© 2017 by Emily Rose Seeber. 

  • Emily Rose Seeber

Make Practicals Meaningful: Using 'crucial experiments' to engage students


What is a crucial experiment?

When Francis Bacon was writing about the scientific method in 1620, he suggested that 'crucial experiments' could be used to distinguish between different theories which were competing against each other. In the 20th Century, Karl Popper build on the idea to distinguish between science and pseudo-science, suggesting that crucial experiments could be used to prove that bad theories were wrong; good theories were simply those that stood up to many crucial experiments and passes time after time. Furthermore, for Popper, a discipline was not a science at all if it could not suggest crucial experiments for testing its theories.

Philosophical justification for the notion that some experiments are more important and valuable than others and can therefore be used to reject some theories and favour others, has been challenged in the 20th Century by Pierre Duhem and others. Duhem suggests that, since we cannot know every detail of any scientific theory, we cannot legitimately compare theories in this way; the one which makes the incorrect prediction could still be the right theory, but for a reason we do not know about yet. Scientists, however, continue to view some experiments as particularly useful as ways of demonstrating that one theory os more likely than another competing one.

Using crucial experiments to engage students in learning

Students are competitive and they like to discuss and argue amongst themselves and with a teacher; they love a battle. Students also love a story. In his book on the cognitive science behind learning, Why Don't Students Like School, Willingham argues that presenting a lesson as a story is a really effective way to ensure that students are really invested in the answer. A conflict, which is a key element of a story, really engages students in the lesson and makes them more likely to remember the learning. If a demonstration or a practical can be used to distinguish between two competing explanations then students are much more likely to see the point of the demonstration. Before the demonstration the students need three things:

  1. They need to know the two competing theories which are being tested against each other and be unable to resolve the conflict by working it out themselves.

  2. They need to make predictions about what will be observed if Theory A is true and what will be observed if Theory B is true.

  3. They need to understand why choosing between A and B is important for making further progress.

Once students have either observed the demonstration or completed the practical and determined which of the competing theories is correct, it is viral that they continue to make more predictions, or write explanations using the correct theory. Memory is the tract of thought, so if the students have spent half their time thinking about each theory and making predictions, they will remember each theory equally well, despite one being wrong and one being right.

Crucial Experiments

There are plenty of simple examples of crucial experiments which can be used in class to promote student engagement and high quality learning. I have provided four examples: three demonstrations and one class practical to provide a framework for developing your own ideas.

A. Demonstrations

The relative reactivity of sodium and lithium

The trend in reactivity of the group 1 metals is covered on every GCSE and IGCSE syllabus and is very important for getting students to understand how the Periodic Table came together and how electronic configuration effects reactivity. Demonstrating the reactivity of lithium and water and then getting students to predict the reactivity of sodium with water without being told what the trend is can be used as a crucial experiment for getting them to compare different ideas about what affects reactivity. There is sufficient challenge for high ability students as they would be expected to evaluate between sodium's higher nuclear charge and the outer shell electron being further away from the nucleus. In this crucial experiment students need to balance out which factor they think is the most important for determining how much energy is required to ionise the sodium.

In order to tease out some of this thinking from students, a concept cartoon activity can be used after the reaction of lithium and water it demonstrated. Students can then evaluate statements such as, "sodium will be less reactive because the electrons are more strongly attracted to the more positive nucleus" and "sodium will react in the same way as lithium because they are both in group 1". Once the students have observed the reaction of sodium and water and confirmed which factor is the most important, they can make detailed predictions about the reaction between potassium and water and use the correct explanations from their original concept cartoon to explain their ideas.

If you do use the demonstrations for this purpose, then it would be a good idea not to focus on the other observations about the reaction at the same time and do the demos again with students focusing on the production of hydrogen and metal hydroxides. This means that the colour change of the indicator is not distracting for students during the crucial experiment.

