Major Sources of Difficulty in Students' Understanding of Basic

Research: Science and Education edited by

Chemical Education Research

Diane M. Bunce The Catholic University of America Washington, D.C. 20064

Major Sources of Difficulty in Students’ Understanding of Basic Inorganic Qualitative Analysis Kim Chwee Daniel Tan,* Ngoh Khang Goh, and Lian Sai Chia National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616 Republic of Singapore; [email protected] David F. Treagust National Key Centre for School Science and Mathematics, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845 Australia

Most students’ introduction to qualitative analysis (QA) is in general chemistry courses at college or university in the United States of America, or as part of analytical chemistry courses for chemistry majors in Australian universities. However, in Singapore, students’ first experience with QA is in grade 10 even though the topic is considered conceptually difficult to understand (1) and there is inadequate time for comprehensive experimental work. In Singapore, high achieving grade 10 students take the national General Certificate of Education Ordinary Level (O-level) chemistry examination that includes a practical examination; inorganic chemistry QA is usually one of the two experiments that students have to perform in the practical examination. Students perform a series of procedures using chemicals, apparatus, and appropriate techniques in QA experiments, observe and record what happens, and make inferences based on their observations. The students find QA a difficult topic possibly because it requires manipulative, observational, recording, and inferential skills, as well as content knowledge (1) that they have studied in topical areas such as acids, bases and salts; oxidation and reduction; reactivity of metals; and periodicity to make sense of the experimental procedures and results.

List 1. Examples of Propositional Knowledge Statements 1. An alkali will react with an ammonium salt to produce a salt, ammonia, and water. 2. An amphoteric oxide/hydroxide is an oxide/hydroxide of a metal that will react with either an acid or an alkali to produce a salt and water. 3. Aqueous ammonia will react with zinc hydroxide, copper(II) hydroxide and silver chloride to produce the respective soluble complex ammines. 4. A precipitation–double decomposition reaction is a chemical reaction that involves the exchange of ions when two or more aqueous solutions of ionic compounds are added together, and results in the formation of a sparingly soluble ionic compound (which precipitates out of the solution). 5. The solubility of a salt in water determines whether it forms a precipitate during double decomposition reactions.

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Students’ difficulties in QA at this level of schooling have been of concern for some time in Singapore, which led Goh, Toh, and Chia (2) to examine how students can be adequately equipped with essential skills in QA, and how these skills can be correctly assessed during practical work. After developing and implementing a schema, the Modified Laboratory Instruction (MLI), for improving the mastery of skills, Goh et al. found that students who had been taught the MLI schema generally did better in a practical test as well as a written alternative-to-practical-test compared to students taught using traditional methods. The MLI students also showed more positive attitudes toward laboratory work as well as greater confidence in their own manipulation of laboratory apparatus. Similar results were obtained by Tsoi (3) who extended the study of MLI to include computer-assisted instruction. Despite the concern of chemistry educators in Singapore to improve the learning opportunities in QA, no study has examined the sources of students’ difficulty in understanding the concepts and propositions involved in QA. The present study helps to address the situation by determining students’ conceptions of QA through interviews. Method and Procedures

Knowledge Base A sound starting point in a study of student conceptions in chemistry is the clarification of the knowledge base of the topic to be studied (4). Thus, a list of propositional knowledge statements (examples are given in List 1) and a concept map (Figure 1) were constructed based on the O-level syllabus, a chemistry textbook, two QA handbooks, and the teaching experience of the first author. The terms “proposition” and “propositional” are used to emphasize the semantic links between two or more concepts to illustrate a specific regularity; concept meanings are developed primarily in the way they are embedded in frameworks of propositions (5). Three tertiary chemistry academics checked the accuracy of the concept map and list of propositional knowledge statements, while four senior secondary chemistry teachers ensured that the contents were appropriate to, and fully met the requirements of, secondary level QA. As can be seen, there is a considerable amount of knowledge, much of it interconnected, that is required for students to develop a conceptual understanding of QA. Tan et al. (1) found that although much of the background chemistry needed for QA is taught in the

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I

ⴚ

KI

Cu

2+

O2

2ⴚ

SO3

H2

2ⴚ

identifies

Cu

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Fe

3+

Ca

reacts further, indicating

Al , Zn

3+

e.g. 2+

amphoteric hydroxide

2+

identifies

2ⴚ SO3

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NaOH (aq)

using

cation

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Insoluble hydroxide

acid

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alkali

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CO2 SO2 NO2

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Cl 2

detects

e.g.

