• Emily Rose Seeber

The Mole: A Different Approach


The mole is generally thought to be a 'threshold concept' in Chemistry. Once students get it, there are all kinds of problems they can understand which were previously denied to them; but without the mole, their understanding of Chemistry will never progress past classifying substances and spotting patterns.

The problem with the mole

A threshold concept is one ‘akin to a portal, opening up a new and previously inaccessible way of thinking about something . . . it represents a transformed way of understanding, or interpreting, or viewing’ (Meyer and Land, 2003). But learning threshold concepts is hard for students; it means blowing apart their current, cosy level of understanding about a topic, and seeing it from a whole now, less cosy perspective.

And this is no less true for learning about the mole. No, not the furry creature, the number, and how it can be used in Chemistry to predict the mass of product from a reaction, et cetera. Students find learning the mole very challenging. They forget the equations, they struggle to use mole ratios, they don't know when to use each specific type of mole calculation (even though to a real chemist, all the different 'types' are actually the same).

On reflection about this problem, I felt that the traditional approach to mole calculations was all ‘bottom-up’, and the students actually needed some ‘top-down’ as well to help them build up a picture. When constantly building up from foundational ideas, i.e. this is what a mole is, here’s one way to use it, and another, and another, if one piece is out of place, the student then struggles to access the rest of the topic, and also rapidly loses confidence in their ability to do Chemistry.

A novel approach

Furthermore, upon reading about how students best learn abstract ideas, I have been struck by two key points: firstly, students need to learn when to use a particular tool as well as how to, so teachers need to interleave problems together so that students can learn this; and secondly, that when teaching abstract concepts they need to be met multiple times from different perspectives. So I completely restructured the way we teach moles calculations so that there is something in each term over the course of the IGCSE from that topic, and each time the additional new content is used in context, allowing the ‘top-down’ perspective.

The scheme roughly looks like the following:

So the aim here is to make sure that students have a really solid understanding of the aspects of mole calculations they have learned previously, which they have properly chunked by using in lots of different examples and contexts, before they learn a new technique. Students have the time to revisit and revise each aspect properly, rather than building on already shaky ground in the following lesson.

Each of the new aspects of the calculation has a distinct practical context which is taught together with the calculation technique to provide another perspective. In the table I have listed one example for each, but in reality we use many more, although some might be demonstrated, and others might be analysing the evidence from a practical write-up rather than the students doing an investigation themselves. (At least one student practical for each of the aspects above.)

How is it working in practice?

So, for example, after students learn about how to calculate an empirical formula from practical data, they then use this technique in a wide variety of different situations before they learn something different about the concept of the mole. They are expected to be able to use this technique fluently as part of their standard set of skills in Chemistry. I actually taught ionic bonding this year from the perspective of being investigative scientists. The students calculated empirical formulae of sodium chloride, magnesium chloride, and aluminium chloride and spotted the patterns themselves. We then did the same for sodium chloride, sodium oxide, and sodium nitride. The question sheet is available here.

In between these key identified points at which students are learning a new technique, previously learned moles content is woven into their other topics so that students are always revising the methods and when they should be used in a range of contexts. An example worksheet which uses relative atomic mass calculations, moles calculations and empirical formulae is available here. Students are then supposed to find moles as a standard part of their chemical toolkit, like being able to evaluate an experimental procedure, rather than a completely separate kind of thing they do for “the moles question in the exam”.

So far, the results have been pretty dramatic. Students across the ability range are confident using simple moles calculations, and are significantly more adept at selecting the right strategy. They are also far more confident, which is a real bonus as this is a topic where a lot of students just seem to ‘give up’ when faced with the traditional approach.

I suppose, at heart, this is just applying the 'spiral curriculum' to a topic which it is not usually associated with, so maybe not that new really. However, moles are usually added in as a big 'mathsy' chunk in Year 10 or 11, and students either love or hate it.

I don't think learning should be like Marmite.

#chemistry #pedagogy #cognitivescience #PCK

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© 2017 by Emily Rose Seeber.