Popular Science Monthly/Volume 21/June 1882/Chemistry in High-Schools

From Wikisource
Jump to navigation Jump to search



HAVE, for some years, been trying to improve the teaching of chemistry in girls' schools. It is, of course, work of the most elementary character. I wished earnestly to make it, so far as it went, inductive study in other words, to train the observing powers to select for themselves the significant facts; and to train the reasoning powers to draw for themselves, with some degree of independence, the more important of the general principles which we call the theory of chemistry.

When I began to teach this subject, about six years ago, the progressive teachers had become dissatisfied with the old plan of book-study or lecture, with experiments by the teacher. The best thing offered as improvement was the performance of experiments by the pupils themselves. This was certainly an important advance; and manipulation is, first or last, essential to any complete knowledge of chemistry. But it is not all that is needed. Mental activity is the important thing.

I will illustrate the plan which has proved best in my own experience, merely saying, first, that even where pupils have studied physics, I have a preliminary drill to make sure the girls are quite clear about the forms of matter, the properties of liquids and gases, attractive and repulsive forces. I do not say a word to them about chemical attraction, because I wish them to study that out for themselves. Sometimes they have already learned by heart a definition from some book on physics; but, as Thomas Carlyle would say, "by the blessing of Heaven they have generally forgotten it."

I think the best experiment to begin with is the evolution of oxygen gas by heating the red oxide of mercury. This is not the easiest or the most convenient way of obtaining oxygen, but it is much the simplest process.[1] I do not usually tell my pupils anything—not even what the substance is which they are to make; but they know I shall question them closely about what they have seen. When the experiment is concluded—the gas tested with a glowing taper; the residue of mercury examined; and a little of the red oxide put into water and stirred, to show that it will not dissolve—I usually ask questions about as follows:

"When the jar was lifted from the water to the shelf, why did not the water fall out? Why did the water afterward sink in the jar? Did you see anything in the jar as the water fell? Why do you think there was anything in the upper part of the jar? What form of matter was it? How do you know that it was not air? How was it like air? How was it unlike air? Had it color or smell? Did it burn? What was it burned? Did the gas have anything to do with the burning?"[2] I then tell them the gas is called oxygen; and I write on my blackboard the name and symbol, with a list of the properties which they have just discovered.

I then proceed to ask: "What remained in the ignition tube at the close of the experiment? What form of matter was it? Did you ever before see anything with that shiny luster? What class of bodies have it?" "Then I tell those who do not already know that it is mercury, and I give the symbol, Hg.

I then say: "Where were the gas and the liquid when that red powder was placed in the tube? What became of the powder? Did it take any force to separate the gas and liquid which you say formed the powder? What was the force? What sort of a force is heat? Do you suppose any force held the oxygen and mercury together? Do you know a general name given to forces which unite bodies? Name some kinds of attraction. Is this force cohesion? Why not? Is it adhesion? Why not?" I finally tell them the force is called chemical attraction, and I call on them to put into words a clear definition of chemical attraction. As they do so, I simply criticise the successive trials they make, until the definition is correct in matter and form; and then, after making them repeat it several times in concert, I write it on the blackboard.

By similar questions, which I will not weary the reader by repeating, I make them draw from the same experiment facts and definitions about elements, simple substances, compounds, oxides, decomposition, etc., etc.

In my life, I have taught a great deal of Latin and English, but I know no such language-lesson as is given when a class, under the fire of a skillful teacher's criticism, slowly perfects a clear logical statement, or definition, for which they have gained the material by using their own senses and reason. My pupils keep note-books, and at every lesson bring me, neatly written out, the substance of the previous lesson.

I have given above only a sample of the general tenor of questions. Sometimes some misapprehension on the part of pupils makes me diverge widely to bring them back on the track.

I endeavor to make the subject as practical as possible by having pupils study the chemistry of common operations. After the above experiment, I usually introduce the subject of air by asking: "Why did the taper at last fail to light again when dipped into the jar? What had become of the oxygen in the jar? Did the remains of the burned paper look like the remains when paper is burned in the air?"

After this I take a bit of sodium and burn it in oxygen. I also oxidize some in the air. I show, by testing, that the oxygen has disappeared from the jar. I test the first oxide with red litmus, both before and after dissolving in water. I let the pupils taste a little of a very dilute solution. I then ask questions about sodium just as I did about oxygen and mercury. I ask what has become of the oxygen and the sodium; what unites them; what the force is called; draw them on to classify the result of the union as a compound and an oxide; draw them on to note the properties of the compound. Then I have them test the oxide formed in the air just as the other was tested. I ask: "Is this a compound? Do you think you know either of its elements? Of which one are you sure? Do you think you know the other element? Why do you think it is oxygen? Where did the oxygen come from? What other reason have you for thinking the air contains oxygen?"

I wait for further experiments before indorsing their partly formed conclusion about oxygen in the air. We next make some study of carbon by burning coal in oxygen. I have them test the result with lime-water. We burn coal in a receiver of air, and test this result also. I have some powdered charcoal heated in contact with the red oxide of mercury, and that result tested. Every particle of information which observation can draw from these experiments is carefully elicited by questions such as I have described.

