Surveying Students' Conceptual and Procedural Knowledge of Acid

Research: Science and Education

Surveying Students’ Conceptual and Procedural Knowledge of Acid–Base Behavior of Substances

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Carles Furió-Más* Departament de Didàctica de les Ciències Experimentals i Socials, Universitat de Valencia, 46071 València, Spain; *[email protected] María-Luisa Calatayud I.E.S. Sorolla, Calle de José María Haro, No. 2, 46022 València, Spain Sergio L. Bárcenas Escuela Preparatoria, Universidad Autónoma de Zacatecas, Zacatecas, México

Acid–base behavior of substances is a primary topic in the high school curriculum and general chemistry courses at university. Acids and bases are common in daily life and are important in industry and biological activities. They also figure prominently in environmental problems and research into possible solutions through scientific and technological developments. The importance of this topic has been accompanied by the identification of learning and teaching difficulties with these concepts (1–8). If the teaching process is to help pupils progress adequately in their knowledge, it is not enough to take into account their ideas prior to instruction. The solutions contributed by the history and epistemology of science to scientific problems must also be taken into account, as it is a way of facilitating understanding (9, 10). Teaching needs to change from being exclusively conceptual and approach scientific research so that the conceptual, procedural, and axiological components of learning are integrated (11). Nowadays the strong link between these three components in science learning cannot be ignored (9). Researchers, such as Adey (12), are concerned about the relationship between learning progression and sequencing of content and consider it important to investigate whether the knowledge of any subject can be organized according to a hierarchy that favors understanding. Although there are many possible solutions to this hierarchy of content, research attempts to study how the student progresses empirically, to explain the sequence of content in the learning process from a psychological viewpoint. Other authors believe that this psychological basis for the curriculum can be complemented by epistemological analysis of the scientific knowledge that students have to (re)construct (13, 14). Thus, this study seeks to assess whether high school students have the necessary knowledge and skills to correctly explain the properties of acids, bases, and salts from their constituent ions or molecules. We also attempt to show whether certain learning difficulties can be explained by this lack of knowledge and skills. Macroscopic and Sub-Microscopic Models Teaching and learning difficulties in chemistry are shown in studies by Wandersee, Mintzes, and Novak (15); Garnett and Hackling (16); and Treagust, Duit, and Nieswandt (17). This research shows that the existence of the different models or representations of the world that are used in chemistry is one of the main issues to be taken into account in the teaching process. In chemistry there are two representative conceptual models that are frequently mixed in the teaching www.JCE.DivCHED.org

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process: the macroscopic and sub-microscopic levels of substance description, interpretation, and interaction (18). Both levels of representation are related as the chemist tries to explain the properties of substances in the macroscopic world with models such as atomic theory of matter where molecules, atoms, ions, and electrons are introduced. Chemists use symbolic representations of these particles, which we will consider, that belong to the microscopic model. These models are included in the acid–base topic in high school: the macroscopic model of acids and bases and Arrhenius or Brönsted sub-microscopic models. Education research (19) underlines the fact that teachers display a linear cumulative view in the construction of scientific knowledge, thus ignoring crises and deep restructurings. Recent research (10, 20) reveals that as consequence of this view students consider that an acid substance is the acid particle (overlapping macro and sub-micro). They also consider that the Brönsted theory is an extended version of the Arrhenius theory. Grade-12 students’ understanding of acid–base behavior can be evaluated by two concept maps that specially include the conceptual and procedural competences that students should acquire by the end of their high school education. The maps are based on the high school chemistry curriculum, chemistry textbooks (21–23), and the teaching experience of the authors of this article. The macroscopic model used to classify substances according to the structure of their constituent particles is shown in Figure 1. The sub-microscopic model that explains the principal macroscopic properties of substances (assumed in Figure 1) is shown in Figure 2. Indicators of Understanding Acid–Base Behavior Indicators of understanding during the learning process are classified to take into account the fact that students have to construct conceptual, procedural, and axiological knowledge to interpret acid–base behavior of substances from their ions and molecules. The complexity of different macro and sub-micro conceptualization models and the way they relate are presented gradually since progression in learning requires starting from a previously learned model before moving on to a more complex one (24). It is obvious that the hierarchy of difficulties will be in accordance with the increase in complexity of levels of representation, so an increase in the failure rate could be expected when the task requires a sub-microscopic description of the phenomenon. This implies that students have to acquire knowledge and skills in the macroscopic conceptualization of the behavior of substances first to be able to classify an ionic or molecular

