By Adam Boxer
A well-crafted explanation lies at the heart of any effective science lesson. A good explanation must take into account a number of factors, but perhaps of foremost importance is the cognitive load placed upon the students. Decades of lab-based research in the cognitive sciences has taught us that students’ working memories are preciously limited. Labelled by Dylan Wiliam as “the single most important thing for teachers to know,” Sweller’s Cognitive Load Theory argues that if students are presented with too much information at once their ability to think, comprehend and learn is severely hampered. The role of the teacher’s explanation becomes to break down the material into the smallest chunks imaginable, into parcels of information which, when carefully sequenced, can be processed by the student and slowly built into a grander cognitive architecture.
In the current orthodoxy of social constructivism, the role of teacher explanation has been side-lined in favour of more “student-centred” approaches like discovery-based learning or peer-to-peer instruction. Stories abound of the denigration of too much “teacher talk” with best practice lauded as that which involves the least teacher instruction. Evidence from the cognitive sciences, observational studies of expert teachers and large-scale longitudinal studies stand in opposition to this orthodoxy.
Explicit Instruction is a highly interactive and dynamic process which prizes teacher exposition, rigorous and extensive student practice and detailed feedback guided by expert subject knowledge. To focus on the first stage of this process, teacher explanations should involve carefully constructed sequences of ideas and clear narrative structure. Generally, it is good practice to break the material into a chunk small enough to comprehend, but big enough to be able to see a wider picture.
An important part of science Explicit Instruction is the role of diagrams and visual representations of abstract ideas. Unfortunately, in the age of powerpoint and google image searches it has become prevalent for science teachers to beam a complete diagram onto the board and then talk through its details. The downside of such an approach is that it can often fall foul of the “split attention effect.” This is where students focus on more than one thing, sometimes on two or three different things, resulting in extremely weakened comprehension. If a teacher beams an image onto a board, the student begins to focus on the image. Their eye wanders around it, trying to find a point of familiarity to latch onto, from which to attempt to deduce and comprehend the whole. Meanwhile, the teacher is talking about the image but their voice drowns out into white noise. This means we have two vital aspects of teacher explanation, the diagram and the voice, working against each other in a cognitive sense.
One way to circumvent this problem is through “live drawing.” Instead of projecting an entire image onto the board, the teacher slowly draws the diagram themselves, silently adding a new section and then talking it through. This way, students’ attention can be completely controlled and managed by the teacher, increasing the chances that they will reap the benefits of the voice and the diagram without their attention being split.
An example of where I do this is when teaching the hydrogen fuel cell. A deceptively complex process, its cyclic nature means it is difficult to break down into smaller components whilst being able to maintain the wider narrative of what is occurring. The different parts work together, are meaningless on their own, and are therefore best explained in one sitting. When teaching such a complex idea, it is even more imperative that the explanation is carefully crafted with the support of diagrams to enable full student attention throughout.
All the below is done using the whiteboard. By definition it is a little tricky to convey through writing what is a dynamic and responsive verbal process, but the below should give you an outline for how I think about constructing explanations.
First, I set the scene by discussing the overall reaction. This is an exothermic process, and I explain that normally this reaction releases energy which can be harnessed for heating, but we could also harness it as part of an electrical circuit.
I then show students the “reactants” at one electrode (I leave discussion of anodes and cathodes for later) and explain where the species come from.
They react to form water and release electrons as below:
Which in turn must be balanced. In this I have used pencil as I will be re-balancing it later. A point of difficulty might be how you know there are two electrons formed, but I teach this having already covered electrolysis and half equations extensively so it does not serve as a hurdle.
I slowly draw the next step, explaining what is occurring as a narrative: “the electrodes generated can now travel through a wire to the other electrode. As they travel through the wire, they generate current which can be used to power a motor. Once they have passed the motor, they join up with oxygen and water to form hydroxide ions”
Now the right hand side needs to be balanced:
Which, as above, means we must balance the electrons, giving us 4e–:
This results in a problem, which is that we have four electrons on the right, but only two on the left. This means we must “re-balance” the left hand equation by entering four electrons:
And the full equation can now be “re-balanced”:
To start completing the cycle, I show the “journey” of the hydroxide ion. Obviously this is a simplification as it is not specifically those OH– ions which move, but it is illustrative.
The process is then repeated for the H2O. Out of the four molecules generated on the left, two become part of the reactants on the right, and two remain as waste.
At this point, we can highlight all the species which do not appear in equal numbers on both sides. So whilst the OH–, e– and 2H2O are part of a cycle, the 2H2, O2 and 2H2O are not.
When we isolate those species we see that we have replicated our overall equation.
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