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SOME FUNDAMENTALS OF LABORATORY INSTRUCTION* HERBERT R. SMITH,LAREVIEW HIGHSCHOOL, CHICAGO, ILLINOIS If a school has a splendidly equipped laboratory, the patrons of it are apt to think that chemistry is well taught there. The good equipment is not necessarily a handicap but it can so fill the mind of a teacher that "the things that are more excellent" are unnoted. Two general faults may be commonly observed in the teaching of chemistry in secondary schools, one of the teacher and the other of the pupil. The topics of chemistry are generally presented to the pupil first through the medium of a text-book, then laboratory exercises follow. Consequently the pupils know most of the results of the experimental work before it is performed, so they find little need of the data of the experiment. They do not object to this useless work because it i s entertaining to mix chemicals, but the experiment is performed and written up in a mechanical way. In this way the pupils get little mental training, particularly in the great value of the experimental method as the best method of finding the truth. They depend on what others say and develop little ability to know for themselves. The text-book approach was discontinued long ago a t Lake View High School, Chicago, when a pupil furnished the eye-opener of writing his experimental record according to the statements of the textbook and ignoring the contradictory results of the materials before him. We suspect that the attractiveness of this method is to be found in the more correct record that the pupil writes. The real purpose of any experiment in training the mind to learn seems to be neglected. Most of the young people in high school and college will avoid using their own minds if they can, and in all too many cases they are permitted to do so. Yet they are being graduated from these institutions. Without close supervision the indinriduul laboratory work is only another way of evading an education. Our data of several years' study show that about 10 per cent of the third- and fourth-year pupils can engage profitably in individual laboratory practice. All the others require training in how to study by the laboratory method, and they must be kept under close supervision if it is accomplished To scatter pupils a t indkidual tables prevents this. What these pupils need most is how to think and learn. The teacher can do this most effectively and a t the same time most economically by keeping them grouped closely together near one table where one or two pupils perform the experiment, while the others observe the results, think out the interfiretation, and write a record of it. The teacher, unhampered by doing demonstration work, gives his whole time to guiding, * Read before the Division of Chemical Education of the A. C. S. at Philadelphia, Pa.. Sept. 10, 1926.
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questioning, and really instructing the group. The whole group can be shown, a t the same time, how to interpret data and learn as they must learn later in life affairs. When a laboratory period conducted by this method ends, every one of the group has all of the points to be learned and has them correctly. The records are written and complete, and the teacher knows that they are correct. The method is certain in its results. As soon as each pupil shows proficiency, he may be intrusted with individual work and, a t times when individual contact with materials is desired, the whole class may work separately with the thorough understanding that the work will be performed in the spirit that has been taught to the group. The text-book being found unsatisfactory as an introduction to laboratory practice, i t is necessary to supply something else to give the pupil a fair knowledge of the laboratory problems. This can be done splendidly in an oral fashion by the teacher in the group method. If an introduction in printed form can be supplied, it can be studied by the pupils in outside time and conserve the needed time of the laboratory period. Another point in the improvement of laboratory instruction is to be found in the emphasis of chemical principles rather than mechanical detail. The early teaching of chemistry was chiefly concerned with the technics of the subject. Few contacts were made with life situations. As time went on the text-books began to grow in size, chiefly with the inclusion of applications of topics to life, a larger consideration of the chemistry of commerce and industry, and finally with the practical aspects of chemistry in the home and daily life in general. This growth toward the practical side of chemistry has wonderfully enriched the subject, yet all that has been done is only a beginning of the possibilities that are opening up before us. Experiments devised to exemplify chemical principles frequently make no contacts with the experience of the pupils and as such are not sufficiently impressed on the pupils' consciousness. To remedy the situation the teacher turns to the chemical side of the work of the home, the shop, and the market place to provide an incentive for the pupil. It is easy to secure the interest of the pupil in this way even if the work has no value. Sometimes we fall into a worse situation in that the chemical work undertaken has little or no contact with chemical princifiles. In avoiding Scylla we fall into Charybdis. The topic, baking powder, is selected to illustrate the point, since it has contact with the lives of everyone. Most of the laboratory manuals in use take up this topic by having pupils make various chemical tests for the probable ingredients of baking powder. These objections are offered. 1. The function of each ingredient of the baking powder and the nature of the chemical action are usually presented in the text-book pre-
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vious to the experimental work so there is nothing vital left for the laboratory study. 2. The experimental work consists of qualitative tests for the components, mere technics that are of value only when their use reveals information of value. Resort to analysis is unnecessary here for the information may be more quickly and certainly learned from the label on the package. Further, the pupils are inexperienced in analysis and every class using this experiment will have some pupils reporting the absence of a component whose presence is declared on the label by the manufacturer. In summarizing the criticism it may be said that the purpose of the experiment is unnecessary and the method of executing it is inefficient. The following procedure is offered as an example of a more worthy method.