Vol. 84 No. 10 October 2007

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Journal of Chemical Education

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Figure 1. Map of properties of ionic and molecular substances (macroscopic model). Note the properties of the atomic covalent and metallic substances were not described as they are not utilized in the study.

substance as an acid or base using empirical knowledge. Second, students need in-depth knowledge of the particles that constitute these substances. Finally, they must know how to relate both models to interpret the properties of acids and bases coherently (dissolving process, electrical conductivity) and to understand the neutralization and hydrolysis processes. The conceptual and procedural knowledge of acid–base behavior of substances that students should gain in a successful learning process is shown in List 1 (on p 1720). This list was based on the high school chemistry curriculum and is organized by macro and sub-micro conceptualization. The curriculum is more extensive but this research is only focused on the conceptual and procedural knowledge of List 1. Experimental Design The diagnostic design is divided into two parts. The first part aims to verify whether the majority of students have the conceptual and procedural knowledge indicated in List 1 and comprises two questionnaires and an interview. Macroscopic and sub-microscopic conceptualization is analyzed simultaneously in the proposed design. Questionnaire Q1 (with three items) deals with the dissociation of substances and was completed by 86 students. Questionnaire Q2 (with eight items), which was given to 60 students, seeks to verify whether students know how to distinguish an atom from an ion and understand the polyatomic ion formula. Interview I-1 on ionic dissociation was carried out on 28 students to assess students’ in-depth thinking. The second part of the design was used to survey whether students make connections between the macro and 1718


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sub-micro conceptualization models and consists of three questionnaires and another interview. Questionnaire Q3 with three items focuses on how ionic and polar molecular substances are dissolved in water and was given to 60 students. Interview I-2, which assesses how students explain the electrical conductivity of aqueous solutions of acids and bases, was applied to 28 students. Questionnaires Q4 and Q5 (two items each) were given to 68 students and aim to verify whether students know how to interpret neutralization and hydrolysis reactions. The questionnaires were completed by grade-12 students (17–18 years old) from a coeducational high school in València between 1996 and 1998. The students had previously studied the concepts of chemical bonding and molecular structures, in keeping with the content sequencing of the high school’s chemistry curriculum and they had also received instruction on elemental acid–base concepts in grade 10. Students were tested after instruction in relation to this material. The interviews lasted between 20 and 30 minutes. They were semi-structured; there was a list of questions to ask, but the interviewer could deviate from the list to pursue any interesting comments or alternative conceptions given by the students. Preliminary examination of the items included in the interviews were conducted and no difficulties of comprehension were found. Three professors of chemistry examined the maps and the list of macro and sub-micro conceptualization knowledge and skills for accuracy, while three chemistry teachers ensured that the content was appropriate for grade 12. Three researchers analyzed the answers to the questionnaire separately to


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Figure 2. Map of properties of ionic and molecular entities of substances (sub-microscopic model). Note the properties of the atomic covalent and metallic substances were not described as they are not utilized in the study.

test that their interpretation coincided. In the case of disagreement over the interpretation of a particular item, it was discarded and a new item was proposed and validated. The results from questionnaires Q1–Q5 are included in the Supplemental Material.W Results Obtained by Applying the Design

Results of Questionnaire Q1 Questionnaire Q1 seeks to verify that students have learned about ionic dissociation to start the topic of acid– base reactions and consists of three items. The aim of item Q1.1 is to find out whether the student recognizes and applies the definition of ionic dissociation that is commonly found in textbooks. The aim of item Q1.2 is to ascertain whether the student knows how these substances are dissociated in water. The goal of item Q1.3 is to learn whether the student recognizes the types of reaction (acidic, basic, or neutral) empirically or by the formula of seven solutions frequently used in the classroom. The majority of students (73%) know the meaning of ionic dissociation of substances in an aqueous solution (item www.JCE.DivCHED.org

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1.1). A typical response is HCl

H+ + Cl−

Ionization of a compound is to divide a substance into its corresponding ions.