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thermal decomposition

such as

Figure 1. Concept map for qualitative analysis in inorganic chemistry.

SO2

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identifies

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Devada's alloy Zn/Al

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reducing agents

using

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react with

e.g.

acidified KMnO 4, K2Cr2O7

reactive metals

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oxidizing agents

using

redox

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chemical reactions

involves

Cu

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I

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physical properties

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solutions prepared using

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Complex ammine

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double decomposition (precipitation)

Qualitative analysis

Research: Science and Education


classroom, many students were not able to make the links between their classroom knowledge and what is needed to successfully understand QA practical work. As a consequence, students adopted a cookbook approach towards QA practical work, which in many instances was encouraged by their teachers so that students could pass the national examination.

Subjects To assess the major sources of students’ difficulties in QA, 51 grade 10 students (15 to 17 years old) from three secondary schools were interviewed. These students had already studied the concepts involved in QA, and had done at least six hours of QA practical work. The students were interviewed in pairs or groups of three, giving them opportunities to talk to each other, to develop, challenge, and clarify ideas (6). This interview method was especially important in this study because the students previously might not have conceptually considered the procedures and reactions in QA. Hence in the interviews, they were forced to start formulating and organizing their thoughts to answer the interview questions, and peer discussions helped in the process. The authors realized that if the students had never thought reflectively about the procedures and reactions, they might invent answers, for example, describing the first thing that came to their head, something that they might not really believe in (7), perhaps making the responses unreliable or invalid. The Interviews There were four sets of interview questions (an example is given in Figure 2), out of which one set would be chosen for each interview. Each set of interview questions covered only a subset of the reactions and procedures in QA, as it was not possible to cover all the different procedures and reactions in one sitting. Time constraints permitted each group of students to be interviewed on only one of the four sets of questions. Each question set had two experiments (Parts A and B) that formed the basis of the interview questions. These experiments were familiar to students, as they were the usual ones performed during practical work and had appeared in past years’ examination papers. To start this part of the interview, students were given a sheet of paper with just the description of the procedures. They were then asked questions such as what they thought the reagents were for, what reactions could possibly occur, what results they would expect, and whether they would carry out any test for gases. Following this, the procedures (except those involving heating, as the interviews were carried out in classrooms) would be carried out either by the interviewer or interviewees, and the interviewees would be allowed to examine the results. They were also given another sheet of paper, this time with both the procedures and the results described, and asked to explain the results. The first author carried out a total of 23 interviews, with each interview lasting between 45 minutes and one hour. The interviews were semi-structured in that although there was a list of questions to ask in each interview, the interviewer could deviate from the list to pursue any interesting comments or alternative conceptions given by the students—this allowed a greater insight into students’ understanding or alternative conceptions of QA. As a result, due to constraints of time, it was possible that not all the questions in each set were diswww.JCE.DivCHED.org

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cussed in the interviews, but what was lost in breadth was compensated by what was gained in depth. The interviews were transcribed verbatim and compared with the concept map (Figure 1) and propositional knowledge statements (List 1) to determine any alternative conceptions on QA. Results and Discussion The data from the interviews provide insights into the major sources of students’ difficulties in understanding QA, namely: formation of precipitates, formation of complex salts, and addition of acid. A summary of sources of difficulties, alternative conceptions, and examples are given in Table 1. In this section, we provide evidence from interviews that support the alternative conceptions identified and also provide

Table 1. Sources of Students’ Difficulties in Understanding Basic Inorganic Concepts Sources of Conceptual Difficulty

Alternative Conceptions

Formation of precipitates

Examples

Precipitation is caused by a more-reactive ion displacing a lessreactive ion.

