What Makes a Good Explanation in Chemistry (and What Makes a Bad One)?
Features of good explanations:
1. Relating of ideas to what is already known
The best explanations build on secure foundations, and link between ideas that students already have so that they can incorporate their new learning into an interconnected web of knowledge. For students to grasp abstract principles, they need top-down and bottom-up learning. Top-down learning is about fitting in students' understanding into their web of knowledge of macro-scale observations, e.g. does the explanation match the practical observations?. Bottom-up means working from first principles using ideas students already have, e.g. particle theory, or their understanding of electrostatic forces of attraction. Using both approaches simultaneously leads to the deepest learning, and both need teachers to explicitly relate the learning to what students already know. (For more on this see by review of A Mind for Numbers in the book reviews section of my website.)
An example of a task in which students link between theory and observation and derive a set of similar linkages for other pairs of elements, e.g. calcium and strontium.
2. A mix of different ways of presenting the explanation
Abstract concepts present one of the biggest challenges to learning, and Chemistry is full of them! Presenting the explanation in a number of different ways can help students to see the deep structure of the concept.
For example, when teaching about bonding, giving explanations in terms of ionic, metallic and covalent bonds, leads students to give their explanations as discretely linked to each individual kind of bond, and they have to learn what the bonds to differently in each kind of structure in order to explain the physical properties. But the deeper structure is an understanding of electrostatic forces, which are present in all three structures, and many of the explanations for the physical properties are very similar.
Getting students to appreciate this deep structure needs to be a goal of giving good explanations. And Make it Stick recommends that we give students a huge range of presentations and examples with different surface structure, to allow them to make the deeper connections. So derive it from first principles, design a practical, do a card sort, try a different investigation, evaluate models, etc.
3. Linking between practical observations and the theory
This is really related to both of the features above, but is particularly crucial in Chemistry, where what I have said above relates to teaching any abstract theory. Students not only need to be able to do this themselves in order to excel at GCSE and A-level, but it is also a really important way of helping students to learn in the first place. Firstly, this is the most obvious way that teachers can help their students with a top-down approach to learning in which they observe (a demo or doing some practical work themselves) and then students brainstorm why they see what they do. Secondly, practical tasks can provide richly different ways of approaching the same theory: students can design investigations, make careful observations, analyse practical data, evaluate investigative work, amongst many other kinds of activities, all of which can contribute to deeper learning.
The combustion of calcium.
4. Tackling misconceptions head on
In Chemistry students have misconceptions about all sorts of things. This is hardly surprising, the explanations can be very abstract. I remember reading about some interesting research done on Physics undergraduate students in the US in which they had radical misconceptions about what they would observe if the string was cut on a pendulum mid-swing, or on a ball being swung around. The students gave explanations in terms of 'impetus', a kind of non-scientific combination of force and momentum. the researchers hypothesised that this was because a simplistic understanding of forces in terms of a simple 'impetus' was sufficient to ensure that they could predict motions of objects sufficiently accurately and quickly in earlier stages of human survival.
Chemistry can be equally challenging: students have vague intuitive notions about why temperature increases the rate of a reaction, or how a catalyst works, but they cannot give rigorous reasoned explanations, or accurate predictions in every circumstance with these vague, woolly notions. Students underlying preconceived ideas, as well as any misconceptions that they have build up during their education need to be dealt with, otherwise they lead students down all sorts of blind alleys, and students won't be able to build up a coherent picture of Chemistry. This means identifying them using diagnostic tools, and getting students to reason their way out of them. Concept cartoons work well for this, as the various different explanations, including ones using misconceptions, can be evaluated in terms of how well they explain the evidence.
5. Using models - but only in the right way to avoid misconceptions
Models can be a great tool for getting students thinking about the abstract ideas in a more concrete way. But models can often lead to misconceptions as they invariably involve a change in context of domain of knowledge and the transfer is never perfect. So how can we use models in teaching and avoid over-simplification/confusion?