However, in item Q1.2, the students have some difficulty in polyprotic acid ionization (57%, 65%, and 55% responded correctly for H3PO4, H2S, and H2SO4, respectively, for the first ionization). Most of the incorrect responses are due to the fact that students do not know how to predict the possible charges in the ions formed from these acids and they did not take into account the fact that the molecules of these compounds are electrically neutral. In item Q1.3, the majority of students recognize H2SO4 as an acid and NaOH as a base (64% and 66% correctly answered, respectively). This is due to both substances being common in their learning. However, lower percentages of correct answers are given for NH3 and CH3COOH (55% and 54%, respectively). This may be due to the presence of H and OH in the formula of NH3 and of CH3COOH, respectively. Students propose ionization without taking into account whether these atomic groups can be ionized or not.


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Research: Science and Education List 1. Competences Concerning Acid–Base Behavior of Substances That Students Should Learn Macroscopic Conceptualization • Students must differentiate between an ionic substance and a non-ionic substance from their properties (specifically, a molecular polar substance). For instance, they have to differentiate both substances by their melting and boiling points, physical state at room temperature, etc. • Students must recognize the similar properties of ionic substances and some molecular polar substances. For example, they both dissolve in water, their solutions conduct electric current, and have properties called colligatives such as boiling point elevation, freezing point depression, etc. • Students must know that acids, bases, and salts are ionic substances or polar molecular substances. In addition to this, they have other specific properties. For example, turning violet vegetable extracts red, corroding metals, and alkali properties are neutralized by acid properties and vice-versa obtaining salts as a new substance. Many of these neutral salts can generate acidic or basic solutions when dissolved in water (hydrolysis). Sub-Microscopic Conceptualization • Students must recognize the entities of an ionic substance (ions) and a polar molecular substance (molecules) from their empirical formula, especially the entities of the most common acids, bases, and salts. • Students must predict the ions that are formed when ionic or molecular polar substances are dissolved and particularly when the polyatomic ions are formed. • Students must distinguish an atom from an elemental ion based on their structural characteristics or their behavior and based on their net charge and volume, respectively. • Students must know the meaning attributed to the formula of a polyatomic ion. That is (i) the subscript to the right of the element’s symbol indicates the number of atoms of that element in the ion; (ii) The superscript is the charge of the whole group of atoms; and (iii) know how the atoms are linked in the polyatomic ion (how to draw a Lewis structure). Relationship between Macro and Sub-Micro Conceptualization • Students must understand how ionic and polar molecular substances dissolve in water in sub-micro terms. • Students must explain what mechanism conducts electrical current in an ionic solution. • Students must know how neutralization between an acid and a base is produced at a sub-microscopic level. • Students must know how the hydrolysis of a salt, when dissolved in water, occurs at a sub-microscopic level. • Students must understand the fragility of an ionic compound in a qualitative way.

A reduction in the success rate is observed when the substances proposed are not so familiar and when some theoretical knowledge of polar molecular substances that are dissolved in water but do not ionize, is required. Such is the case with CH3OH (23%). Similar results are obtained for substances that dissolve in water and, moreover, do ionize, such as CH3NH2 (25%). In both cases, half the incorrect answers ionized CH3OH as CH3+ and OH− and CH3NH2 as CH3NH− and H+. This failure rate increases in the case of SO2 with only 7% answering correctly. This can be explained because SO2 does not have H or OH in its formula. Furthermore, identifying SO2 as sulphurous acid anhydride is a prerequisite for solving this question (SO2 + H2O → H2SO3 → H+ + HSO3−). The majority of students may have applied a theory similar to Arrhenius (they have already been exposed to this theory) where all substances containing H atoms (ionizable or not) in their formula are acids and those that contain OH are bases. However, they have not used the operational definitions of an acid or base from the properties of the substances (such as for example how ionic and polar molecular substances are dissolved, the electrical conductivity of acid and base solutions). Students have difficulties dissociating substances when they have to apply theoretical knowledge to find out whether ionic dissociation is possible or not. In addition, we applied the chi-squared test to the results obtained assuming the null hypothesis that 50% of an1720