When aqueous sodium hydroxide is added to an unknown solution, a precipitate is formed because the morereactive sodium ion displaces the lessreactive, unknown cation.

Ammonia is included in the reactivity series.

When aqueous ammonia is added to an unknown solution, a precipitate is formed because the morereactive, unknown cation displaces the lessreactive ammonium ion.

Formation of complex salts

Precipitate dissolves in excess alkali.

When excess aqueous sodium hydroxide is added to a zinc hydroxide precipitate, the precipitate dissolves in the excess alkali added. The fact that the precipitate first appeared when aqueous sodium hydroxide is added is not taken into account.

Addition of acid

Neutralization involves redox reactions.

Iron(III) hydroxide is oxidized by nitric(V) acid.

Order of addition of acid is important.

Carbonates cannot be determined by the addition of aqueous barium nitrate(V) to the unknown solution, followed by dilute nitric(V) acid; the acid has to be added directly to the unknown solution.

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comments from the extant literature that might explain these findings. The discussion admits that other explanations may be possible.

Formation of Precipitates Many cations and anions are identified in O-level QA through the formation of precipitates in reactions involving the exchange of ions. For example, certain cations are identified by the color of the insoluble hydroxide formed when aqueous ammonia or sodium hydroxide is added to an

unknown solution, and whether the precipitate reacts further with excess alkali. Anions such as chloride and iodide are identified through the use of aqueous silver nitrate(V) or lead(II) nitrate(V), while sulfate(VI) is identified by reaction with aqueous barium salts. Many students had difficulty explaining the reactions that occurred in the identification of cations. • Alternative conception: Precipitation is caused by a more-reactive ion displacing a less-reactive ion

Part A Test Number

Procedure Descriptions

Observations

a

To a portion of R add aqueous sodium hydroxide until a change is seen.

b

Add excess of aqueous sodium hydroxide to the mixture from (a).

c

Add excess of dilute nitric(V) acid to the mixture from (b).

1. Why is aqueous sodium hydroxide used? 2. What results can you expect from carrying out procedure (a)? Why? 3. Why is excess aqueous sodium hydroxide used? 4. What result can be expected from carrying out procedure (b)? 5. Why is dilute nitric(V) acid added?

Part B Test Number

Procedure Descriptions

Observations

a

To a portion of R add aqueous sodium hydroxide until a change is seen.

A white ppt is obtained.

b

Add excess of aqueous sodium hydroxide to the mixture from (a).

White ppt soluble in excess reagent to give a colorless solution.

c

Add excess of dilute nitric(V) acid to the mixture from (b).

White ppt reappears and dissolves on addition of excess acid to give a colorless solution.

6. What is the anion present in the white precipitate? 7. Why is the white precipitate obtained? 8. Why is the white precipitate soluble in excess reagent? 9. Does the white precipitate really dissolve? 10. Can you deduce the identity of the cation? 11. Can you give a reason for your answer? 12. Why did the precipitate reappear? 13. Why did the precipitate disappear again? Figure 2. Student interview questions, set 1.