Firstly, I think that students should be trained to evaluate models as part of using them. For example, 'is using different shaped bits of pasta a good model to use for elements, mixtures and compounds?', 'is a greenhouse a good model for the greenhouse effect?', or 'is an escalator a good model for equilibrium reactions?'. Secondly, the models used need to be carefully chosen by teachers so that they challenge, rather than reinforce, students' misconceptions. The time spent in a department meeting brainstorming the models that you will use, and suggesting misconceptions that students will already have and what they could pick up, is time extremely well spent.
The free RSC course on Developing and Using Models, (link HERE) provides really excellent guidance on how to use models well as part of Chemistry teaching.
6. Building in complexity and sophistication in stages
This is all about scaffolding. We are all aware as teachers of the need to scaffold learning so that students build up their picture of the learning in manageable chunks so they don't just give up on a task that is too vast for them. I've written in detail on how to scaffold learning for students, so I won't repeat myself, but how an abstract concept will be scaffolded is the most important aspect of lesson design (in my opinion). This is what determines whether or not you can carry all of the students with you through all of the conceptual content.
It is always a sign of a really excellent lesson when students can explain something really challenging at the end, because they have been guided, almost imperceptibly, through the learning process. Furthermore, it gives students a huge sense of achievement in their learning, inspiring confidence and chemists of the future!
7. Allowing students to piece together some of the aspects themselves
Finally, allowing students to play a part in the explanatory sequence themselves can give them a real sense of ownership over their learning. For example, students could be given a bad explanation and asked to explain why it is poor, or asked to put together pieces of the explanation from different sources (some from a practical, some from a flowchart, some from research), and asked to work individually or as a group, to assemble the pieces and write their own explanation. This kind of process is linked with improved student recall, as they are invested in the reasoning, they focus their mind harder, and the pieces of the information are then 'chunked' effectively, improving students grasp of the topic.
Features of bad explanations:
1. Giving particles agency, or feelings
One kind of explanation which really grates on me is when teachers assign agency to particles. I'm sure that most of us have heard phrases like:
"the chlorine wants to gain an electron to have a full outer shell",
"noble gases are unreactive because they have full outer shells: they are happy atoms", or;
"the cyanide wants to attack the electron deficient carbon because it is a nucleophile".
None of these explanations make any sense. Atoms are not agents, they don't have free will, and they are not persons of any kind, they don't have feelings. All these explanations do is create misconceptions that particles do have feelings. Often these kinds of explanations are used as simple ways of getting students to follow the pattern of reactivity without diving into the depths of the explanation, but this is a poor excuse for outright lies.
This is picked up by Keith Taber in his work on chemical misconceptions, as a key student belief and one that teachers must avoid. His diagnostic worksheet on Hydrogen and Fluorine (below) can be used to challenge students who think in terms of 'happy atoms'.
2. Explanations which only work in a limited number of cases
Some explanations are very limited, and will quickly lead to teachers making up all kinds of exceptions to the rules. Such as the 'explanations' that the third shell can only take up to eight electrons, that when non-metals react they gain electrons, or that chemical reactions give products which have full outer shells. Although all of these explanations is safe at GCSE, if student are carrying on to A-level they are not. And any intellectually curious GCSE student will also spot the holes in these kinds of explanations. They are limited in what they can explain, and they are limiting students' potential to understand the Chemistry at a deeper level.
3. Mathematical and logical derivations
A lot of Chemistry can be derived from first principles using maths and logic, and it can be tempting to give students some of these elegant solutions. But, although they are beautiful to those of us who already understand and grasp the abstract principles, these kinds of 'proofs' are terrifying for students.
Firstly, and trivially, it automatically segregates between the mathematical and non-mathematical students. And all of the latter group of students' fears and preconceived ideas about "I can't do maths" take over and it's 'fight or flight' for many of them: they will just clock off from the learning.
Secondly, although this appears to satisfy some of the conditions above (building on what is already known and building complexity in stages), this is only on the surface. Students find learning abstract concepts extremely difficult (see my review of Make It Stick in the Book Reviews section of my website), and they need to see the same abstract concept in numerous different real-world situations in order to grasp it. Deriving the new learning from old learning does not bring any real-life examples in which students can contextualise the abstract principles they are being expected to learn.
If you have any other ideas of features of good or bad explanations in Chemistry which you feel I have unfairly overlooked, please do feel free to contact me!