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swers would be correct and 50% incorrect (and blanks), finding that almost all percentages are statistically significant with a P value of less than 0.05. Together with the percentages for each item, Table 1 in the Supplemental MaterialW presents χ2 with a degree of freedom and those corresponding to P. The same statistical method was applied to the percentages in Tables 2–5 obtained from the answers to the items in questionnaires Q2, Q3, Q4, and Q5, respectively.

Results of Questionnaire Q2 Questionnaire Q2 has eight items and was given to 60 students. Items Q2.1 and Q2.2 are aimed at discerning whether students can differentiate between some of the structural and functional characteristics of atoms and ions. Students show better results in their understanding of the concept of ion, as 68% of them consider that the electron configurations of Na and Na+ (item Q2.1) are different and they differ by one electron. In this case students have correctly understood electronic configurations and this allows them to differentiate between Na and Na+. The majority of incorrect answers confuse atom with ion (3, 25). Worse results are obtained in item Q2.2, where 54% recognize the properties of Cl and Cl− as being different, and half of the incorrect responses confuse atom with ion and may have thought that the ion and atom of an element behaves in the same way (26). In addition to this, they attributed macroscopic properties to an ion, as shown in the following example:


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Research: Science and Education Although chlorine forms negative ions, the properties of Cl and Cl− are the same, they are the same element.

The aim of items Q2.3–2.8 is to find out whether students can fully represent H2SO4 ionization and whether they understand the meaning of the symbols used in sulfuric acid ionization. The complete ionization of sulfuric acid (item Q2.3) was written correctly by 46% of students, but only 17% show that three ions for each molecule of sulfuric acid ionized are produced (item Q2.4). The majority of incorrect responses to this item suggest that two ions are produced (H+ and SO42−). Maybe students confuse which types of ions (two in this case) are present with their relative number. Almost half the sample left item Q2.4 blank (46%). Low percentages of correct responses were obtained in items Q2.5–2.8: 17% (How many atoms are there in a sulfate ion?); 32% (How are the atoms in the sulfate ion attached?); 22% (What electrical charge does the sulfate ion have?); and 10% (Where does the electrical charge belong in the sulfate ion?), respectively. High percentages of blank responses are obtained in these items (68%, 61%, 66%, and 81%, respectively). Note that the major difficulty occurs in item Q2.8 according to these results. An example of an incorrect response (9%) is The electrical charge (2−) belongs to oxygen since it is more electronegative than the other elements.

These results show that half the students know how to write the formula for the sulfate ion but the majority ignore the meaning of both the subscript to the right of the element and also the superscript in a polyatomic ion. The superscript is the charge on the whole group of atoms.

Results from Interview I-1 Interview I-1 was designed to study students’ thinking in-depth and to test the results obtained in questionnaire Q2. It has three questions with similar content to some items in Q2 and was given to 28 students (List 2). A majority of the students interviewed (86%) confused atom with ion in question 1, as shown in the following example: Miguel: A positive sodium ion is one which has lost the outermost electron, and so it has a net positive charge and the chloride ion has a net negative charge because it has 7 valence electrons and it gains one electron to achieve the same configuration as the nearest rare gas.