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Fourteen students said that the formation of precipitates, for example, when aqueous sodium hydroxide was added to an unknown solution, was due to a more-reactive cation displacing a less-reactive one. Similar findings were reported by Boo (8) in her study of older grade 11 and 12 students (16– 19 years old). In O-level chemistry, students are taught that displacement is a redox reaction in which a more-reactive element reduces a less-reactive element in its compound, resulting in the formation of the less-reactive element and the compound of the more-reactive element. However, many students used the term “displace” to describe reactions in which the exchange of ions occurred, resulting in the formation of precipitates. For example, in Set 1 Part A (Figure 2), when students were asked why the white precipitate was formed, 11 of them said that the sodium ions displaced the unknown cation, resulting in the formation of the precipitate. The following excerpts illustrate the students’ ideas. Student 9 (S9): Does it have anything to do with its…being a reactive metal? Interviewer (I): Why do you say that? S9: Because the metal at the top displaces the bottom (less-reactive) metal. So maybe because sodium is a reactive metal, it will be able to displace those below it. Rather than you get something like iron or lead that’s not as reactive as sodium and may not be able to displace the other metal. I: OK. Can you explain how the results in the experiment tie in with what you said about displacement? S9: I think the metal is less reactive than sodium. That’s why sodium hydroxide is able to displace it and form a new insoluble salt. I: OK…so what actually happens that gives rise to this insoluble substance? S8: Is displacement possible? I: Why do you say that? S8: Because…because sodium is a metal higher in the reactivity series. So er…any cation that is below it may be displaced. I: …if a precipitate is formed, can you tell me why…or what leads to its formation? S33: Displacement. I: Why displacement? S33: Why displacement…the more-reactive one displaces the less reactive. S34: Sodium…displaces the… S33: Sodium is very reactive…so it displaces things under it in the reactivity series. Reif and Larkin (9) think that scientific terms are commonly taught to students, but the use of these terms are rarely explicitly elaborated by teachers, and students seldom think about or clarify their meanings. Thus, students do not fully understand the meaning of the terms, are unable to explain the use of the terms explicitly, and cannot use them correctly. This could explain the students’ confusion over of the term “displace” or “displacement”. Another possible reason could be that the students could not differentiate between the ion and atom of an element (10), and may have thought that the ion and atom of an element behaves in the same way. A third reason could be that students are also required to memorize the reactivity series. Thus, what they learned in reactivity of metals could have become more prominent compared to www.JCE.DivCHED.org

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what they learned about double decomposition and cause students to inappropriately apply knowledge from reactivity and displacement, to explain double decomposition. • Alternative conception: Ammonia is included in the reactivity series In Set 4 of the interview questions, aqueous ammonia was used instead of aqueous sodium hydroxide to determine the identity of the unknown cation [iron(II)]. Student 22 believed that the unknown cation displaced the ammonium ion because it was more reactive. I: What sort of reaction happens that produces the solid? S22: Precipitation … discharge … displacement … displacement reaction. I: Can you elaborate further on what you mean by displacement? S22: Displacement that…cation is more reactive than the ammonium ion…cation. I: OK. S22: And thus it will displace the ammonium cation to form…a… hydroxide. Even though ammonia is not a metal and is not included in the reactivity series, student 22 still applied the concepts of reactivity and displacement to the use of aqueous ammonia, giving another example of the “more-reactive cation displaces less-reactive cation” alternative conception. Interestingly, when students were asked questions on why aqueous barium chloride or silver nitrate(V) could be used to identify anions, none of them mentioned anything about reactivity and displacement even though exchanges of ions resulting in the formation of precipitates also were involved. The following excerpts of interviews illustrate student conceptions of the processes involved in the identification of anions through double decomposition reactions. I: What sort of reaction occurs to give you a white solid? S22: A…precipitation. I: OK. Can you describe what happened? S22: Because…it’s like…both are aqueous solutions…so the sulfate ion will react with the barium ion to form barium sulfate. Then that barium sulfate will become a precipitate which is insoluble…and therefore precipitates out. I: OK…why is the solid formed…how is the solid formed? S 41: When silver reacts with the chloride ion…forms silver chloride, an insoluble salt. Thus it seemed that the students interviewed only associated reactivity and displacement with cations and not anions. For example, students 22 and 41 applied the “a morereactive ion displaces a less-reactive ion” alternative conception to explain the formation of precipitates using alkalis, but they could give acceptable answers as to why precipitates were formed when aqueous barium nitrate(V) and silver nitrate(V) were used. This seemed to support the hypothesis that learning about the reactivity of metals interfered with the understanding of reactions involved in the identification of cations. This also could show that the students lack an adequate conceptual framework of relevant chemical reactions; they might have a collection of scientific concepts and alternative conceptions, and choose to use different ones in different situations (11).

