Asia-Pacific Forum on Science Learning and Teaching, Volume 11, Issue 1, Foreword (Jun., 2010)
John K. GILBERT

The role of visual representations in the learning and teaching of science: An introduction
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The scope of the symbolic mode of representation

Although the ‘symbolic’ is undoubtedly one of the major modes of representation, it is not clear whether or not symbols should be classified as visual or non-stimuli. For the sake of completeness of coverage, if for no other strong reason, they are included here and treated as if they were visual stimuli.

Mathematical representation of all forms is used widely, becoming increasingly important in science as the sophistication level of the models employed rises e.g. algebraic equations, sets, calculus. This is a separate and extensive branch of knowledge and cannot be addressed here.

However, chemistry has evolved a set of symbols that are widely used across the other sciences. The chemical elements are given symbolic labels, some self-evident e.g. ‘H’ for Hydrogen, some very evidently derived from Latin e.g. ‘Pb’ for Lead (after ‘Plumbum’). Symbols are given to the particular units of quantity, for example ‘mol.’, and of concentration , for example ‘mol.dm-3’. For students of chemistry, the most demanding system of symbolic representations is the ‘chemical equation’, where a number of interlocking conventions apply (Taber 2009). The challenge arises from the many sub-forms that are in use in textbooks. Students are often initially taught chemical equations in a way that is derived from speech e.g.

Sodium hydroxide + hydrochloric acid → sodium chloride + water

However, in the standard IUPAC convention, this reaction should be:

OH‾(aq) + H+(aq) → H2O(l)

which represents a major intellectual leap from the spoken version. Even with the ‘spectator’ ions left in, this example might be given as:

Na+(aq) + OH‾(aq) + H+(aq) + Cl‾(aq) →Na+(aq) + Cl‾(aq) + H2O(l)

‘Molecular equations’ (which must lead to misconceptions at the submicro level) are also given:

NaOH(aq) + HCl(aq) → NaCl(aq) +  H2O(l)

frequently even without the ‘state’ symbols:

NaOH + HCl →NaCl + H2O

There seems to be an ‘educational consensus’ that ‘reversibility’ symbols are omitted, except in cases where the reaction has a finite equilibrium constant. Even here, the convenient conventions of word processors have lead to a change in the reversibility symbol:

N2(g) + 3H2(g) ↔ 2NH3(g)

The indication of precipitation adds a complication in relevant cases

Ag+(aq) + Cl‾(aq) →AgCl(s)

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)    ΔH = - 890 kJmol‾1

as does  the notion of ‘electrode potential’ in half-cell equations

MnO4(aq) + 8H+(aq)  + 5e‾  → Mn2+(aq) + 4H2O   Eθ = +1.52 V

Whilst a complete IUPAC convention set represents a clear code of representation, frequent experience of partial – or even incorrect – systems must cloud students’ learning.