This student talks about the positive sodium ion but is in fact referring to the sodium atom and the same can be said for the chloride ion. A similar result was found in item Q2.3 (Table 2 in the Supplemental MaterialW), where only 46% of students interviewed wrote the complete ionization of H2SO4 correctly. The following are examples of incorrect responses: Mar: H3

O+

+ SO4

2−

Laia: H+ + SO4− there are hydrogen and sulfate ions Alejandro: H2+ + SO4− ions and water

These mistakes may be due to the fact that students use the simplest ionization. The molecule is divided into two ions, one positive and the other negative. Neither the law of conservation of mass (Laia) nor the electrically neutral behavior of the molecule of compound are taken into account by some students (Mar). www.JCE.DivCHED.org

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List 2. Interview I-1 Questions on Ionic Dissociation (1) What is a positive ion, Na+, and a negative ion, Cl−? (2) What particles (atoms, molecules, or ions) are produced in an aqueous solution of sulfuric acid (H2SO4)? (3) Where does the charge belong in the sulfate ion SO42−?

Only 14% of the students interviewed correctly answered the third question by suggesting that the charge belongs to the whole group of atoms. This result is similar to those mentioned above for item Q2.8, where only 10% of students were successful. The majority of incorrect responses show that the negative charge is attributed to one of two types of atoms present in the ion formula, as shown in the followings examples: Antonio: to sulfur…to ion sulfur Alejandro: the negative charge belongs to oxygen since it is more electronegative than sulfur

In these cases, students state that the charge is not the whole group of atoms. One of them argues that as the charge is negative, it must belong to the most electronegative atom. Results from the interview are similar to those obtained from questionnaire Q2 and both show that prerequisites such as being able to differentiate between an atom and an ion have not been acquired by approximately half of the students interviewed. The students did not know how many atoms there were in the polyatomic ion either, as was the case in questionnaire Q2.

Results of Questionnaire Q3 Following the previous results, if students have not understood the ion concept, a poor success rate in explaining the macroscopic properties of these ionic substances using the sub-microscopic model could be expected. First, we asked a sample of 60 students how ionic and polar molecular substances dissolve using questionnaire Q3 with three items. Items Q3.1 and Q3.2 are proposed to find out whether the students know that an ionic compound is composed of many ions of different charges and how they interpret the solution of most acids, bases, and salts in water. In item Q3.1, 32% of students clearly differentiate between atom and ion, since they draw charged atoms to explain how a KCl crystal is formed. However, 27% of students draw potassium and chlorine atoms, thereby confusing atom and ion. There are a high percentage of blank responses (41%). The percentage of correct responses was expected to be low in item Q3.2 (12%), since the students have to explain the dissolving process of KCl. An example of a correct response is When a crystal of K+ and Cl− ions is placed in water, water molecules are attracted to the ions by ion–dipole forces. A metal ion that is bonded to several water molecules is said to be hydrated.

Twelve percent of almost correct answers suggest the following: Water molecules are placed between ions, and they dissolve.

In this case students do not refer to electrostatic interactions to explain the dissolving process. Students transfer the mac-


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roscopic properties of substances (KCl crystal dissolves) to ions (10). Fifteen percent of students clearly confuse atoms and ions, according to the results obtained for item Q3.1 (and items Q2.1 and Q2.2), for instance, KCl is a crystal with a three-dimensional unit cell. When it is dissolved several water molecules are bonded to crystal atoms.

A high percentage of blank responses are given (44%). Students have studied the dissolving process of ionic substances in a bond topic before, however this has been forgotten by almost half the students. Surprisingly, only 32% of students responded correctly when asked whether water was composed of H+ and OH− (item Q3.3). Here is an example of a correct response: Water is a molecular substance with O⫺H covalent bond.

Half the incorrect responses read as follows: Water is an ionic substance, so it will be formed by H+ and OH−.

Both KCl and H2O are familiar compounds to students, since they are common examples in the classroom; KCl as an ionic solid and H2O as a molecular substance. Students can associate water with equilibrium H2O H+ + OH− and they do not realize that the equilibrium constant value is quite low, reflecting minimal ionization in water.