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Formation and Reactions of Complex Ions Grade 10 chemistry students were required to know that oxides and hydroxides of aluminum, lead, and zinc would react with excess aqueous sodium hydroxide to form complex salts, respectively. However, the syllabus did not require them to know that zinc hydroxide, copper(II) hydroxide, and silver chloride would react with excess aqueous ammonia to form the respective soluble ammines. These reactions were similar to the reactions of the amphoteric oxides/hydroxides with sodium hydroxide, which students encountered frequently in the QA practical work. In many experiments, students were instructed to add acids to reverse the formation of complex ions, but again, they were not required to know the reactions involved. This could create an awkward situation in that students might just follow these common instructions without understanding what they were doing. This proved to be true during the interviews when students had to answer questions on the formation and reaction of complex ions; 11 students admitted that they had no idea what reaction had taken place or what a complex ion was. Generally, the students struggled to answer the questions posed, and several alternative ideas were proposed, such as “the precipitate merely dissolved in the excess reagent added” and “displacement reactions”. • Alternative conception: The precipitate dissolves in excess alkali Many students did not seem to understand the disappearance of insoluble zinc hydroxide in excess aqueous sodium hydroxide. They believed that the precipitate merely dissolved when excess aqueous sodium hydroxide was added; they did not take into account that the precipitate actually appeared with the initial addition of the alkali. Solomonidou and Stavridou (12) argued that people usually “ignore the possibility that a substance must sometimes interact chemically with one or more other substance(s) and/or energy to produce new substances(s) completely different from the initial one(s)” (p 383). They also contended that “the learning of language by children may suggest that substances are inert objects” (p 384) that do not interact with other substances or energy. Ribeiro, Pereira, and Maskill (13) found perceptually dominated thinking guided students’ understanding of the word “reaction”; if the students did not see a new substance being formed, they tended not to refer to the change as a reaction. The mere disappearance of the precipitate without visible signs of reactions such as color changes, evolution of gases, or release of energy could mislead students into thinking that the precipitate just dissolved. They would be comfortable with that idea because they focus on the initial state of the precipitate without taking into account the final state as Solomonidou and Stavridou (12) proposed, and also how the precipitate was obtained in the first place. The students’ conceptions were illustrated by the following excerpts of interviews in which they were asked why the precipitate disappeared when excess sodium hydroxide was added. I: My point is that if the precipitate is insoluble in the first place, why should it be soluble later? S11: Maybe you add extra solvent…there’s more liquid for the substance formed to dissolve. I: OK, so you’re saying there’s more solvent, that’s why… 730


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S11: I think the reason why we get a precipitate first…is maybe…there is a limited amount of solvent…that’s why some of it dissolves in the solvent, whereas the rest did not dissolve…that’s why when you add more…then it will dissolve…I think it is like that…I think that’s how it works. I: OK…is a precipitate insoluble? S13: Yes. I: Then why should it dissolve in excess? S13: Because before adding excess, that solution perhaps could be concentrated, so…it gives out a precipitate. By adding excess sodium hydroxide, you’re giving more volume for the… you’re creating more space for precipitate to actually dissolve in it. Student 13 believed that the precipitate dissolved in the additional aqueous sodium hydroxide added because more solvent added meant more space or volume for the precipitate to dissolve. Student 11 believed that the size of the particle and space between water molecules played a major role in dissolution as the following excerpt illustrates. S11: I think whether something will dissolve depends on, I think from our textbook, the space…the molecular space between the water, right? So if let’s say sand, the atom, the size of the atom is bigger than the molecule of the water, that’s why it cannot sink between them, so it cannot dissolve …something like that. Ebenezer and Erickson (14) also found that some students thought that “substances do not dissolve because they do not find sufficient space in the dissolving medium” (p 192) and that size of the solute affects its solubility.

Addition of Acid Several students had difficulties with procedures involving the addition of acids and bases as they were unaware of the reactions taking place. They had learned the reactions in the chapter on acids, bases, and salts yet could not make the link between what they had learned and the procedures they performed. • Alternative conception: Neutralization involves redox reactions Several students were uncertain about when acids behaved as acids and when they acted as oxidizing or reducing agents. Others commented that neutralization involved redox. Many students could not give the correct answer when asked why a precipitate reappeared and then disappeared when acid was added to a mixture containing excess alkali and a complex salt. These reactions were common in the procedures that the students carry out; unfortunately, they did not understand what they were doing. The following interview excerpts highlight the students’ difficulties. I: OK, now can you tell me then what will happen if I add acid? S26: Maybe…because the oxide may be reduced…or something like that. I: Reduced by? S26: If you add in the acid…is it reduced or oxidized? I’m not too sure whether it is reduced or oxidized. I: OK…by nitric acid…or acids in general? S26: I think…acids in general.

