THE LECTURE-DEMONSTRATION METHOD IN HIGH-SCHOOL CHEMISTRY DONVAN HORNS,TUCSON HIGH SCHOOL, TUCSON, ARIZONA
A careful search of the literature bearing on the presentation of demonstrations in high-school chemistry to serve as the basis of an experimental comparison of the demonstration method with individual laboratory work,' has revealed that little has been published on the technic of this very important method. Every chemistry instructor must of necessity acquire some skill in giving demonstrations and gain a certain knowledge of what to do and what not to do. A ,number of books have been published containing collections of experiments for demonstration, but the method as a method is not clearly defined. D a v i s ~ nin , ~his introduction to a collectiou of experiments for demonstration, gives an excellent discussion which any one can read with profit. Nevertheless there appears t o be no standard for the giving of demonstrations. What material is available has not been put in form where the inexperienced teacher can get a t it readily, and even those of experience do not always know where to get the information. The widespread discussion of the relative merits of the lectnre-demonstration and individual methods for laboratory work has led to considerable experimental work. The reader will find available the work of wile^,^ Carpenters4Nash and Phillip~,~ AnibeL6and Knox.' The conduct of individual laboratory work is treated fully in the various textbooks on method, yet in spite of the interest in the demonstration method, comparatively little has been written about it for tliose who need the information most. Every high-school instructor realizes the importance of a well-planned and executed demonstration. There is nothing better for clearing up a point under discussion or for teaching a new principle. However, the instructor who is just beginning his work has probably had little in his 1 Van Home, Don, "An Experimental Comparison of Demonstration and Individual Laboratory Methods in High-School Chemistry." A thesis far the M. A. degree submitted to the School of Education, University of Southern California, 1929. a Davisan, H. F., "A Collection of Chemical Lecture Experiments," The Chemical Catalog Co., 1926. 3 Wiley, William H.. "An Experimental Comparison of Methods in Teaching HighSchool Chemistry," I. Educ. Psychol., 9, 181 (April, 1918). 4 Carpenter, W. W., "Comparison of Different Methods on the Basis of Results Secured in High-School Chemistry," Doctor's dissertation, Columbia University. 1926. Teachers' College Publications. 6 Nash and Phillips, "A Study of the Relative Value of Three Methods of Teaching High-School Chemistry," J. E d u . Res., 15, 371 (May, 1927). 8 Anibel, Fred G., "Comparative Eflediveness of the Lecture Method and the Individual Laboratow Method," 5.Ed%. Res., 13, 355 (May, 1926). 7 Knox, W. W., "Demonstration Method versus the Laboratory Method of Teaching High-School Chemistry," Sch. Rev.,35,376 (May, 1927). 109
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college, university, or normal school training to guide him in performing demonstrations. With the idea of aiding the inexperienced teacher and perhaps giving suggestions to those of experience, the writer of this paper offers the results of his own experience with the lecture-demonstration method. Experiments for Demonstration 1. Use Demonstrations at the Beginning of the Course.-In highschool science the students are a t first very much lacking in the necessary skill for individual laboratory work. Therefore the very first experiments can advantageously be given as demonstrations, the students repeating them afterward if desired. Such exercises include the cutting, bending. and fusing of glass tubing. The insertion of glass tubing in stoppers, the handlmg of reagent bottles, the clamping of glassware, the application of heat to apparatus-all represent details best taught by demonstration. The printed directions in the manual can never be quite so effective as an actual view of the procedure. 2. Use for Dangerous Experiments.-There are a few exercises in every high-school laboratory manual which have an element of danger for the student. Probably the most conspicuous is the preparation of chlorine. The usual laboratory does not have ventilating ducts capable of carrying away all fumes and as a rnle there are not enough hoods for even groups of students to work. Therefore if the instructor demonstrates in such cases there will be less danger of injury and no protests from anxious parents. 3. Use for Difficult Experiments.-All instructors come to know that certain experiments are especially diffiplt for students. For example, the preparation of nitrous oxide requires that ammonium nitrate be heated with a steady flame a t just the right temperature to get a flow of gas and yet avoid an explosive decomposition. In this and similar cases, the instructor with supposedly superior manipulative skill should demonstrate the preparation. 4. Use for Experiments Too Long for Hour Periods.-In many schools the six-period-a-day schedule allows only one hour for laboratory work a t any time. Under such conditions, the instructor is faced with the dilemma of hasty work on one hand and of insnfficient time for thorough work on the other. Many of rhe longer experiments cannot be completed by the average student in an hour, but they can be demonstrated with ease. Not all high-school students can prepare several bottles of oxygen and examine the properties of the gas in one hour. The instructor should be able to do i t with proper preparation. 5. Use to Teach Principles.-It is the writer's experience that experiments which are intended to teach principles should be demonstrated. At least the first of such experiments should be. For example, finding the