Results from Interview I-2 A second interview I-2 (comprising four questions) was carried out on 28 students to find out how they explain the electrical conductivity of aqueous solutions of acids, bases, and salts. First, the students were asked to predict whether the solution of substances mentioned in the question would conduct electricity. Later, they carry out the experiment (with a glass containing the solution, a light bulb, a battery, and two electrodes) and finally they explain what occurs (List 3). Correct answers suggesting that distilled water (question 1) would not conduct electricity were received from 82% of the students interviewed. Students’ explanations range from stating that water is a molecular substance to those indicating the absence of ions. The few incorrect responses say that water is not a conductor because it has no free electrons, as illustrated by the following excerpts: Interviewer (I): Do you expect distilled water to conduct electricity? Student 2 (S2): When we connect the battery and there is only distilled water in the glass, the electric current does not circulate. I: Why? S2: There are no electrons in the medium … the light bulb does not light up. It remains off.

The functional fixedness mechanism used in a metallic conductor appears and students associate the conductivity of substances to free electrons regardless of the type of substance. A total of 18 out of 28 students (64%) interviewed answered question 2 correctly, since they suggest that methanol will not conduct electricity, whereas only 14% explain their response, stating that methanol is a molecular substance and although it dissolves in water it does not ionize. The rest of students do not explain their response. This result is simi-

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List 3. Interview I-2 Questions on Electrical Conductivity of Substances or Solutions. (1) Do you expect distilled water to conduct electricity? (2) Do you expect methanol to conduct electricity? (3) Do you expect the aqueous solution of sulfuric acid to conduct electricity? (4) Do you expect the aqueous solution of sodium chloride to conduct electricity?

lar to those mentioned above for methanol in item 3 in Table 1W on whether this reaction would show acid, basic, or neutral behavior. The following excerpts from the interview highlight the reasoning behind half the incorrect responses: Interviewer (I): Do you expect methanol to conduct electricity? Mar (S3): Well, yes, I do. I: Why? S3: It is an ionic compound and it dissociates in water then the electrons are free and they conduct electric current. I: Can you write the ionization of methanol? S3: Two parts CH3+ and OH−.

On the one hand this student believes that methanol is an ionic substance and that it is ionized as a basic substance in line with item Q1.3 in Table 1.W On the other hand, the idea that electrolyte electrical conductivity in water is due to free electrons formed when the substance is dissociated reappears. A majority (75%) of the students interviewed answered question 3 correctly, since they suggested that sulfuric acid would conduct electricity. The following interview excerpts are from students who believe that sulfuric acid would not act as a conductor: I: Do you expect the aqueous solution of sulfuric acid to conduct electricity? S9: I don’t think the aqueous solution of sulfuric acid will conduct electricity.

The experiment is carried out by the student and the light bulb included in the circuit lights up. This student carried out the experiment, reflected on the result and changed his response. Sixty-eight percent of the students interviewed answered that the aqueous solution of NaCl (question 4) would conduct electrical current and more than half (52%) argued that it was due to the existence of ions in the solution without giving a correct explanation about the mechanism of electrical conductivity, as shown by the following excerpts: S3: Sodium chloride dissociates completely into ions in water, so this solution is a good conductors of electricity. I: Can you explain this? S3: Ions….electrons flow the electrode negative to the electrode positive and this succeeds if the sodium and chloride ions interact, but don’t know exactly how. I: What happens to ions? S3: When the ions are oxidized or reduced, they produce electrons and close the circuit. Sodium atom flow to negative electrode and chlorine to positive electrode.


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This student talks about “sodium atom” when actually referring to the positive sodium ion and the same can be said for the “chlorine” (atom). This student confuses atom with ion. The majority of incorrect responses to this question are along similar lines, as shown by the following excerpts from the interviews: I: Can you describe what happens in the circuit if we dissolve sodium chloride NaCl in water? S3: Sodium chloride dissociates completely into ions in water. So, electrons are free and they conduct electric current.

