ANALYSIS IN FIRST-YEAR CHEMISTRY * G . T. FRANKLIN. LANE TECHNICAL HIGHSCHOOL, C ~ C A GILLINOIS O,
The discussions of this paper are intended to apply to first-year chemistry of schools of secondary character. It is largely a matter of the outgrowth of the experience of the writer in teaching chemistry in schools of this character. One of such experience is frequently reminded of the difficulties in the way of the slow student and the superiiciality of the quicker witted ones. General notions develop with much slowness. The patience of the teacher is frequently lost in the attempt to get him to understand the nature of a "test." Such simple ideas as indicator tests for acid or alkali are only understood after considerable experience in the use of the indicator. Beads of a few highly colored metallic salts are made. If his knowledge goes no further, he has acquired some information. If he is given a salt whose identity is unknown, and makes it into a borax bead, and it matches one he has previously made, he has improved his knowledge. If he is given one that does not yield any color in the bead, he is puzzled. One feels instinctively that considerable experimenting of a varied character is necessary for him to understand the nature of the test. After the pupils have completed preliminary tests with borax beads and blowpipe with cobalt nitrate, it is the custom of the writer to give unknowns with instructions that they can be found by one of the two methods. The reactions to negative tests are appreciated more by the teacher than by the pupil, but i t seems necessary to do such work. Another case of this kind may make the point of view plainer. The pupil is asked to shake thoroughly a mixture of water and table salt until the water is saturated. The solution is then poured off from any undissolved crystals and heated to boiling. A few more crystals are added. He is asked to write his observation and make a conclusion. In spite of the fact that he has been instructed by experiment concerning the meaning of "saturated" and "unsaturated" solutions, his answers are often without meaning. The numbers that fail to get the idea are alarmingly large. There is a suggestion that more work involving the thinking processes is needed covering a larger period of the pupil's activities. Textbooks of a scientificnature invariably give information and then ask that the information he used in the solution of problems. The value of the information is rated on the basis as to whether the student can use it in solving the problems. Are our laboratory instructions dodging this matter? I think most of us agree that the laboratory is the heart of the subject. To emphasize heavily other aspects of the subject leads to a dangerous field of a sort of "pseudoscience," the cultural value of which might be better obtained in other fields. Analysis has long been recognized as the logical basis for problems in laboratory. It is quite true that * Condensed and adapted from a paper read before the Division of Chemical Education of the Chicago Section of the American Chemical Society, September 28. 1928. 361
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most courses include much work of the nature of analysis; however, it is largely of a preliminary nature and informational rather than of the problem type. The pupil habitually puts together chemicals that produce reactions, often of a striking character. He is told that certain ones represent tests for ions. If he happens to do the work poorly, he sees the work done by his neighbor, takes ,note of it, and goes on. The laboratory is altogether a place of information gathering, a place to confirm what has been read in the books or may he a t the wish of the pupil. It is rarely called to the attention of the pupil that the laboratory may be used as a means to solve problems. His work is too "socialized." A method of giving each pupil a problem independent of his neighbors should have its place. It must by its nature be more or less supplementary.to the usual courses. To provide needed supplementary work, the writer has devised and adopted for his classes various short schemes. A scheme for the analysis of bakmg powders is described in a recent issue of School Science and Mathematics.' Another scheme, which has been under process of development for some time, is discussed in the paragraphs following. Toward the end of the year there is a need felt for a review of a laboratory nature. The courses generally provide for standard tests for such ions as chloride, bromide, sulfate, etc. To give a pupil an unknown involving one of these ions and ask him to test for an answer until he finds i t is a questionable thing to do. It is rather unreasonable to ask him to detect nitrate by the usual test when his unknowb may be an iodide or bromide. It is likewise expecting him to do too much determine the halogens by the use of sulfuric acid alone. Worst of all, he is working without any special plan, unless i t is provided by special instructions. The following scheme has been devised to meet this need. Analysis Involving Some Acid Ions
A . One of the following ions is included in the problem: C1- ( ). PO4--- ( ), Br- ( ), S04-- ( ), I- ( ), S-- ( ), SOa-- ( ), NOa( ), COa-- ( ), Boa--- ( ). Insert the number of the experiment which discusses the respective test in the parentheses following the ion formula. B. I. Cover a portion of the unknown with concentrated sulfuric acid. Record your observation and then proceed to the next. 11. Make a water solution of a portion of the unknown, filtering off any undissolved residue. Add silver nitrate solution to a portion of the filtrate. A precipitate, insoluble in nitric acid, indicates C1-, Br-, or I-. A precipitate, white and soluble in nitric acid, may be regarded as negative. A "A Simple, Practical Problem in Analysis," G. T.Franklin, 29, 414-8 (April, 1929).