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equivalent weight of a metal like magnesium by reaction with an acid and measurement of the hydrogen evolved, can be demonstrated with due attention to the procedure and the mathematics involved. After one such example, the student is better equipped to do careful weighing and manipulation, and has gained some appreciation of the mathematical foundation of chemistry. Class Preparation 1. Assignment of Work in Advance.-From experience the writer has come to believe that the best results are secured from a demonstration when textbook material or outside reading is assigned in advance. Some educational authorities contend that the student should come with an open mind and learn by seeing and doing. This may be the best way to learn, but the high pressure under which the modern school system works makes this plan rather impractical. I t may be well to test the students a t the beginning of the period by a series of oral questions or by some foxm of short answer test, trying to bring out the essential points of the material to be covered in the demonstration. 2. Assignment of Special Tasks to Pupils.-Many times students can be trusted to prepare materials and assist in the demonstration. The writer has had considerable success with the plan of selecting two of the best experimenters in each class and giving them some special training so that they can do most of the actual work. Student aid during the conduct of the experiment gives the instructor a better chance to conduct the discussion and questioning. Q
Preparation of Apparatus and Materials 1. Have AU Material Needed.-Nothing is more embarrassing to an instructor than to find that just a t some critical moment he lacks a necessary reagent or piece of apparatus. High-school students are very keen to sense any lack of organization. To stop proceedings while sendmg for the material may destroy a large part of the effect of the demonstration. Beside loss of attention at the time, there is a lowered respect for the ability of the teacher. Therefore, it is desirable to check the list of materials very carefully and make sure that everything needed is on hand. teacher of some experience is 2. Make Sure That It Will Work.-A perhaps justified in assuming that he does not need to try the demonstration out in advance. The inexperienced instructor, however, needs to go through the whole procedure beforehand. That is the only way to make sure that everything will go off smoothly and that the desired results will be obtained. Even the teacher of experience should not neglect this precaution. I t takes time and energy which the teacher may be reluctant to apply, but there are so many factors involved in even the simplest
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demonstration that it is somewhat hazardous to attempt it without a preliminary trial. "Keeping demonstrations up to the highest standards that the school permits, demands heroic effort and often self-sacrifice on the part of the husy t e a ~ h e r . " ~ 3. Make It Visible to All.-Do not start the demonstration unless every one is able to see clearly. Where the lecture room does not have seats raised in tiers, rearrange chairs so that every one has an unobstructed view. Lack of attention to this factor leads to disciplmary troubles as well as failure to learn. 4. Have Apparatus Clean and Neat.-In high-school science we should teach habits of neatness and precision. The best possible example is a neat arrangement of apparatus with an array of shining glassware and orderly portions of materials. I t seems trite to mention cleanliness, yet the writer feels that the lack of it is one of the most annoying features of high-school teaching. Set your students an example in this matter. 5. Make the Demonstration Short.-Do not drag out and prolong unnecessarily any procedure. It is difficult to hold the attention of students without i n t m p t i o n for a full period. If necessary, shorten the experiment as given. If a long pause for heating or the like is encountered, have appropriate questions or discussion ready. Conducting the Demonstration 1. Explain Apparatus and Method Used.