THE USE OF PROBLEMS IN TEACHING COLLOID CHEMISTRY F. 0.ANDEREGG, PURDUE UNIVERSITY, LAPAYETTE, INDIANA
In administering an elementary course in physical chemistry, the selection of material for presentation constitutes a very nice problem. Of all the various branches of physical chemistry, none has a greater interest for the student and none gives him more valuable training than colloid chemistry. The great range of phenomena covered by the principles of colloid chemistry and their close connection with most operations of everyday life make their study one of especial appeal to most students. Since most of the laws of colloids that are suitable for teaching in an elementary course are qualitative in character, numerical problems cannot be depended upon to drive them home. To satisfy this deficiency, a list of problems has been collected which can be given, a few at a time, for the student to analyze and hand in. A reasonably complete analysis of the various factors which contribute to a given result is a t first quite difficult to the average student and he is apt to become discouraged. However, the practical nature of the problems, themselves, together with oery full discussions of the problems in class and encouragement from the instructor, result in the student gradually acquiring a much greater ability to handle new problems. These exercises also work very well in graduate courses in colloid chemistry. The first one in the list has been treated by Bancroft.' In addition to the question of simple displacem'ent of the air film on the canvas fibers, the question of the action of any sizisg or waterproofing on the wetting must be considered. Hydrophobic substances aid in the shedding of water because of the difficulty of replacing their air film by one of water. The explanation of waterwings involves the opposite effect; the fabric is thoroughly wet with water and the surface tension of the water between the fibers tends to prevent the air from escaping. Problem 31 refers to the common occurrence of iron in most stones which on heating often become red owing to the growth in size of iron oxide particles a t the elevated temperature and a resultant change in structural color. The principles covered in the other problems should be evident to any persons familiar with the more common laws of colloid chemistry. 1. A tent roof is touched during a rain. A drop from a paddle may roll on the surface of the water for an instant. 3. A jet of water breaks up. 4. Waterwings. 5. Making a good mercury barometer. 6 . . Under-sea painting. 7. "Everhat," sold by drug stores consists apparently of roasted iron filings. A little water is added to produce the heat. 2.
Bancroft. ''Applied Colloid Chemistry," McGraw-Hill, 1921, p. 71.
8. "Magnesia" pipe coverings may contain more than 90% of voids. 9. A turbulent flow encounters more resistance than slip or telescopic flow. 10. Turbulent flow gives better heat transfer. 11. A good soldering (or brazing or welding) flux. 12. Make a list of examples where union occurs between pieces of metal without solder, cement, etc. 13. Asphalt sticks t o the edge of the cutting knife. 1 Rubber erasing, wet and dry. 15. Drilling a hole in glass. 16. Grinding with a wet sandstone. 17. A drop of water falling on silk may leave a ring. 18. A good shoe-polish. 19. Prepared paste, Argerol, etc., should be added to water-not vice versa. 20. Bone fertilizer is hard t o wet. 21. "Only 95% of mercury vapor is usually condensed." 22. Good chalk crayons are made from plaster of Paris. 23. Adding 50% bentonite to soap often improves its detergent power. 24. Jewellers use fine sawdust in cleaning. 25. Wall paper cleaning. 26. Red mercuric oxide on grinding turns yellow. 2 i . The color of very fresh iron rust. 28. The metallographist's heat tinting. 29. Colors of carborundum. 30. "They smoked till the air was blue." 31. Most stones acquire what color on heating? 32. What colors may be produced by adding varying amounts of lamp black to white lead ground in linseed oil? 33. Freezing of fresh concrete. i' 31. Freezing of a tender plant. 35. Freezing of a hardy plant. 36. Very rapid freezing of ice cream. 37. Very rapid thawing of ice cream. 38. Very rapid thawing of a hardy plant. 39. The effect of a slag, containing much iron, as a fertilizer. 40. The removal of coal tar fiom the skin. 41. A paper (cigarette) sticks tenaciously t o the lip. 42. The removal of ordinary ink stains from cloth. -13. The de-inkina .of newswoers. . . 44. Making cow's milk digestible for infants 5 Ammonia is added t o rubber latex before shipping. 46. The effect of time and temperature of heating ferric oxide on its solubility in hydrochloric acid. 4 . A good putty. 48. Dehydrating sewage sludge. ~~
