Students love a good demo. For many, the most memorable part of their Chemistry lessons will be the biggest explosion, the densest smoke, the brightest colour changes and the most fruit-some stinks.

I recently read an article in the Royal Society of Chemistry’s Education in Chemistry magazine that triggered a bit of a realisation (not only about how I teach chemistry but also about how I might approach other areas of science-or even other subjects entirely should the need arise).

Much of chemistry – and certainly most of the interesting bits – happens at the visible, macroscopic level. However, the explanations for these bangs and pongs usually occur at an unseen or abstract level. Furthermore, the description of these unseen interactions is presented using a daunting language of symbols and equations. Mastering chemistry requires mastering all three:

  • Observable: “Wow, it’s gone blue!”
  • Atomic/Molecular: “The particles of the red compound have reacted with the particles of the green compound to make a new compound that is blue.”
  • Symbolic: red solution + green solution → blue solution

Juggling these three levels is something that I’ve often thought about but never tried to articulate. It was with some embarrassment that I therefore recently learned that this had been identified by Alex Johnstone, a globally recognised expert in chemistry education, nearly 30 years ago.

Johnstone had suggested that the ideal curriculum would enable chemists to “view our subject on at least three levels” and “jump freely from level to level in a series of mental gymnastics” (Johnstone, 1982). In later publications, Johnstone represented the idea as a triangle, sometimes referred to as the “chemistry triplet” but more commonly known as Johnstone’s triangle (Talanquer, 2011).

A template for Johnstone’s triangle, taken from the Royal Society of Chemistry (Hofgartner, 2019),  is shown below:

Using the triangle, students are able to slowly build their understanding of each of the three levels separately before being guided in how to make links. Additionally, expert learners might be provided with a partially-filled triangle and asked to use their understanding of the underlying concept to fill in the blanks.

Keith Taber from the University of Cambridge has written extensively on the significance of triangulation of understanding and its implications on teaching in chemistry:

…Ventures into the triangle should be about relating previously taught material, and should be modelled carefully by the teacher before students are asked to lead expeditions there; and such explorations should initially be undertaken with carefully structured support. The aim, after all is not to avoid students being in the triangle, but to make sure that whilst there they can appreciate what teachers like Johnstone already recognise as ‘a beautiful, integrated view of chemistry’

(Taber, 2013).

The resurgence of interest in Johnstone’s Triangle is undoubtedly linked to developments in understanding of cognitive science. The linking of observable to unobservable phenomena imposes a heavy cognitive load. “Expert” chemists will move fluidly (pun intended) between macroscopic, microscopic and symbolic representations, while novice learners may struggle to hold all three components long enough to establish links (Johnstone, 2000).

Taber and others have described how novice chemists need to be carefully guided in populating the triangle in a clear sequence, starting from a concrete observation, through a molecular explanation, finally arriving at a symbolic representation. As ever, model examples are extremely useful.

I’m undoubtedly very late to the party but it occurs to me that perhaps these challenges are less specific to chemistry than I had previously assumed? A similar model may be useful in any number of other heavily loaded subjects where observable outcomes need to be linked to underlying theories and representations. The building of the Berlin wall will have an observable component (the wall is built), a symbolic representation (division between East and West) and a “microscopic”/unseen explanation (the myriad interactions and events that precipitate the macroscopic).

Without knowing very much about any of the following, I can think of a few scenarios that might usefully be split into and taught as triplets:

  • In English: Mercutio’s death. The lines of dialogue (macroscopic); the thoughts and feelings of the individuals (microscopic); the characters and their allegiances (symbolic).
  • In Geography: Climate change. The observable phenomena (macroscopic); the individual contributing actions (microscopic); the figures, statistics and quantification (symbolic).
  • In DT: Building a table. The overall function and shape of the object (macrospic); the dimensions and design of components (microscopic); the drawings and plans (symbolic).
  • In PE: Performance in netball: The team performance (macroscopic); the performance of individual players (microscopic); the drills and exercises completed in training (symbolic).

Problems with using this model for teaching chemistry (and, I would imagine, any other subject) arise when concepts can’t be strictly compartmentalised into the three domains. However, as an introduction to a subject, or as a means of scaffolding complicated relationships for novice learners, I think it might be very useful.

There are many examples of how this type of thinking about topics in chemistry can support other techniques such as flipped learning; a student might be asked to complete one of the apices of the triangle before a lesson and then be guided in populating the other two, and building the links between them, during the lesson. Already, there are nice examples emerging of this flipped learning and how it proved useful during lock-down (Petillion & McNeil, 2020).

Hofgartner, R. (2019, June 5). Develop deeper understanding with models. Education in Chemistry, Sep 2019.

Johnstone, A. H. (1982). Macro and microchemistry. The School Science Review, 64, 377–379.

Johnstone, A. H. (2000). Teaching of chemistry – Logical or psychological? Chemical Education Research and Practice, 1(1), 9–15.

Petillion, R. J., & McNeil, W. S. (2020). Johnstone’s Triangle as a Pedagogical Framework for Flipped-Class Instructional Videos in Introductory Chemistry. Journal of Chemical Education, 97(6), 1536–1542.

Taber, K. S. (2013). Revisiting the chemistry triplet: drawing upon the nature of chemical knowledge and the psychology of learning to inform chemistry education. Chemistry Education Research and Practice, 14(2), 156–168.

Talanquer, V. (2011). Macro, Submicro, and Symbolic: The many faces of the chemistry “triplet.” International Journal of Science Education, 33(2), 179–195.

By Nick Gower

Head of Chemistry

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