HIV virus among chimps, say, two million years ago, devastating the
population at that time,
so that only the few virus-resistant chimps survived? Why or why not?
- The crusades brought rats and the bubonic plague back to Europe
from the Mediterranean basin, resulting in untold human agony and
depopulation in Europe. Did this resultant depopulation serve to
enhance the value of human life, leading inevitably to specialization
of labor, banking and commerce, inherited wealth, leisure time, and
[among those fortunate few] time and resources for scientific
inquiry? Does science drive society, or does science drive
Interesting ideas, Carl! See you next time!
Notes taken by Ben Stark.