-If the apparatus is at all complicated or unfamiliar, it is well to begin the demonstration with an explanation of such points. Encburage questions which are to the point. Perhaps the student is puzzled by some detail which is perfectly obvious to the instructor. During the course of the experiment name each reagent as used. Students are always curious about the chemicals used, and the writer has found it a good plan to pass a labeled bottle around the class so that all may see. 2. Call Attention to Important Things.-Many times the production of an unusual color or a vigorous chemical action of some kind will ohscnre the important phenomenon for the inexperienced observer. For example, if the vigorous action of the zinc and sulfuric acid which produces hydrogen draws their attention to such an extent that they do not notice the effect which the hydrogen is having on copper oxide, then tell them what to see. By this we do not mean that the thinking should be done for the class. As far as possible draw out the proper inferences by judicious questioning. 3. Talk and Work at the Same Time.-At times where he does not trust the actual work to a student, the instructor will often find it necessary to keep up a running fire of comment and explanation while his hands are 8 Smith. Alexander, and Hall, E. H., "The Teaching of Chemistry and Physics in Secondary Schools," Songmans, Green and Company, 1902, p. 135.
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busy with the manipulation of apparatus. In fact, he must acquire the ability to do two things at once. Here again is an illustration of the necessity for practice and rehearsal. The demonstration, like a theatrical performance, requires careful staging and production. The instructor must needs be something of an actor. case there is a stage wait at 4. Enconrage Logical Discussion.-In some point in the proceedings, utilize this time to the best advantage. Guide the discussion into the proper field. This may be a practical application of the principles involved or a generalization from the facts revealed. The great advantage of the demonstration method lies in the fact that conditions can be controlled for the class as a whole and thinking can be directed along the right path. At the end of the experiment every effortshould be made to bring out the essential facts. Try to cultivate the ability to generalize. If there is any such thing as the scientific method, here is the place where it can be taught and learned. But while every effort should be made to encourage discussion, care must be taken that some individuals do not sidetrack the group. The high-school mind is likely to go off at a tangent if not guided aright. In this connection we urge that a written record be kept of the demonstration. This record may take any one of the several forms in use for laboratory notebooks. Whatever it may be, a written record is essential for retention and assimilation of the facts. The record should grow out of the observation and discussion. A Typical Demonstration In order to illustrate some of the points mentioned above, the writer wishes to present an outline of a demonstration which he has used with success. No claim is made for originality, but an attempt is made to indicate how points which bother the high-school student can be cleared up by demonstration and discussion. It would not be worthwhile to give the questions and comments in full, for the outline is intended to be suggestive only. Purpose: To Study Water of Crystallization.-As an introduction, mention any instances of crystallization which the class has had a chance to observe. The formation of frost crystals and the form of snow flakes will usually be familiar even if their previous laboratory experience furnishes no example. Mention the f a d that water seems to be necessary for the formation of some crystals. Apparatus.-Horn pan balance, lead shot, watch glass, ring stand, iron ring, mortar and pestle, tongs, test tube holder, Bunsen burner, wire gauze, 12 test tubes (preferably large Pyrex tubes, 25 X 250 mm.), test tube rack, 10 sheets filter paper, 15 an. Materials.-Anhydrous copper sulfate, crystallized sodium sulfate, copper sulfate, sodium chloride, sodium carbonate, potassium chlorate,
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potassium sulfate, potassium nitrate, potassium aluminum sulfate (alum), barium chloride, cobalt chloride; cobalt chloride solution (saturated); gypsum crystals. Procedure.