CHAPTER 5 RESULTS SHOWING SCOPE AND QUALITY OF RESEARCH ON
6.3 DIFFICULTIES WITH GENERAL DEFINITIONS
This section concerns student difficulties with distinguishing definitions of acid and base, and with other definitions in the topic.
6.3.1 Difficulty S4: Acid and base definitions are not distinguished
Some students, even in senior secondary classes, interchange the definitions for acids and bases, as reported in numerous studies below (see Table 6.4).
Table 6.4 Summary of research on interchanged acid-base definitions
Reported student conception Educational level of students Incidence Author(s)
Acid-base definitions interchanged Senior secondary 7% Linke & Venz (1979) Same definition given for both acid &
base
Senior secondary 4% Linke & Venz (1979)
Acid-base definitions interchanged Senior secondary Not available Vidyapati &
Seetharamappa (1995) OH- ions are found in acids Senior secondary high
achiever
Not applicable
Ross & Munby (1991) Acids can be alkaline or neutral Junior secondary Not available Toplis (1996)
Acid is an acceptor of hydrogen ions. Senior secondary 10% Ouertatani et al. (2007) Acid is a donor of hydroxide ions. Senior secondary 10% Ouertatani et al. (2007) Base is an acceptor of hydroxide ions. Senior secondary 20% Ouertatani et al. (2007)
The relatively small prevalences (4% and 7%) reported by Linke and Venz (1979)above could suggest that these might simply be mistakes, which are easily corrected, rather than genuine conceptual difficulties (Abimbola, 1988). However the higher incidences reported by Ouertatani et al. (2007), particularly with the definition of a base, indicates otherwise. So this is evidently not a trivial difficulty, and it needs further investigation. Towards this end, questions such as the follow need addressing: “What links do students need in order to conceptualize these definitions?”; “Why are they unable to form links between the definitions and other knowledge?”; and, “What aspect of the definitions are students confusing – the hydrogen and hydroxide ions, or the words acceptor and donor, or perhaps superimposing the acceptor / donor aspects of the Brønsted model onto the Arrhenius model?” There could even be confusion with the Lewis model if students have heard of acids as electron pair acceptors and bases as electron pair donors. The description of the difficulty arising from the author’s descriptions is still exceptionally vague, not indicating its essence at all. As a result I can only classify it is as Emergent – Level 2 – despite its having been reported in five educational contexts. Further research should probe which conceptual links are missing for these students but, in the interim, propositional knowledge should include at least the definitions for both acid and base according to both theoretical models, as follow:
• Arrhenius acids are substances that release hydrogen ions in aqueous solution. (2.2.1)
• Arrhenius bases are substances that release hydroxide ions in aqueous solution. (3.2.1)
• Brønsted model: acids are molecules or ions that can release a proton (hydrogen ion).
(2.3.1.1)
• Brønsted model: bases are molecules or ions that can accept a proton (hydrogen ion).
(3.3.1.1)
The student conception given by Toplis (1996) requires mapping to further propositional statements (3.1 now modified to include alkalis) that concern operational knowledge of both neutral and alkaline solutions.
• Neutral substances are neither acidic nor basic. (5.1)
• Basic substances (or alkalis) give basic (or alkaline) solutions. (3.1)
6.3.2 Difficulty S5: Alkali is another word for base
Two research studies show a conception indicating that students transfer a concept from the Arrhenius model inappropriately onto the Brønsted model. A student interviewed in Schmidt and Volke’s (2003) study responded: “Water as an alkali is difficult to conceive” and Toplis (1998) reported similarly. The difficulty description given above arises directly from this data.
The term alkali applies in the chemistry context of substances and so has no place in the Brønsted model (see Section 3.3.3). Consequently, this difficulty maps in one step to the following propositional statements, which go beyond merely defining a base according to the two theoretical models, in an attempt to show the boundaries between two conceptions of bases.
• Arrhenius model: bases are substances that release hydroxide ions in aqueous solution.
(3.2.1)
• Alkali is an alternative term for Arrhenius bases. (3.2.1.1)
• Arrhenius bases do not include Brønsted bases (3.3.2.2) such as water. (3.2.2.2.1)
• Brønsted bases are molecules or ions that can accept a proton (hydrogen ion)(3.3.1.1)
• Brønsted bases include the molecules H2O, NH3 and, ions OH-, HCOO–, CH3COO–, CN–, and S2– (3.3.2.1)
• Brønsted bases do not include Arrhenius bases (3.3.2.2)
Few research details are given by Toplis and Schmidt and Volke did not report pursuing the difficulty beyond interviews with a few students. Consequently the difficulty, alkali is another word for base can be classified only as Level 2, or Emergent. Other authors have not built on this work and so further research is needed to verify that the description is stable across other
contexts. Furthermore, the research reported on this difficulty does not indicate whether the teachers concerned used mixed models and so caused the difficulty or whether the students themselves were unable to differentiate two models they had been taught. Accordingly, while this propositional knowledge may not be sufficient to address the difficulty, it represents a minimum of scientifically correct propositions that are necessary.
