chemical principles exemplified
ROBERT C. PLUMB Worcester Polytechnic Institute Worcester, Massachusetts 01 609
Rainy Weekends and Heterogeneous Nucleation Illustrating principles of nucleation of phase transitions
Informatio?~,providecl by Dr. Vincent J. Schaefer, Professor of Atmospheric Sciences, SUNY, Albany, New Yo&, and Ralph H. Frederick, ESSA, Silver Spring, Maryland Does it really rain more frequently on weekends than during the weeli? The frustrated sportsman may feel that it does, but the fact is that i t rains more during the weeli than on weekends! This has been verified by independent studies in England, France, and the USA.l The greatest daily precipitation is in the Tuesday through Friday period. The seven-day cycle is noticeable during the cool season, but not apparent during the summer season. Since the seven-day cycle of the week is man-made and does not occur naturally (the closest natural cycle is the 291/2 day lunar cycle which can pass through maxima and minima every second weeliend but which averages out every few years), one must look to man's influence on the xi-eather for the explanation. The chemical principles involved are well known Surface energy effects become significant when a particle is small, because of the large ratio of surface area to volume. The resulting instability of small particles causes phase transitions from gas to liquid and solid, liquid to solid, or solution to solid, to be "sluggish" producing laboratory phenomena such as supercooling and supersaturation. If suitable foreign materials (dust, stirring rods) are present, heterogeileous nucleation on these foreign surfaces may take place, bypassing the energy barrier to homogeneous nucleation imposed by the inherent instability of small particles. Alternatively, particles of the solid phase introduced by the experimenter will serve as seed crystals.
Those readers who are conceriled with bridging the gap between the inherent interests of students and the specialized abitrnct subject matter of chemistry are invited to contribute to this column. The exempla-that is brief anecdotes about materials and phenomena which exemplify chemical principles-show how the scientific principles with which chemistry instruction is concerned have impact on all aspects of the world in which we live. Teachers who use exempla in their classroom instruction-lectures, exams, problem assignments-find they are a potent device for enriching their courses, building motivation and increasing comprehension of abstract principles, and the myth of "irrelevancy" is demolished. 1 FRF;DERICI~, R. H., Second National Conference on Weather Modification, April, 1970. Bull. Amer. Meteorol. Soc., 49, 4 ; 50, 411; 51, 335; 51, 337. Natwr, 225,190 (1970).
15
1
cool season
----warm season
1
1
13 SUN MON TUES WED THU FRI SAT
Percent of total precipitation b y day-of-the-week for period 191 2-61. (Average of 2 2 stations in Eastern United States.)
Apparently the seven-day cycle is another example of heterogeneous nucleation-the now well known work of Bernard Vonnegut on cloud seeding with particles of silver iodide (l~eterogeneousnucleation), and the earlier success of Vincent Scllaefer in using the low temperature of dry ice to seed clouds (homogeneous nucleation), have demonstrated that nuclei introduced into supersaturated air can produce rain. The nuclei responsible for the observed seven-day cycle of precipitation are not well known, but combustion products from leaded gasoline (are more miles logged by commuters than by Sunday drivers?) and particulate matter in industrial stack gases released during the work week are prime suspects. The more stable cloud formations and decreased convection in the winter probably make man's effects on the weather more apparent during the cool seasons. I'erhaps the most astounding (and currently debated2) weather anomaly is prccipitation in La Porte, Indiana, where 7-11 in./year more rainfall is alleged to result from atmospheric effluent of steel worlis. For much of the country, man's pollution of the environment has a measurable effect on the weather and, as far as weeliends are conccrned, one on I\-liich most people would 100liwith favor. Sickle-Cell Anemia-Where
Solubility Becomes
a Matter of Life or Death! lllusfrating chemical principles controlling solubilify
Contribution by Professor A. R. Patton Colorado State University When you breathe, the oxygen is carried from the lungs to the tissues by the red blood cells. The red part is the iron-protein compound called hemoglobin. Sickle-cell anemia is a disease in which as much as 85% of the hemoglobin in the red cells crystallizes from solution. This insoluble hemoglobin distorts the cells into a sickle shape,3clogs capillaries causing blood clots, Volume 47, Number 7 0, October 7 970
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691
and deprives tissue of oxygen. The disease is hereditary and if both parents carry defective genes almost all the child's hemoglobin will be defective, and he will not survive more than a few years after birth. The chemical explar~ationof siclcle-cell anemia lies in the effect of polar and nonpolar groups on the water solubility of organic compounds. The borderline between solubility and insolubility of hemoglobin depends upon just one amino acid in a molecule made up of nearly three hundred. Normal hemoglobin is soluble because it has in a certain place a glutamic acid residue with a polar ionic group
692
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Journal o f Chemical Education
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I n siclcle-cell hemoglobin, as a result of the genetic defect, there is a valine residue where glutamic acid should be. Valine contributes the nonpolar hydrocarbon group -CH-CH, I CH,
This causes siclile-cell hemoglobin to be insoluble. The result is nearly always fatal.