-After stating the purpose of the experiment, and giving any instances of crystallization with which the class may already be familiar, exhibit to them the crystallized and anhydrous copper sulfate. Original bottles with the labels intact are very convenient to pass around the class. See if any one can suggest the reason for the very decided difference in physical properties. Crush a small piece of the crystallized copper sulfate in the mortar and heat gently in a test tube. By question and discussion bring out the fact that "anhydrous" means without water and that the two materials shown are both copper sulfate and differ from each other only in the water content. Following this, pour out a sample of sodium sulfate on paper or send a bottle around the class for inspection. Counterpoise about 20 grams of the same material on a watch glass on one pan of the horn pan balance. Leave this whole proceeding with the rest of the experiment. Heat a small amount of crystallized sodium sulfate in a test tube. Make sure'that the students realize that water is driven off. Take a small amount of sodium chloride in a test tube and heat. If sufficient heat is applied to melt the material, make sure every one understands that no water is formed. A surprisingly large number of students will call the liquid water. Let the liquid sodium chloride cool, and see if it is not apparent to them that the liquid was not water. In succession, heat small amodts of sodium carbonate, potassium chlorate, potassium sulfate, and potassium litrate. Call attention to the fact that sodium carbonate is commonly known as "washing soda" and see if any one has ever noticed the behavior of the compound when exposed to the air. If any bright student makes the observation that potassium compounds apparently do not contain water of crystallization, remind the class that generalization without considering a sufficient number of cases is a rather dangerous procedure. The next substance "alum" when heated will give off large amounts of water and show the stitdents that potassium compounds may contain water of crystallization. The swellimg and bubbling of the material as the water passes off will undoubtedly be the subject of considerable comment. Mention the fact that fused alum is used for medical purposes. Follow these examples with the heating of barium chloride and cobalt chloride. In the case of cobalt chloride, call attention to any noticeable color changes. A very interesting demonstration of color change due to differentdegrees of hydration may be shown by taking several sheets of filter paper and applying them to the mouth of a bottle of saturated cobalt chloride solu-
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tion. Manipulate the liquid so as to get a sheet of the paper with a spot in the center slightly moistened with the solution. Getting the paper too wet simply means a longer time for drying. Dry the paper over a small flame of the burner. The color change will catch the attention of all. Mention the novel barometers sometimes prepared from this compound which show color changes corresponding to humidity changes in the air. A demonstration of "sympathetic ink" can also be worked out and inserted here if desired. Heat some gypsum crystals in a test tube and note any changes. Explain that "Plaster of Paris" is prepared by the partial dehydration of gypsum and that the "setting" of the plaster is due to taking up again of water of crystallization. In conclusion, call attention to the fact that the side of the horn pan balance containing the crystallized sodium sulfate is considerably lighter than a t the beginniig of the period. Connect this fact with the water given off by heating this same compound a t an early stage of the experiment. As a final procedure, try to get the students to summarize the results into one statement which shall be precise and yet inclusive of all facts. Place of Demonstration Method 1. Demonstration Cannot Replace Individual Laboratory Work Entirely.-From the above discussion, the inference might be drawn that the writer believes that all laboratory work should be given as demonstrations. The writer is firmly convinced that the demonstration method cannot be used in high-school chemistfy to the exclusion of individual work. There are certain values to be acquired from personal work which cannot be secured in any other way. Whatever value the demonstration method may have, we would not be justified in presenting all science work by that method. 2. More Demonstration Needed in Average High-School Course.T i e is a considerable factor in high-school science teaching and by the rather extensive use of the demonstration method a greater variety of material can be presented than can be done by individual work and more time secured for drill and review. Certain experiments can undoudtedly be presented better and a saving in cost is possible. 3. Experimental Work Necessary to Find Proper Balance.-Precise experiments are necessary to determine just what proportion of demonstration and individual work is best to use. The types of experiments best adapted to demonstration should be carefully considered. Here is a fruitful field for the experimenters in the field of educational method.
Summary 1. The writer discusses the need for an outline of the technic of giving
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demonstrations in order to aid chemistry instructors, especially those of little experience. 2. The experiments adapted to demonstration, the preparation and conduct of the demonstration are discussed. 3. By way of illustration, a typical demonstration (water of crystallization) is outlined. 4. The writer suggests the need of experimental work to determine the relative place of the demonstration method in the teaching of high-school chemistry.