6.3.3 Difficulty S6: Amphoteric species are neither acid nor base.
The student difficulty with amphoteric species goes beyond merely not knowing the concept label or definition (Bradley & Mosimege, 1998) or not recognising aluminium or zinc hydroxides as possible proton donors (Furió-Más et al., 2007). Kousathana et al. (2005) showed that two multiple-choice items with small differences elicited different student responses. The first question asked which species could not act as an amphiprotic (that is amphoteric) substance and the second asked which species could not act as both an acid and a base in the Brønsted model. In both cases the answer (HCOO–) and the distractors (H2O, HCO3
–
and HS–) were the same. Although for both items students seemed to prefer to give no answer rather than choose any of the options, performance was much better with the term amphiprotic, 70% against 49%. The authors speculate that students had created a new class of substances, so that substances are classified as acids, bases or amphoteric substances; in other words the three are mutually exclusive. This conception can be mapped to the propositional knowledge given in the IUPAC ‘Gold book’ (McNaught & Wilkinson, 1997), namely:
• Amphoteric species are those that can behave both as an acid and a base. (4.1)
• Amphoteric properties depend upon the context in which the species is investigated. (4.1.1)
The evidence of student difficulties from Kousathana et al. (2005) together with the corresponding propositional knowledge suggests that students do not understand another critical aspect of the Brønsted model; specifically, that acids and bases are so classified in relative rather than absolute terms, according to the context of the reaction (see Section 3.3.3). In response, the Brønsted definitions already given as propositional statements for Difficulties S2.1, S2.2, S4 and S5 were modified to emphasise this aspect, and emphasising that there must be a suitable acceptor for, or donor of the proton present. Furthermore, bearing in mind the students’ mutually exclusive conception it was judged more appropriate to give lists of examples of Brønsted acids and bases, which included some items common to both, rather than a separate list of amphoteric species which might be seen as separate from acids and bases.
Consequently, two types of propositional knowledge were involved in this difficulty; firstly explicit definitions of acid and bases and secondly lists of examples for acids and bases
expanded so as to include the examples introduced in the research on this difficulty.
Accordingly, the difficulty mapped to the following propositional statements.
• Examples of amphoteric substances include Al(OH)3 and Zn(OH)2 (4.2.1)
• In aqueous solution, amphoteric hydroxides can form either hydrogen or hydroxide ions.
(4.1.2)
• Molecules or ions are classified as Brønsted acids when they release a proton (hydrogen ion) to a base. (2.3.1.1)
• Molecules or ions are classified as Brønsted bases when they accept a proton (hydrogen ion) from an acid. (3.3.1.1)
• Al(OH)3 and Zn(OH)2 may act as acids in certain reactions. (2.3.2.2)
• Arrhenius bases: examples include NaOH, Al(OH)3 and Zn(OH)2 (3.2.2.1.1)
• Brønsted acids: examples include the molecule H2O and ions: NH4
+, HCO3
– and HS– (2.3.2.2)
• Brønsted bases: examples include the molecule H2O and ions: OH–, CH3COO–, HCOO–, CN–, S2–, HCO3
–, HS– (3.3.2.1)
Reversing these propositional statements suggests the following description of the difficulty:
Species can be classified as acids or bases or amphoteric. However, when mapped back to the difficulty data, it was clear that this description did not show the mutually exclusive nature of the conception identified by Kousathana et al. (2005). Accordingly, the description was further modified to: Amphoteric species are neither acid nor base. The classification is at Level 3 because it has only been studied in only a limited way, one educational context. Further research should use open-ended methods to verify whether students do see these three categories as mutually exclusive.
As a teaching exercise, it would be useful for students to fill in examples of acids and bases onto a diagram such as in Figure 6.1 given below. Some species may only be able to act as acids (e.g. HCl or NH4
+), some might only be able to act as bases (e.g. CO3
2– and HCOO–), while others could fall into the common classification and be termed amphoteric.
Figure 6.1 The relationship between species classified as acid, base or amphoteric
acids amphoteric bases
species