Featured article
De
Vos, W., & Pilot, A. (2001). Acids and bases in layers: The stratal
structure of an ancient topic. Journal of Chemical Education, 78(4),
494–499
This
paper poses challenging questions for contemporary curriculum developers. At
what point does the curriculum evolve to account for new scientific
developments? How long after a new development in science should it impact upon
the school-level curriculum? Historically science curricula across the world
have evolved to include new discoveries such as DNA. Are more recent
developments appropriate and important to include or are they to be considered
as “beyond” school science?
A common concern of teachers, and their
students, is that the science curriculum is “too full”. Does this thinking
limit the addition of newer chemistry ideas? Does the methodology of constant addition
rather than restructuring limit the contemporary relevance of what students
study at school?
In
this paper the authors explore the presentation of the topic of acids and bases
in school textbooks through time. The textbook chapters are effectively a proxy
for chemistry curriculum content at the time of publication.
The
authors propose a connection between developing scientific views about the
nature of acids and bases through history with the presentation of this core
topic in school textbooks.
The
authors use the metaphor in the title of this paper “A stratal structure of an
ancient topic” to encapsulate how evolving ideas about acids and bases became
gradually added (and not superseded) in the chemistry curriculum. It is likely that
the following quote will resonate with anyone involved with curriculum
development to this day.
“Apparently
it was much easier to add something to the existing curriculum than to remove
something from it or to restructure it”.
Rather like an archaeologist digging into the layers of history, ideas about acids and alkalis can be traced through time from the earliest to the most modern layer at the time in which the paper was published.
Image by kp yamu Jayanath from Pixabay
In
each layer the authors describe a “context”. This puts into perspective the way
in which acids and alkalis were perceived.
The
authors categorise six different layers which are outline below.
Layer 1: Craft context
Acids,
bases, alkalis and neutralisation have been known about for hundreds of years.
Modern nomenclature was not used and this early knowledge related more to forms
of "recipe”. For this reason, the authors named this layer as being based
in a “craft context”.
Acids
and bases have been included in the chemistry curriculum since the early 19th
century. This “craft layer”, according to the authors, forms the bottom (first)
layer.
Layer 2: Synthesis context
The trigger
for this second layer was the work of Antoine Lavoisier. Following discovery of
the critical role of oxygen in combustion there was as shift in thinking to
consider the synthesis of acids. Lavoisier introduced a systematic naming
approach for inorganic acids by linking the name to the element from which each
acid could be formed. For example, sulfuric acid was formed from sulfur.
Lavoisier
categorised bases as deriving from metallic elements via their oxides. Acids
reacted with bases to form salts and water.
The
authors contrast the highly practical approach of the craft context with the
more theoretical approach of the “synthesis context”.
19th
century chemistry courses adopted this synthesis context through the inclusion
of a chapter on acids, bases and salts. This provided a systematic overview for
students of then contemporary inorganic chemistry.
Layer 3: Analytical context
The authors describe how the introduction of the periodic
table in the second half of the 19th century led to acids and bases
losing their central position in the theory of inorganic chemistry. However, a
new reason was found to be important to keep them in the chemistry curriculum.
Half-way through the 19th century (and later in
the chemistry curriculum) the understanding of reactions occurring in fixed
proportions resulted in the development of analytical chemistry.
Ideas such as neutralisation and end point (as well as
gram-equivalent weights and normality) became a core part of chemistry
education. Apparatus such as pipettes, burettes and conical flasks became key
to school studies of chemistry.
Layer 4: Arrhenius context
Towards the end of the 19th century, the authors
explain, physical chemistry began to emerge. The ionic theory was developed by
Arrhenius. This familiar theory stated that in aqueous solutions acids and
bases are ionised (completely if strong and partially if weak).
As a result, in the first half to the 20th
century the topic of acids, bases and salt became used as an illustration of
ionic and equilibrium theories.
A new definition of an acid emerged as a “hydrogen
containing substance that in aqueous solution produces hydrogen ions in
solution”. The new definition of a base was a substance containing a hydroxyl
group that in aqueous solution produces hydroxide ions.
New concepts were introduced to the curriculum such as Ka,
pH, pKa, pKb and pKw alongside calculations
involving logarithms.
Layer 5: Brønsted context
The authors decided to make this as a separate layer due to
the Brønsted-Lowry and Lewis theories referring to protons and electrons
(atomic structure).
The authors suggest that the shift from the Arrhenius
context to this context was substantial.
In this context, acids and bases are no longer described in
terms of substances. For example, sodium hydroxide is no longer the base. The
base is the hydroxide ion because it can accept a proton.
The authors take this further to explain that a
neutralisation reaction is now defined in terms of its mechanism (transfer of a
proton from the acid to the base).
Layer 6: Application context
This layer was contemporaneous with the writing of the
paper. The authors describe how in recent years there have been no new
theoretical layers added to chapters in chemistry textbooks. They suggest that
this is because acids and bases are no longer a key area of modern chemical
research.
The new layer, the authors suggest, arose from an
educational viewpoint of the importance of the making clear the relevance of
the acids and bases topic to students and society.
At this point textbooks started to make connections to the neutralisation
of acid in the stomach or the environmental issue of acid rain. Analytical
chemistry became set in more relevant topics such as analysis of products from
a supermarket.
Implications for teaching and the curriculum
As a result of their detailed analysis and categorisation
the authors’ discussion revolves around the problem that these different
contexts are not always clearly distinguished in school textbooks. Even more
problematically the authors give examples of where mixed contexts are used.
A very simple example of this is the use of the word “acid”
as a noun. It can have three different meanings depending upon the context. In
the craft context an acidic solution IS the acid. A bottle contains
“hydrochloric acid”.
In the synthesis, analytical and Arrhenius contexts the
substance in the bottle is a solution of an acid. Finally, in the Brønsted
context an acid is defined in terms of particles so “hydrochloric acid” is said
to contain the acid hydronium ion.
It is clear to see that unless these differences are made
clear in teaching there is a lot of scope for confusion.
The authors conclude with the following remark.
“The stratal structure is not a result of a well thought out
pedagogical strategy. Instead, it is a product of a historic process, and accumulation
of successive, separate decisions.”
The authors caution that this stratal structuring is not
restricted to the chapter on acids and bases and suggest that it could be a
reason why so many people claim to have “never understood anything of
chemistry”.
This issues a challenge to modern curriculum development. At
what point in educational history will anyone have the courage to tackle the traditional
and worldwide fundamental chemistry curriculum to ensure a properly planned
progression for students of the future.
Reflective questions
What does the word acid mean when you are teaching students
aged 11?
At what point in the curriculum does the context and hence
meaning of an acid change?
Is a mixed meaning of the word acid ever used and how could
this be avoided or be made more explicit?
What could curriculum developers do to support a more
coherent approach to the teaching of acids and bases in school?