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Students 25 and 26 and the interviewer were discussing the addition of nitric(V) acid to a mixture containing excess aqueous ammonia and a precipitate of iron(III) hydroxide. Student 26 believed that a redox reaction had taken place. Students were taught that nitric(V) acid was an oxidizing agent, so that might be why student 26 thought that a redox reaction had occurred. Another reason might be that student 26 knew that hydrogen ions were involved in reactions of acid and that a definition of reduction was the addition of hydrogen to a substance. Thus, she might have thought that when an acid was used, hydrogen ions were “added” to the other reactant, reducing it. This assumption was supported by one student who suggested during a QA practical session that concentrated and dilute hydrochloric acids were reducing agents because they contained hydrogen ions. When given the same situation, student 33 stated that a redox reaction could be involved. I: So what sort of reaction happened? S33: What sort of reaction happened? The iron hydroxide is being oxidized. I: Iron hydroxide is oxidized by? S33: By the nitric acid. I: To form? S33: To form iron nitrate and water. I: Why do you use the word oxidized? S33: Because it has an increase in oxygen but a loss in hydrogen. When excess nitric(V) acid is added to iron(III) hydroxide [Fe(OH)3], a solution containing mainly iron(III), nitrate(V), and hydrogen ions is obtained. The chemical equation describing the reaction is: 3HNO3 ⫹ Fe(OH)3 → Fe(NO3)3 ⫹ 3H2O From the formulae of the compounds involved, one could be easily misled to think that hydrogen was “lost” from the second reactant and that there was an “increase” in the number of oxygen particles in the first product. Looking at it this way, and knowing that an oxidation reaction involved the loss of hydrogen from a compound or the addition of oxygen to a compound, one could understand why student 33 had believed that iron(III) hydroxide was oxidized by nitric(V) acid. Schmidt (15) also found that high school students thought the reaction between magnesium oxide/hydroxide and hydrochloric acid was a redox reaction because oxygen was present in the reactants. If student 33 had used the oxidation number model of redox, he would realize that no redox reaction had occurred (16–17). Thus, student 33 could have arrived at this alternative conception of neutralization by not understanding what actually happened in a neutralization reaction, and using inappropriate models of redox for the situation. • Alternative conception: Order of addition of acid is important Aqueous barium chloride/nitrate(V), aqueous lead(II) nitrate(V), and aqueous silver nitrate(V) are used to identify certain unknown anions. To distinguish the anions that form precipitates with barium, silver, or lead(II), students are instructed to add dilute acid before or after adding the barium, silver, or lead(II) solutions. This is because carbonates and sulfate(IV) ions react with the acids liberating carbon dioxide www.JCE.DivCHED.org