The reactivity of aluminium

Placing aluminium in the reactivity series is another opportunity for a crucial experiment. Students can suggest that aluminium is either very unreactive (it does not appear to react; everyday aluminium foil is extremely inert), or aluminium is fairly reactive (demonstrating and then comparing the reactions of iron and iodine with aluminium with iodine can be used to give weight to this theory). Students can then plan a series of different displacement reactions and make predications about what they would observe in each reaction depending on where aluminium sits, e.g. above or below magnesium.

Some displacement reactions can then be carried out on order to place aluminium in the reactivity series. This example from the RSC is excellent, particularly as it helps the students to understand why aluminium appears to be so inert. The dancing flames demo is a similar idea. The results of this demonstration, combined with the more dramatic thermite reaction, can then be used to confirm the true reactivity of aluminium and students can then move on to writing equations or evaluating practicals in which they know where aluminium sits on the reactivity series. It is worth noting that the thermite reaction should be done towards the end of the lesson rather than at the start, as otherwise students are likely to be thinking about how awesome the demo was, rather than about the reason why aluminium appears to be unreactive. It functions much better as a plenary to confirm the students choice of their, and consolidate their understanding of displacement reactions than as a starter to pique their interest and then distract them from the learning for the rest of the lesson.

Oxygen vs. Phlogiston

Although I no longer teacher KS3 Chemistry, I used to teach a section on the discovery of oxygen and rejection of phlogiston theory, and it is an excellent opportunity to use crucial experiments to illustrate how they have been used throughout the history of science. One thing to be careful about is that students do not spend too much of their lesson time puzzling about phlogiston as otherwise they will remember more about phlogiston than the scientific method or about the behaviour of oxygen. Once again the RSC have produced a series of demonstrations which are crucial experiments to help students evaluate the competing theories about burning.

In the past I have often used the simple burning iron wool on a balance demonstration shown above to act as a crucial experiment between the two theories. Firstly because it is incredibly visual and all of the students, regardless of their mathematical reasoning skills, can understand that if the balance goes up the iron is getting lighter and must be losing phlogiston, and if the balance goes down then the iron is getting heavier and must be gaining oxygen.

B. A Class Practical

Inductive effect vs. Mesomeric effect

An example of an A-level class practical using the idea of a crucial experiment in Chemistry might be students compering the reactivity of phenol and benzene (this is one of my own, adapted from the CLEAPSS Safer Chemicals Safer Reactions booklet). Students are directed to understand that there are competing ideas about how the -OH group affects the reactivity of the benzene ring; the oxygen is more electronegative than carbon so it is electron withdrawing (negative inductive effect), but the oxygen has long pairs which can donate into the ring (positive mesomeric effect). Deciding which is the most important factor can be the subject of a crucial experiment.

Students are provided with sufficient information about the competing theories to make a prediction about the behaviour of phenol in each simple test: the addition of bromine water and heating with dilute nitric acid. They then carry out these reactions as part of a series of tests on a safer phenol derivative in order to determine whether or not the mesomeric effect is more important to the reactivity of phenol than the inductive effect or not. Once again, after the experiment, a post-lab should be given to students in order to give them time for thinking about the correct theory only and reinforcing their memory (and understanding) of what is really going on.

Using crucial experiments to guide the learning is a way of ensuring that students are really thinking about the reactions they are observing and linking their observations to theory. This is important for top-down, big-picture learning which is an essential aspect of neural chunking.

It is so important that when we carry out practical work and demonstrations, students are fully engaged in the learning and they are not just excuses for them to have a chat and get away without having to think very hard. Otherwise carrying out practical work and demonstrations would be a bit of a waste of time; and none of us want that to be true. Chemistry is a practical subject after all. Setting up a conflict between different theories which can be resolved, a crucial experiment, is one way in which we can ensure that experimental work is exciting and informative, but also thought-provoking and meaningful.

There are plenty of other examples of crucial experiments that can be used in chemistry teaching. If you have any great ideas which you think I have unfairly missed out, please feel free to contact me through my ABOUT page.

Images from Education in Chemistry and Royal Society of Chemistry Learn websites.

#pedagogy #science #practical #historyofscience #PCK

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