At this point I usually inform them that the red oxide of mercury is sometimes made by heating mercury a long time in contact with air. They commonly by this time consider the evidence of oxygen in the air pretty conclusive. I next lead them to think about the air we breathe: whether it comes from our lungs unchanged; to think of some way of testing whether it contains free oxygen. I have them test the breath with lime-water, discuss the effect of the union of carbon and oxygen, especially the heat.

In all this I tell them very little. I become greatly interested in seeing how much I can get them to do for themselves. I simply try to stimulate and get them on the right track. At this point I usually ask them whether they think the air contains anything besides oxygen, and set them to discussing ways of getting at the other element in air.

Devising experiments is a very important part of chemical training, and, where the pupil sees beforehand some question to settle, he can be made to do it. By rousing him to think, criticising his crude plan, and showing, or making him think of, its defects, it can be done. Pupils will devise the well-known experiment of burning away oxygen from air, but of course they must be told that phosphorus is the best combustible for their purpose. By taking up the various requirements of the experiment separately, they will suggest nearly everything.

But before the experiment is actually carried out, to prevent the confusion which would arise from the vapor of phosphoric anhydride, I have them make a little study of phosphorus. It is examined, burned in oxygen, burned in air, the anhydride noted, its great affinity for water, its behavior to litmus both before and after union with water, its taste noted, etc. After this we use phosphorus to help us study the composition of air. The girls note (approximately) the proportion of oxygen to nitrogen.

We test the air for carbonic acid; discuss the moisture in it, etc.; and then I have them make some study of water. To do this I first put a bit of sodium in a very small cage of wire gauze, and thrust it under a little water. The result is tested, and shown to be the same compound they before knew. When they are satisfied that the oxygen must come from the water, we collect hydrogen and examine it. I have them also note the new method of decomposition. Then we have the proof by synthesis, burning hydrogen and collecting a little of the water.

As we proceed, my pupils begin to think ahead of questions, and their perceptions grow sharper.[3]

If, after this, we decompose water by a battery, the students will at once recognize the process for themselves as decomposition; and it confirms their previous analysis of water. But, if I begin experimenting by battery decomposition, they can not study out for themselves the rationale of the process. Of course, the teacher can explain and point out and make it understood, but they take it all pretty much upon authority, and their minds are far less active and independent. And so, of making oxygen at first with potassium chlorate. The chlorate is a less simple substance than the red oxide of mercury; and the presence of the binoxide of manganese, with the catalysis, complicates the process.

While directing this experimental study I do not tell them any of the facts which come on testimony, unless, like the fact about the making of the red oxide of mercury, it is a necessary step in some chain of reasoning which they can make out mainly for themselves.

The precautions necessary in using such substances as sodium make it unwise and imprudent to set careless young folks to handling them. One accident would bring lasting disrepute on our chemical study.

Showy experiments are demoralizing, though they excite for the time a sensational interest. But, when young folks really think for themselves, they are so pleased with it that they can take the highest interest in a very simple process. There is an experiment which I learned from that capital book, "Eliot and Storer's Chemistry," which illustrates a good many things I have said. It is designed to show the great diffusibility of hydrogen. A tube, closed at one end with plaster of Paris, is filled with hydrogen, and put in a tumbler of water for a day or two. The water first rises in the tube, then sinks to the level of that in the tumbler, in consequence of hydrogen escaping faster than air comes in. When I first taught chemistry, my pupils took no interest in this experiment. When I tried making them discuss the changes, and discover for themselves the property of hydrogen which causes them (which they do with all ease), they find it more interesting than the burning of phosphorus in oxygen. This experiment shows, too, how genuine inductive teaching must necessarily be oral teaching, for a text-book merely tells the philosophy of the changes, which is precisely the thing the pupils ought not to be told.

When chemistry is taught inductively, the order in which the subject is presented becomes important. It is of the highest consequence that the more dependent parts of the science should not be put forward in the beginning. I do not think the order of our American text-books so good as that of Stockhardt.

I will state, in a very few words, the order which seems to me best. I usually make the pupil study, first, the individual properties of the thirty chief elements, taking up no compounds but oxides and hydrogen acids. The pupil should test the oxides with red or blue litmus; note the acid or basic taste; note which are insoluble in water. The difference between hydrates and anhydrides should be clearly brought out, and the part which hydrogen plays. After this survey, the pupil, for himself, without prompting, divides the elements into their two great classes.

Then, after some little study of sulphides and the other binary compounds, the principal acids and bases should be shown in their concentrated form. After this, a number of them should be combined to form salts, and, in doing this, it should be brought out very clearly (by, not for the pupil) how the bases replace the hydrogen of the acid. There should also be some general study of crystallization.

It would be easy to multiply suggestions, but it has been my purpose in this brief paper simply to describe what I have tried, and give only the results of experience.

  1. I always, of course, have ready a quantity previously otherwise made.
  2. I find that pupils will at first, of themselves, make the somewhat conventional distinction between "combustibles" and "supporters of combustion." For a while I let this pass.
  3. In final review, I employ the topical method of recitation, but this method is too loose for investigation, which must be held down to accuracy, by well-considered questions, taking up one point at a time.