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and sulfur dioxide, which can be detected and identified. If the precipitate remains unreacted, then it can be either barium sulfate(VI), or halides of silver, or lead(II). However, students 1 and 2 believed that the order of addition of acid was important; if acid was added after the addition of the barium solutions, then carbonates and sulfate(IV) ions could not be detected. I: OK now…looking at that, can a carbonate be present and detected using that test? S1: No, because you are already reacting it with barium nitrate. I: And? S1: Then if you were to add in the nitric acid later, it will not be able to show whether… S2: Maybe it’s because if you were to react carbonate with barium nitrate, you will get barium carbonate, which is insoluble, right? I mean acidified barium nitrate, but I think it’s how you’re going to differentiate between barium sulfate or barium nitrate [carbonate?] because barium sulfate is also an insoluble salt. So in this case you wouldn’t be able to know what to write down. I mean you won’t be able to conclude whether it is a sulfate that you’re getting or a carbonate that you are getting because both are white precipitates. Students 1 and 2 seemed to believe that if acid was added to the unknown anion prior to the addition of the barium solution, only then would any carbonate present react with the acid; to them the barium solution interferes with the reaction between acid and carbonate preventing any liberation of carbon dioxide. Instructional Implications This study reveals that the secondary students interviewed in the study did not understand several important reactions and procedures in O-level QA. Although the students carried out procedures to identify cations and anions frequently in QA practical work and learned the theory involved in the topic acids, bases, and salts, it seems that many of the students interviewed could not or did not relate the theory that they had studied in the classroom to what they were doing in the laboratory. McDermott (18) contends that one of the causes of students’ lack of understanding of chemistry is the failure to integrate knowledge. One reason for this could be that lessons were frequently seen by students as isolated events with no connections to the previous lessons, experiments, or topics (19, 20), so the students lacked appropriate frameworks that could guide their learning. One way to help students develop the appropriate framework for QA is through gaining first-hand knowledge of the materials and “a feel for the phenomena” (21, p 34) involved in QA. This instinctive knowledge will help students “build up personal constructs” (p 35) that will help them in “acquiring theoretical understanding of the underlying concepts later” (p 46). Hodson (22) also concurred that familiarization of the physical world precedes making sense of it, and Barrow (23) stated that students need to be exposed to reagents and the transformation of the reagents to make them “part of the body of experience that can be drawn automatically, when higher level thoughts are processed” (p 453). Woolnough and Allsop (21) gave examples of “experiences” in their book (p 59), and those relevant to QA include

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studying chemical changes with color changes, precipitate formation, and gas evolution. In the introductory QA practical sessions, students should spend some time experiencing relevant reactions, thinking about and discussing them. The teacher could then introduce the theory behind the phenomena by describing what happened at the micro-level and giving the relevant equations. Students should be better able to construct meaning from what the teacher says after experiencing the phenomena, thinking and talking about them. This method is similar to the orientation, elicitation of ideas, and restructuring of ideas in the constructivist teaching sequence proposed by Driver and Oldham (24). These are the phases where students are given the opportunity to develop a sense of purpose for learning the topic, make their ideas explicit by bringing the ideas to conscious awareness, and clarify these ideas through discussion and comparison with the alternative views of others. The teacher can also use Vee heuristics (25) in laboratory instruction—teachers and students are required to give explicit consideration to the aim(s) of the experiment, observations to be made (and the recording of such observations), the relevant concepts involved, and the knowledge claims derived from the experiment. In the case of QA, students are usually taught the relevant content knowledge before they start on the series of experiments, so the task of the teacher is to help students make the links (18) between the content knowledge that they have already learned and what they are doing in the experiments. This should help students understand the rationale behind the procedures and reagents used in the experiment (26), something on which current practice places insufficient emphasis (27). Greater care, as the study shows, is required in teaching about formation of precipitates in QA. Teachers need to help students differentiate between reactions involving exchange of ions and displacement. Students could carry out simple experiments to compare displacement, for example, zinc and aqueous copper(II) sulfate(VI) and reactions involving exchange of ions—aqueous sodium hydroxide and copper(II) sulfate(VI). Teachers could then describe the exchange of ions resulting in the formation of precipitate, and reinforce it by writing the net ionic equation. Animation sequences depicting the exchange of ions and precipitation at the micro-level could be shown to students to help promote greater understanding of the processes and help them differentiate these processes from displacement reactions. Garnett et al. (4), and Harrison and Treagust (28) contend that the use of multimedia simulations can provide students with concrete microlevel representations of chemical structures and reactions, and Dechsri, Jones, and Heikkinen (29) noted that images were more easily recalled than words and could act as an “easily recalled conceptual peg for abstract concepts” (29, p 892). As an extension, students could compare the reaction between aqueous sodium hydroxide and zinc sulfate(VI) and that between aqueous sodium hydroxide and copper(II) sulfate(VI) to study complex salt formation. Water and excess sodium hydroxide could be added separately to the amphoteric zinc hydroxide to determine whether the precipitate dissolves or reacts with the sodium hydroxide. Teachers need to write down the equilibrium reaction resulting in the formation of the complex salt, as well as how acid, if added,