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precipitate, not white and soluble in nitric acid, is a test for one of the ions listed above. 111. Make a nitrate test, using a small portion of the filtrate prepared in (11). A colored ring indicates NOs-, I-, or Br-. Determine by other tests which it is. IV. Make tests for S01--, PO4--, and Boa---, using separate portions of the filtrate prepared in (11). Part A must be used to confirm your test in (I), (11), (111) in case (IV) is negative. NOTE: Chemicals often contain traces of chloride and sulfate as impurities. Unless tests for these ions are decidedly positive the result should be merely held in mind for further evidence. Metallic sulfites invariably contain sufficient sulfate to yield a good sulfate test. Sulfur dioxide may cloud limewater as well as carbon dioxide. Do not be satisfied with a probable result. If at the conclusion of the analysis you have obtained positive tests for more than one ion, repeat the tests for these ions. If you are still unable to decide, include all in your report as indicated below. V. Make a detailed outline of your report as follows: REAGBNI'
RESULT
CONCLYS~ON
Girt ion indicated present by positive test or ions indicated abrent by negative test)
..
Special Test
I t is my custom to include in part A of the notes the number of the experiment which has given the pupil preliminary information about the test. In case loose-leaf notes are used and not returned to the pupils, the numbers could not be used unless books were available, in which case it would provide a very valuable exercise for the pupils to do it. It would not be a bad idea to have the pupils write out and have available standard tests for use while doing the experiment, repeating those forgotten, before beginning the analysis. Too much emphasis cannot be placed upon requiring the pupil to refer to and do the special test for the ion indicated present in I, 11, or 111. He will necessarily have to look up the tests included in IV. To the pupil the correct answer assumes too much value and bow he solves the problem is kept in the background. It is the business of the teacher to bring the background to the foreground. A container for each substance used to provide unknowns is kept as part of permanent equipment. I t is sometimes necessary to camouflage with other substances that do not interfere with the tests to prevent matcb-
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ing colors. If time and space afford, a large number of containers would be the ideal. As to how the scheme works out, suppose a pupil is given a bromide. He adds sulfuric acid as directed and may or may not recognize bromine vapors formed. He may write "fuming" or "colored vapors or both, or "bromine indicated," depending upon how much he remembers. He then proceeds to the next and adds silver nitrate after making a solution, followed by nitric acid. He then writes "Cl-, Br-, or I- indicated." He then makes a test for nitrate ion by the usual method. He records "colored ring," "indicates NOa-, Br-, or I-." If he is thinkmg, he will observe that chloride ion is eliminated. He now proceeds to the end of the experiment, obtaining negative results for the remainder. He now looks up the experiment in which Br- and I- are discussed, in case he Kas forgotten, and proceeds with chlorine water and chloroform to confirm his observations. If he receives a sulfate compound, he is not so interested, especially if i t is a rather insoluble one like calcium sulfate. It tests his patience and willingness to work without obtaining any clue until near the end. An interesting side-light to the study is the pupil's reactions to the tests. Often he is imbued with the idea that one test for an ion is final and unimpeachable. It is a warning not to drill too hard on these tests during the preliminary work and leave the pupil to formulate his idea of the test through the analytical study. C
How Men Learn Probed by Cornell Experiments. How the human hrain learns not t o do things, which is as important a branch of education for life as learning t o do other things, has been probed by experiments conducted a t Cornell University by Dr. A. I,. Winsor of the Royal Education Department. Using the procedure of collecting saliva, which the Russian physiologist, Prof. I. P. Pavlov, first used to show that a dog's mouth will water mare freely when it hears a bell and associates the sound with eating, Dr. Winsor placed food hefore a hungry man and measured the increased flow of saliva which the sight of food caused. When the man continued to look a t the food and was given no chance to eat, the saliva flow diminished. The hrain had recognized that the food signal war false. The same negative learning process occurs when a child jumps a t the sound of a nearby train whistle. After hearing the whistle many times, instead of jumping more intensively his muscles no longer respond to the shrieking noise. The alarming sound is recognized as boding no h a m . From a series of experiments, Dr. Winsor concludes that psychologists have heretofore underrated the significance of the negative learning process. The law of learning assumes that repeated stimulation of the eye, ear, nerves, or glands will cause a response t o bemme more firmly fixed. Yet repetition in many circumstances serves t o decrease or inhibit the response, so that a habit is formed hut it is the opposite of what would ordinarily be expected. Whether an individual develops a positive or negative habit in responding to a situation depends on the biological significance of the situation, Dr. Winsor's experiments indicate.-Science Seruice