Again the students transfer the electrical conductivity model for metals to that for ionic compounds. Garnett and Treagust (27) argue that grade-12 students have difficulty with electrical circuits and oxidation–reduction reactions owing to overlapping daily and scientific language. According to the results of this research, the problem is not a question of terms, but rather a deeper lying semantic error. Students know that electrons move in the external circuit but they do not know the meaning of ion and consequently, do not know how it relates to the electron. Students fail to differentiate between the ion and atom of an element.

Results of Questionnaire Q4 Questionnaire Q4 (items Q4.1 and Q4.2) was given to 68 students. The aim of item Q4.1 is to find out what sort of neutralization concept is used by students. In other words, students can recognize the process as neutralization of acid and base properties (macroscopic conceptual model), as a reaction between hydronium and hydroxide ions (Arrhenius conceptual model), or as a protonic transference from acid to base (Brönsted and Lowry conceptual model). The second part of item 4.1 is designed to see how they test, by means of an experiment, that this process is neutralization. The aim of item Q4.2 is to learn what ideas students use to identify that reactions are neutralizations between the five given reactions (List 4) and how they interpret them. First of all, it is worth highlighting that one third of students do not correctly answer these questions. In item Q4.1 (Explain what happens when an acid and a base are mixed), most students (44%) use the macroscopic level to define neutralization by the following: acid + base → salt + water. A theoretical interpretation is given by 26% of students (11% according to the Arrhenius model and 15% according to the Brönsted model). Only a third of the sample says that indicators can be used to discover when an acid is neutralized by a base. Students applied the Arrhenius and Brönsted models in the correct responses of 33% and 25% to items Q4.2.a and item Q4.2.b, respectively (List 4). On average, 29% of students responded correctly to these items, which is approximately the same percentage observed for item Q4.1 (26% of students gave a microscopic interpretation of the neutralization reaction). Students recognized two bases in the first part of these reactions, Al(OH)3 and Zn(OH)2 and also the OH− ions. However, they did not consider the possibility that these compounds could dissociate to give protons and therefore did not classify these reactions as neutralization reactions. The low percentage (11%) of correct responses to Q4.2.c (List 4) is explained by the absence of H and OH in the formulas for reactants and H2O in products. On analyzing the results for Q4.2.d, 9% of correct responses, and Q4.2.e, 3% of correct responses, we can see that most students believed www.JCE.DivCHED.org

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List 4. Chemical equations from questionnaire 4, question 2.

(a) Al(OH)3(s) + OH−(aq) → AlO2−(aq) + 2H2O(l) (b) Zn(OH)2(s) + 2OH−(aq) → ZnO22−(aq) + 2H2O(l) (c) SiO2(s) + CaO(s) → CaSiO3(s) (d) HCl(g) + CH3OH(l) → CH3Cl(g) + H2O(g) (e) NH3(g) + CH3OH(g) → CH3NH2(g) + H2O(l)

these reactions were neutralization reactions because they fit the chemical equation: acid + base → salt + water. According to results from questionnaire Q1, almost all students know that HCl is an acid and more than half know that NH3 is a base. Only 23% know that methanol is a polar molecular substance that dissolves in water but does not ionize. However, in these reactions students, as in interview I-2, consider methanol to be an ionic substance and so the OH group of its formula is a hydroxide ion. These results could show that students have learned a representation of neutralization “acid + base → salt + water” by heart and that the concept of dissociation has not been understood.

