…is a chapter in one of my books:
Taber, K. S. (2019). The Nature of the Chemical Concept: Constructing chemical knowledge in teaching and learning. Cambridge: Royal Society of Chemistry.
Contents:
- 5.1 Chemistry as a Natural and Empirical Science
- 5.1.1 The Nature of the Natural
- 5.1.2 The Wider Context of Scientific Discovery
- 5.2 Chemistry as an Empirical Science Depends on Imagination as well as Benchwork
- 5.3 Thinking about Chemical 'Discoveries'
- 5.3.1 Conceptualisation Is Shaped by the Cognitive Apparatus
- 5.3.2 Constructivism
- 5.4 The Discovery of Potassium: Imagining a New Substance
- 5.4.1 The Creation of Potassium
- 5.4.2 The Construction and Development of the {Potassium} Concept
- 5.4.3 The Shifting {Potassium} Concept
- 5.4.4 Potassium sans Isotopes
- 5.4.5 Developing the {Metal} Concept
- References
The natural
"It is not uncommon for a distinction to be presumed in everyday discourse between what is natural and what is man-made: yet those things made by people working in science are certainly within the natural realm, rather than the result of some magical or supernatural activity…The term 'chemicals' may imply, for some, artificial, manufactured substances. So, for example, some people consider that they will be healthier if they eat foods that do not contain any chemicals. The cynical chemist might respond that they would not be healthy, but simply starve – as all foods are comprised of chemical substances."
Taber, 2019, pp.69-70.
Discovering the synthetic
Can we 'discover' new substances that did not exist before we made them?
"It is likely that some compounds that have been synthesised by chemists never existed in the universe before they were prepared in a laboratory. If so, these compounds were not discovered in nature so much as discovered in the imagination (as a possibility) and only then realised, constructed, in vitro. The possibility is 'discovered' in the mind of a chemist, and then the compound as a natural substance is in effect the confirmation of a hypothesis. Some discoveries may be serendipitous (mauveine comes to mind), but this description still applies: the chemist imagines what the unexpected product could be, and then carries out analytical procedures to test the idea."
Taber, 2019, pp.71-72.
The discovery of potassium
"Potassium was first isolated as an element in 1807 when Humphry Davy electrolysed potash (potassium hydroxide)" (p.75), so, Sir Humphry Davy discovered potassium. Yet of course the concept he formed at the point of making that discovery was not the same as potassium concept as best understood today. The chapter explores this idea that the concept of potassium has changed over time – blurring the notion that potassium (as understand today) was discovered at one point in time.
"Davy suspected he could isolate elements from potash, but he was not looking for potassium as such. There was no concept {potassium} that would allow him to be seeking a substance that would be so conceptualised. Indeed, he is reported to have suspected that potash might contain phosphorus, or sulphur 'united to' nitrogen (Siegfried, 1963, p. 249). So, Davy was successful in isolating an element from potash, but it was a new element he had not anticipated. Davy also isolated sodium, another new metallic element. However, categorising potassium and sodium as metals, something taken for granted by chemists today, was not as straightforward given the state of chemical knowledge of the time"
Taber, 2019, p.76.
We might even say that potassium was not around to be discovered, and in a sense it was invented?
"it is probably fair to say not only that (the concept) {potassium} did not exist on earth prior to Davy's work, but also that (the metal) potassium did not exist on earth prior to Davy's work. …Given the high reactivity of potassium, it seems very unlikely any of the pure metal had ever existed 'naturally' on earth (or indeed anywhere else in the universe), in the sense that gold, silver and copper may be found 'native'. Potassium, like all the heavy elements, is considered to be have been formed in supernovae – where the conditions are hardly suitable for pure metal to condense…
So, in a sense Davy did not so much discover (the elemental substance) potassium as create it. This raises the questions of whether we should consider the element potassium actually existed before the first sample of the elemental substance potassium was created."
Taber, 2019, p.77.
This reflects two sometimes confused ways of using the term element – as the elemental substance, or as an element being in a sense present otherwise (e.g. is the element oxygen present in a sample of pure water?)
Potassiums?
Davy's (concept of) potassium was not understood as being a 'mixture; of isotopic forms,
"…Aston showed in 1921 that the … atomic mass of c.40 for potassium, neatly approximating a whole number, appeared to be due to a mixture of (at least) two isotopes – the spectrum suggested these were K39 and K41. Further work suggested that neither of these appeared to be the source of the radioactivity of potassium. It was found that potassium also contained the isotope K40 at "about 1 part in 8300" (Harkins and Liggett, 1923, p. 178) and it was reported that the K40 isotope "is responsible for the entire known activity of potassium'"
Taber, 2019, p.79.
It is not being suggested that Davy be denied the claim to discovering potassium because he was not in a position to characterise its isotopic composition – the point is rather that scientific concepts often develop over extended periods of time (when we sometimes expect learners to acquire authentic modern versions of them almost instantly).
"The {potassium} concept of contemporary chemistry evolved, with modifications, from the concept {potassium} deriving from Davy's own investigations."
Taber, 2019, p.81.
As another example, new technology and techniques, not available to Davy, allowed the determination of crystal structures
"the evidence suggested that potassium had a body-centred cubic lattice (a structure inherently less compact than the face-centred cubic or hexagonal close-packed structures found in many metal crystals). So, again, something new was learnt about potassium, so that the chemical concept {potassium} was again modified"
Taber, 2019, p.82.
Changing metals
The chapter also consider how a pre-existing concept, of metals was challenged when potassium and sodium were discovered. As these low density, low melting, highly reactive materials were not typical of the substance then recognised as metals it was possible that the metal concept could have been applied in making a discrimination that these new substance were something other than metals (as the category metals does not exist in nature and force it self upon us, but is rather one the ways chemists decide it is useful to group things. (Indeed, engineers or materials scientists may have a rather different 'metals' concept!)
So, Davy and other scientists had to decide if there was sufficient analogy between these new substances and the existing examples of metals to admit sodium and potassium as metals – and so somewhat modify the concept metals (not just by add new examples, but in terms of the range of properties metals could have),
"The implication here is that 'metals' was a class of bodies with certain common characteristics. Analogy is a common technique used by scientists, that involves looking for structural similarities in different systems…. If the properties of these new substances were mapped onto those typical of those substances already admitted into the category … then the potential analogues (potassium, sodium) failed to map onto the target in terms of one important property, i.e. density, but mapped well onto the target in terms of other properties considered relevant, so potassium and sodium 'should' – on balance – be considered metals by analogy with the other substances considered as metals.
…the identification of these relatively low-density substances as metals modified the chemical concept {metals} somewhat … by admitting two more elements that were less prototypically metals.
Taber, 2019, pp.84-85
This book is part of the RSC Advances in Chemistry Education series.