Bread Introduction.-For fifty centuries the world stood still-waiting to be fed. The great masses of population, men, women, and children, toiled through the long days to ohtain a hare subsistence of food and a hovel for a home. Wh+n there was bread enough t o satisfy hunger, that day was called a feast day for there were many days of fasting. In this age of the achievements of science we ask "What shall we have for dinner today?" and proceed to gratify our appetites with whatever pleases the palate, even if i t does not accord with the laws of nutrition. The one great cause of this transformation of conditions is labor-saving machinery. The steel-faced plow which uprwts the prairie sod, the cultivator by which one man can destroy the weeds on ten acres of ground in a day, and the harvester and thresher which rapidly reap the harvest and give back t o modem man the golden wheat by many hundred-fold. W ~ t hmodem machines one man now produces as much food as the labor of hundreds in primitive times. B u t the metal to make the machinery must first he won from Nature through the science of chemistry. The initial or fundamental work of all material progress is performed by the chemist. His work makes possible the steel plow to supplant the sharpened stick, the steel hoe for the clam shell, the binder for the sickle, the thresher for the flail, the roller mill far the pounding stone, and finally the leavening baking powder to transform the wheat flour into light bread in a few minutes. This multiplication of the productive power of man makes the cereal grains abundant and. therefore, cheap. The same cause operates to lessen the cost of the essentials of life. No longer does each one work arduously from sun t o sun for food and the neceasities of life. There is leisure time for recreation and the cultivation of the finer arts. Baking powder was invented as a means of leavening bread quicker and more economically than with yeast. The various kinds differ mainly in the use of a different substance as the acid principle. In this manner three different classes of powders are known. 1. Cream of Tarlnr. This d a s s contains potassium bitartrate and sometimes tartaric acid as well. The cost of this ingredient makes this powder the most expensive. I t is changed to Rochelle salt, a drug, in the process of bread making. 2. Phosfihate. An acid phosphate, usually calcium superphosphate, is used. I t is cheaper than the bitartrate and while the sodium and calcium phosphates formed in the bread are drugs, there is possibility of them being used in the body as mineral foods.
3. Alum. Some one of the alums, generally sodium alum, is used. It is very cheap and slow acting, but has no advantage otherwise over the other kinds. The aluminum hydroxide and the salts formed have no possible values as foods. Many persons abject to the use of aluminum mmpounds in foods. Yeast leaves no undesirable residues in the bread so i t is greatly preferred far leavening purposes. About 2 per cent of the carbohydrates are decomposed by t h e diastase and zymase in the leavening process. Starch readily takes up moisture so i t is always put in baking powders to protect the mixture from action when exposed to the moisture in the air. Some manufacturers appre~iatethis fact so much that they put in a very liberal supply. I n such cases the starch becomes a filler t o cheapen the powder.
Experiment on Baking Powder Purpose: To study the chemical reactions of the principal ingredients of baking powder. I. The Leavening Agent. Put a large pinch of sodium bicarbonate in a clean testtube, dissolve i t with a little water, and test the solution with red litmus paper (?). Put a few drops of sulfuric acid in the solution, note the effect, and test the gas in the tube by a drop of lime water on the end af a stirring rod (?). Questions. 1. , Solutions of a d d salts usually contain the hydrogen ion. What ion is shown by the litmus test? 2. Explain its presence by enlarging this statement: Hydrolysis takes place to a greater degree than the ionization of the a d d salt. 3. Write the equation of acid reacting with the sodium bicarbonate. 11. The A c d Principle. I n separate watch glasses or beakers put a large pinch of potassium bitartrate, monocaldum phosphate, and any one of the alums such as sodium aluminum sulfate. Press a piece of blue litmus dawn on each substance while dry. Examine for effect (?). Put a little water on each of the substances (?). Tabulate the dry and wet litmus tests. 4. Why do dry substances (molecules) not affect litmus? 5. What ion is shown to be present in each case? 6. Explain its formation in each Ease. (The first two substances are acid salts. Hydrolysis takes place in t h e salution of the last.) Complete these equations: NaHCOa KHC~HIOS --+ RNaC&Oa f ? ? NaHCOs CaH&?O,)r +CaHPO, NalHPO. ? ? N ~ H C O I NanS04 AIx(SOJa A1(OH)r NarSO, ? ? 7. Why do all baking powders contain sodium bicarbonate? 8. Why must all baking powders contain a substance of acid nature? 9. Why does exposure to air for a time make baking powder less effective? 10. Why is starch an ingredient of all baking powders? 11. Which baking powder leaves the least objectionable residues in the bread? 12. Why is yeast a better means of leavening bread than baking powder?
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111. Comparison of Baking Powders. Obtain several different types of baking powders. Fill s dry cartridge shell with each, quickly insert them under separate testtubes filled with and inverted aver hot water (?). Compare the different powders in a tabular form regarding components, speed of action, and available carbon dioxide. See the label on the can for the components. 13. What powders are rapid in their chemical action?
14. Why
should dough containing such powders be put into the oven quidrly after
mixing? 15. Should the oven be slow or fast in baking bread containing a rapid baking powder? Why?
The points of value of such a procedure are that: (1) A specially prepared introduction opens up the topic to the pupil's mind and a t the same time the service side of chemistry is emphasized as it operates in life situations. Information not obtainable by laboratory study is herein provided so that the pupil may begin to synthesize the knowledge of the topic. (2) In the experimental work the properties of the two main ingredients of baking powder are studied with the manner of their interaction to produce carbon dioxide. (3) Fundamentals are stressed by specific questions requiring the pupil to consider the experimental data and learn from these facts. (4) Three types of baking powder are compared as to cost, rate of action, and wholesomeness. (5) Leavening with baking powder is compared with leavening with yeast. (6) The chemical principles of neutralization, ionization, and hydrolysis are applied and illustrated. (7) Parts I, 11, and I11 are fundamental and required of all pupils. Additional parts are provided for the more proficient pupils so that they need not be kept a t the same level as the slower ones. (8) A summary with exercises for drill on fundamentals is provided as a review for a following day. Here again advanced work is provided for the more apt pupils. (9) Practical points in the use of baking powders are developed with study of the chemical principles so that the pupils cannot fail to apply these principles in life situations. (10) Instead of practice with technics of little value the pupils are trained to think from facts, learn chemical principles, and to apply them in daily life.
