A chemistry class to begin the year’s work (reprint from 1980)

Has teaching chemistry changed in the last 35 years? This December 1980 article in Chem 13 News describes how a teacher begins his school year. We would be interested to hear readers’ reactions to this 35-year old approach to a chemistry class. It certainly sounds like guided inquiry was at work. It was originally reprinted from the New Jersey Science Teachers Association Bulletin, Volume 26, Number 1, August 1980.

At the beginning of a school year there is always a good deal of bookkeeping to be attended to: forms to be filled out by each pupil, laboratory desks to be assigned, textbooks to be distributed in exchange for book receipts, and so on. Along with this, it is customary to discuss briefly the nature and value of the subject to be studied, and its place in the education of the pupils — a combination of orientation and pep-talk, illustrated with demonstrations. After this usual introduction, I have for several years started my chemistry classes in a somewhat unconventional way, which I find interesting and which may be of interest to others. Following is an account of the method used and some of its results.

In each class, near the close of the day’s chemistry period, I took a battery jar, put a candle in it (fastening it upright with a few drops of melted candle wax), poured in some water (but not enough to cover the candle), lighted the candle, and lowered over it an inverted bottle until the bottle touched the water. Each pupil was then asked to turn in, as homework for the next day, a report on what happened and why.

The reports were interesting. Some pupils had noticed that bubbles escaped at first; all had noticed that the candle went out; most of them had noticed that water rose in the bottle; a few thought that the rising water put the candle out (although it had not risen far enough to do so). Next day in class, when the reports were read and discussed, the importance of accurate observation needed no further emphasis. The demonstration was then repeated, so that all could agree on the observable facts.

As for the second and more difficult part of the question, the explanation most commonly given was that the candle went out because it had used up all the oxygen in the bottle, and that the water rose to take the place of the oxygen used. This explanation raised the question, “Where did the oxygen go?” After discussion, demonstration reading, and experiment concerning what happens to oxygen when a candle burns, and after some learning about carbon dioxide and water vapor, the question still remained, “Why should the volume of gas in the bottle be less after the burning than it was before?” This led to further work (in class, after school, and at home) on condensation of water vapor, solubility of carbon dioxide, and other factors. Some pupils in their original explanations had already reached this point, although usually not clearly or fully.

By this time the affair of the candle had been going on for two or three days, and it was becoming more involved all the time. We proved, by the usual method, that there was carbon dioxide in the bottle after the candle had gone out. But one of the pupils demonstrated that there was acid in the water, and suggested that at least some carbon dioxide must have dissolved. Every attempted explanation seemed to lead to something else.

About this point, one of the boys threw a bombshell into the discussion when he quoted from our textbook the statement that a candle would go out when the amount of oxygen in the air went down to 17 per cent. This seemed surprising to many, and some took the trouble to experiment in order to find out if there really was oxygen remaining in the bottle after the candle had gone out. They found that there was. At least, they said that iron filings placed in that gas rusted in the usual way. Others, now more critical than at first, wanted to know how the experiment was carried out. It appeared that iron filings had been pushed up through the water and were therefore wet. It was suggested that the iron filings might have obtained oxygen from the water, which was believed because of its formula to contain oxygen. The experimenter countered by putting iron filings in water and leaving them submerged; no appreciable rusting occurred, while during the same period of time rusting became evident on iron filings in the gas remaining in the bottle after the candle went out.

We were now up against the necessity of explaining how the consumption of 4 per cent of the original air (21 per cent oxygen when the candle started to burn, 17 per cent when it stopped — if we could assume that the 17 per cent figure given by our text was accurate) could lead to a reduction of apparently one-fifth or even one-fourth in the volume of gas in the bottle. We were beginning to question simple explanations.

At this stage the more inquiring pupils began to delve into the quantitative features of the question. There was measuring and calculating of volume, and allowing for the volume of the candle, and waiting for the gas to cool, and trying to catch the bubbles that escaped from the bottle, and so on. One boy determined to analyze the gas remaining after the candle went out, particularly to see for himself how much oxygen remained. He spent most of his spare time for several weeks in devising and testing homemade apparatus for the purpose. He learned something about pyrogallol, and a good deal about the practical difficulties that can arise and the sources of inaccuracy that need to be overcome. Others tried out candles of different lengths, and bottles of different sizes, testing their own hypotheses.

When we finally went on to more conventional subject matter, the pupils had reached various results in their thinking, depending on their interests, equipment, and previous preparation. All were interested, but some were willing to let the matter drop after a few days. A few were more confused than challenged by the complications that appeared, and wanted me to tell them the answer and get it over with. But I did not claim to know all the answers, though I was willing to help them with their thinking. We did get together on some explanations that seemed to be satisfactory as far as they went; but certainly no one of these was the full answer to our problem. Some pupils were not satisfied, and kept bringing up further questions, day after day, of their own accord. Some questions could be answered at the time; others were cleared up later in the school year, as we learned more chemistry and some may still be unanswered. But life’s problems are like that: one cannot always demand and receive complete and final answers. And there seems to be no reason for arranging to give our pupils the opposite impression. All agreed that there was much more in the experiment than appeared to a casual observer, and that it would take a great deal of time, effort, and knowledge, and maybe even a little wisdom, to get to the bottom of the subject.

Later in the school year one of the boys brought in a newspaper clipping which described this very experiment, and gave the explanation that water rose up to take the place of the oxygen used up. The last line of the clipping read, “Simple, isn’t it?” That got a laugh from the class.