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will affect the equilibrium, resulting in the reappearance of the precipitate and the eventual reaction of the precipitate with acid. As for the order of addition of acids, students should be given experiments to compare the effects of adding acids before and after the introduction of barium, silver, and lead(II) solutions to the unknown sample, and work out the reactions that occur. Literature Cited 1. Tan, K. C. D.; Goh, N. K.; Chia, L. S.; Treagust, D. F. Res. Sci. Tech. Educ. 2001, 223–234. 2. Goh, N. K.; Toh, K. A.; Chia, L. S. J. Chem. Educ. 1989, 66, 430–432. 3. Tsoi, M. F. Effects of Different Instructional Methods on Interpretation Skills in Chemical Qualitative Analysis. M.Ed. Dissertation, National University of Singapore, Singapore, 1994. 4. Garnett, P. J.; Garnett, P. J.; Hackling, M. W. Stud. Sci. Educ. 1995, 25, 69–95. 5. Novak, J. D.; Gowin, D. B.; Johansen, G. T. Sci. Educ. 1983, 67, 625–645. 6. Duit, R.; Treagust, D. F.; Mansfield, H. In Improving Teaching and Learning in Science and Mathematics; Treagust, D. F., Duit, R., Fraser, B. J., Eds.; Teachers College Press: New York, 1996; pp 17–31. 7. Johnson, P.; Gott, R. Sci. Educ. 1996, 80, 561–577. 8. Boo, H. K. J. Res. Sci. Teach. 1998, 35, 569–581. 9. Reif, F.; Larkin, J. H. J. Res. Sci. Teach. 1991, 28, 733–760. 10. Nakhleh, M. B.; Krajcik, J. S. J. Res. Sci. Teach. 1994, 31, 1077–1096. 11. Palmer, D. H. Sci. Educ. 1999, 83, 639–653. 12. Solomonidou, C.; Stavridou, H. Sci. Educ. 2000, 84, 382– 400. 13. Riberio, M. G. T. C.; Pereira, D. J. V. C.; Maskill, R. Int. J. Sci. Educ. 1990, 12, 391–401. 14. Ebenezer, J. V.; Erickson, G. L. Sci. Educ. 1996, 80, 181–201. 15. Schmidt H.-J. Chem Educ. Res. Pract. Eur. 2000, 1, 17–26. 16. Garnett, P. J.; Garnett, P. J.; Treagust, D. F. Int. J. Sci. Educ. 1990, 12, 147–156. 17. Sisler, H. H.; VanderWerf, C. A. J. Chem. Educ. 1980, 57, 42–44. 18. McDermott, D. P. J. Chem. Educ. 1988, 65, 539–540. 19. Berry, A.; Mulhall, P.; Gunstone, R.; Loughran, J. Aust. Sci. Teach. J. 1999, 45, 27–31. 20. Duit, R.; Treagust, D. F. In Improving Science Education; Fraser, B. J., Walberg, H. J., Eds.; The National Society for the Study of Education: Chicago, Illinois, 1995; pp 46–69. 21. Woolnough, B; Allsop, T. Practical Work in Science; Cambridge University Press: Cambridge, UK, 1985. 22. Hodson, D. Stud. Sci. Educ. 1993, 22, 85–142. 23. Barrow, G. M. J. Chem. Educ. 1991, 68, 449–453. 24. Driver, R.; Oldham, V. Stud. Sci. Educ. 1986, 13, 105–122. 25. Nakhleh, M. B. J. Chem. Educ. 1994, 71, 201–205. 26. Gabel, D. L. J. Chem. Educ. 1999, 76, 548–554. 27. Osborne, J. Sch. Sci. Rev. 1993, 75, 117–123. 28. Harrison, A. G.; Treagust, D. F. Sch. Sci. Math. 1998, 98, 420– 429. 29. Dechsri, P.; Jones, L. L.; Heikkinen, H. W. J. Res. Sci. Teach. 1997, 34, 891–904.

Vol. 81 No. 5 May 2004

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Major Sources of Difficulty in Students' Understanding of Basic

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