Results of Questionnaire Q5 Questionnaire Q5 (items Q5.1 and Q5.2) was given to 90 students. The aim of item Q5.1 is find out how students define hydrolysis. We consider both macroscopic and submicroscopic explanations to be correct responses. The aim of item Q5.2 is to see whether or not students distinguish hydrolysis from neutralization. Only 10% of the students responded correctly to item Q5.1. Half the incorrect responses do not specify if the reactor that undergoes hydrolysis has to be a salt (macroscopic level) or ions (sub-microscopic level). Thus, students are unable to interpret what happens between the particles when the salt dissolves in water. In other words, students do not know that the hydrolysis of salts (macroscopic level) is due to proton transfer between cations or anions and H2O molecules (sub-microscopic level). Thus it seems that these students mixed macroscopic and sub-microscopic levels when describing and interpreting substances and their interactions, as research has highlighted (18, 20). A high percentage of incorrect (38%) and blank (39%) responses were obtained for item Q5.2.a on the hydrolysis reaction between cations NH4+ and H2O molecules. Students, according to results for item Q5.1, did not associate hydrolysis with a reaction between ions and H2O molecules. Half the incorrect responses (35%) classified carbonate ion hydrolysis (item Q5.2.b) as a neutralization process because this process is a chemical equilibrium where double arrows do not indicate reversibility but equality. Thus, the chemical equation acid + base → salt + water can also be written as salt + water → acid + base as in Q5.2.b. In item Q5.2.c, all the incorrect responses indicate that HCl ionization is hydrolysis since it is the reaction between a substance and water according to the definition of hydrolysis given by students in item Q5.1. Conclusions and Teaching Implications This study reveals that the grade-12 students interviewed have some knowledge of macroscopic conceptualization, but less about sub-microscopic conceptualization of acid–base be-


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havior of substances. Moreover, students fail to correctly answer the question about the relationship between macro and sub-micro conceptualization (List 1). This could explain some of the following learning difficulties students have: 1. A large number of students know the meaning of ionic dissociation of substances in an aqueous solution, but find it difficult to apply theoretical knowledge to discover whether ionic dissociation is possible or not. 2. Students were unable to differentiate between an ionic and polar molecular substance from their macroscopic properties and struggle to classify substances as acid or base. For instance, some said that water was an ionic substance (50% of incorrect responses), while only 7% of them indicated the acid behavior of an aqueous solution of SO2. 3. Acid–base behavior is predicted by students from the compound formula, as they have little empirical knowledge of acid–base behavior of substances. Thus, they associate the existence of H or OH in the formula of substance with acid or basic reactions respectively, without differentiating between atoms (or atom group) or ions. This explains the low percentages of correct responses obtained for the acid–base behavior of methanol 23% and methylamine 25%. 4. Students did not understand what the subscript and superscript in the polyatomic ion formula meant, so they said that the ion charge was on the nearest atom, which is usually the oxygen atom, the most electronegative atom (9%). 5. One out of three students did not differentiate between the ion and atom of an element. They therefore have difficulties interpreting the macroscopic properties of substances such as how ionic substances (e.g., KCl) dissolve in water (44% responded incorrectly). 6. A large number of students interviewed know that the aqueous solutions of ionic compounds conduct electrical current but they did not know how to explain it and a third of them believe that NaCl dissolves in water, forming ions and that there are also free electrons that conduct electric current, but ions do not conduct electricity. 7. A large number of students could not differentiate between an ionic and polar molecular substance from their constituent particles. Therefore they did not differentiate between the ionic dissociation process (when ionic substances dissolve in water), the ionization process (when polar molecular substances dissolve in water and form ions), the neutralization process (an average of 24% correct responses), or the hydrolysis process (an average of 15% correct responses). 8. Students sometimes confuse macro and sub-micro conceptualization models when trying to explain a property. For instance, 25% of them attribute macroscopic properties to an atom or ion.

These results show that improvements in teaching and learning of chemistry require teachers to have a thorough knowledge of the discipline, especially with regard to how scientific problems have been uncovered and solved in the history and the epistemology of chemistry. Furthermore, teachers must be aware of and be able to use efficient strategies in promoting the conceptual change that science education is currently undergoing. There are projects that attempt to enhance students’ scientific understanding by developing the knowledge and skills required “to do science” (28, 29). 1724


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Finally, progression and learning in science is a major issue for those involved in science education with important implications for teaching. Careful thought must be given to the way the curriculum is structured (24, 30) so that it may be presented in a suitable manner to be readily assimilated. This curriculum should take into account the knowledge prerequisites necessary to construct more complex theories from previously learned models. In addition, students should be tested to check that they have these prerequisites in order to improve their progress in learning scientific concepts. Supplemental Material The results from questionnaires Q1–Q5 are available in this issue of JCE Online. W

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