Good - it's gone wrong;Secondary;Science amp; Technology

31st December 1999 at 00:00
The dodgy results obtained with many classic chemistry experiments can be exploited to stimulate scientific thinking, says Tim Griffiths.

A fresh approach to some of the old experiments can help develop students' scientific questioning and discussion. Consider the classic candle experiment to show the volume composition of oxygen in air, which must feature in almost every science course written in the past 100 years at least. How can an experiment that is so flawed scientifically be enshrined so firmly in our methodology? Most teachers focus on what has been taken out of the air during the combustion process. Few ask about what has been put back, since to do so makes the result look even worse than it usually is. Achieving a rough appreciation that something is taken out of the air when burning takes place does give the experiment validity but for me the real justification for doing it lies in a discussion of its many flaws and deficiencies. It is a process that I often refer back to in key stage 4, reminding students of what they saw when they were younger and encouraging them to be critical of it.

I remember in the 1960s - when safety in science lessons was less stringent - the classic "bell jar" experiment being demonstrated with phosphorus burning in air in a sealed bell jar, after ignition by a warmed knitting needle pushed down from above. Maybe there was some clever sleight-of-hand by the teacher, but I seem to remember that the method did give a pretty good approximation of 20 per cent oxygen in the air. (Diagram 1) This variation on the experiment, despite being environmentally unfriendly, had the advantage of producing a solid oxide, with no complication caused by the volume of the chemicals produced as products of burning. When, as a young science teacher, I came to repeat the experiment using a floating candle I soon learned that it was not possible to replicate the results easily. After a few abortive attempts to get the "right" answer I turned my mind to the reasons why it did not work, and have used this to advantage in class discussion many times since.

There are various problems with the candle experiment and a class can identify many of them with a little prompting. A variation worth trying is to use a crunched-up kitchen towel in an evaporating basin instead of the candle. (Diagram 2) The greater convection current produced can give a quite different result from the candle version. Students can be encouraged to think about what happens to the oxygen when it is used up, and this can lead to further investigations on the relative solubility of oxygen and carbon dioxide. More advanced students could take this further by writing possible equations for the experiment and looking at the volumes of gases involved.

Those seeking to obtain a good, simple and reliable figure for the amount of oxygen in the air should allow iron filings to rust over a period of time. Students can be encouraged to explore why this gives an answer closer to the accepted "book answer" for the volume composition of air. The experiment can be developed still further by trying other metals such as powdered magnesium. (Diagram 3) An experiment which had its origins in the 1960s involves the use of the gas syringe, which was first introduced into schools as part of Nuffield Chemistry. One experiment with these syringes sought to give a better answer to the amount of oxygen in the air, by passing a known volume of air over heated copper turnings until all the oxygen has been converted to copper oxide. I have still to obtain a good answer from this method and suspect that it needs much patience and a very good supply of fine copper turnings to work in practice. (Diagram 4) Having bought these expensive but useful syringes - the bane of most laboratory technicians - it was sensible to make good use of them and it was found that with a well-adjusted gas syringe it was easy to investigate the rate at which different gases effused through a pin hole. (Diagram 5) The times taken for this to happen have a correlation with the relative molecular mass of the gases used. It is only in more recent years that I have begun to wonder about this experiment, bearing in mind that the size of the smallest pin hole is still huge compared with the dimensions of the molecules concerned. Any competent GCSE scientist will tell you that a mole of any gas occupies the same volume, suggesting that the gas volume is more to do with the distance apart of the particles than their actual size. Why, then, is there such a significant difference in the times taken for the gases to escape from the syringes?

Seeking to find a range of experiments to shed light on the behaviour of particles led me to set up a series of balloons, filled to approximately the same dimensions with different gases. This was done in the expectation that one filled with hydrogen would "leak" out of holes in the balloon rubber more quickly than other gases since the particles concerned were very small, as was proved in the experiment on gas effusion from a gas syringe. In practice, the rate at which a carbon-dioxide-filled balloon went down exceeded anyone's expectations, leading to some excellent research by a chemistry group at GCSE level, involving good practical work - and much contact with members of the rubber industry. The answer seems to be that a strange mechanism exists whereby carbon dioxide actually "dissolves" in the rubber, setting up a concentration gradient across the membrane. It is essentially a chemical process rather than a physical one. I suspect that there is an idea here which could be worthy of investigation at a research level.

Another golden oldie which is excellent for developing discussion is to look at the strange way in which water can behave. This can also be a superb way of energising KS4 students at the beginning of a revision course, bringing together many strands of knowledge. A good starting point is to cut a large block of ice with a weighted wire. (Diagram 6) Students will come up with some fascinating reasons for the mechanism for this experiment (regelation) and a skilful teacher can often use partially right ideas to help to develop a working theory. Much can be teased out about particles, matter and changes of state from this. Students are familiar with friction causing heat and, for many, this is their route into understanding the process, but the idea needs development if the process of refreezing is to be understood as well. Using different thickness of wire attached to identical weights is a useful development in coming to a conclusion about the experiment, or the process can be done with different weights. There is much scope for a penetrative Science 1 study here. Incidentally, many students show real awe and wonder at this seemingly magical process and it can be a good time to look at other odd features about water and to consider what would happen to our planet if, like other materials, water became more dense as it froze.

This leads naturally to icebergs and flotation. When teaching density and flotation, an interesting diversion is to find a variety of plastic objects which have densities a little either side that of water. When they are put into a large tank of water, some will float and, of course, some will sink. The challenge is to make those which sink rise to the surface. This can be accomplished by adding salt. Again, there are many possible avenues for further investigation based on this and students will learn much about the nature of dissolving, density and flotation. Another brain-teaser our science department developed is to make up a "cocktail" of oil, water and coloured ice cubes and to encourage students to explain what happens as the cubes melt. With the current re-emergence of the 1960s "lava lamps", this goes down particularly well. (Diagram 7) Much of the fun of science lies in following up possible explanations and valuing student contributions. A climate needs to be established in which students can be adventurous in exploring models. Many "wrong" explanations can be discussed and end up being more helpful as a learning experience than many "right" ones - and many flawed experiments can be turned to advantage by the imaginative teacher.

Tim Griffiths is deputy head at Amery Hill School, Alton, Hampshire

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