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The three types of knowledge in chemistry
Department of Psychology,
University of Glasgow.
Alex Johnstone (see references below) pointed out that in Chemistry, students
must learn in three different representations at once, and how to inter-relate
each new concept or fact in all three domains:
- Macroscopic: descriptive and functional
(e.g. how chemical phenomena appear to the senses, colour, smell,
- Formal or representational (the equations used to represent reactions).
- Molecular, "submicro", explanatory:
the invisible but 3-dimensional world of molecules' shapes and their dynamic
motions, interactions, and kinetics.
There are several points about this.
- The third of the above is generally the hardest for students, and least
well dealt with in teaching. It requires special attention educationally.
It is a distinctive feature, and problem, of chemistry.
- However it is not necessary to teach all three domains to everyone,
even though the combination and their inter-relationships are the essence of
chemistry to experts. There are several reasons for this.
- The obvious, and probably necessary, starting point for learners is
connections to aspects of the world they already know . You shouldn't
teach the others without the macro; but you might teach the macro without the
- The earliest, and so in important ways the most original, chemists, of
course started here. So the macro may not be the latest knowledge, but it
must be real science nevertheless. Thus it is foundational: not in the
logico-deductive sense of a set of axioms from which other things can (now with
hindsight) be derived, but in both the historical and more importantly the
educational sense that the formal and microscopic are value-less (to humans)
without the macro, but the reverse is not true.
- You can teach and practise the scientific method solely in that domain:
isolating variables, doing experiments.
- If you then want to teach some systematic theory, then  is the next
step. This does NOT necessarily have to be grounded in the micro.
Thermodynamics, famously, can be and has been grounded axiomatically without
any reference to the micro.
- What is good for, and what delights, experts, is NOT necessarily what is
good for, and delights, beginners.
(It is natural to be "learner-centered" in the simplistic sense of treating
learners as you would wish to be treated now, but in fact this is not always
best for them. More attention to and respect for what a learner knows, and
the way that puts them in a different intellectual place than you are in now,
leads to a different response.)
- Every subject (as far as I can see) requires relating two domains,
comparable to  and  above: macro and formal. This appears, for
instance, in the
as the relationship between the public/abstract and personal/perceptual
aspects of a subject. Johnstone's insight into the nature of learning
chemistry is a constraint on how general that is. It seems some subjects
require more than two aspects to be addressed and inter-related, at least at
the HE level of the subject.
A.H. Johnstone (1982)
"Macro- and microchemistry"
School Science Review vol.64 pp.377-379
A.H. Johnstone (1991)
"Why is Science Difficult to Learn? Things are Seldom What They Seem"
Journal of Computer Assisted Learning vol.7, 75-83.
A.H. Johnstone (1993)
"The development of chemistry teaching"
Journal of Chemical Education vol.70 no.9 pp.701-705
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