Chemistry students' conceptions of solubility: A phenomenography

LEARNING

James Stewart and Peter W. Hewson, Section Editors

Chemistrv Students’ Conceptions of Solubility: A Phenomenography

JAZLIN V. EBENEZER Faculty of Education, Department of Curriculum: Mathematics and Natural Sciences, The University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada.

GAALEN L. ERICKSON Faculty of Education, Department of Curriculum Studies, The University of British Columbia, Vancouver, British Columbia V6T 124, Canada; e-mail: [email protected]

This study identified a number of conceptions of solubility elicited from grade 11 students in individual interviews. These conceptions were grouped into six categories related to the students’ preferred explanations for solubility phenomena: (a) physical transformation from solid to liquid; (b) chemical transformation of solute; (c) density of solute; (d) amount of space available in solution; (e) properties of solute; and (0 size of solute particles. The findings from this study raised three general issues regarding chemistry learning. The first of these is the critical role played by students’ “everyday knowledge” in their understanding and interpretation of solution phenomena. A second issue was the tendency for students to extend their understanding of properties of materials at the macroscopic level to the microscopic level. And, finally, the issue of the discrepancy between the meanings implied by the students’ language and the teacher’s intended meanings through their use of a technical vocabulary. This article argues that an understanding of the typical conceptions used by students should form an integral component of chemistry teaching, both as points of origin for lesson planning and for the development of curricular materials. It also calls for more collaborative work between teachers and researchers to help facilitate a better understanding of student learning for all concerned. 0 1996 John Wiley & Sons, Inc.

Science Education 80(2): 181-201 (1996) 0 1996 John Wiley & Sons, Inc. CCC OO36-8326/96/02018 1-2 1

182 EBENEZER AND ERICKSON

CONTEXT OF CHEMISTRY TEACHING

Andrea was in a dazed and confused state one evening when her aunt found her desperately trying to make sense of a chapter on solutions in her grade 11 chemistry course. Scattered in front of Andrea were her chemistry textbook, pages and pages of neat notes given by the teacher, and chapter test review sheets on solution chemistry. The review sheets included questions such as: “Why does a NaCl solution conduct an electric current but a sugar solution does not?”; “Write equations for the dissolving of ammonium nitrate, lithium sulfate and calcium phosphate”; and “Calculate the concentration of all ions if 0.2 moles of FeCl, and 0.2 moles of MgC1, are dissolved to make 1 .O liter of solution.”

Andrea was a capable student who had worked hard and been maintaining an A av- erage throughout the difficult course, so it was surprising to see her in such a state of incomprehension. “I’m trying to make sense of all this balancing stuff (i.e., the sym- bols for the elements, ions, and their respective states), but visually and mentally it is making me dizzy. I just don’t understand!” To Andrea a complete balanced ionic equation was visually complicated and confusing because she did not understand why or how the elements acquired positive or negative charges and how this related to balancing ionic equations; for example, when should calcium nitrate be written as Ca(NO,), and when should it be written as Ca, (aq) + NO; (aq)? Her teacher’s notes contained the explanations for ionic equations but these notes did not help her to understand the equations in front of her. Also, she did not connect the teacher’s demonstration on the conductivity of various salts the previous day to ionic equations.

The foregoing vignette is an example of the predicament of many chemistry students when teachers organize their instructional activities from an expert’s point of view -giving detailed notes, conducting labs, and demonstrating experiments-all of which presuppose some important conceptual knowledge on the part of their students. But what is the nature of this knowledge and how might it influence chemistry instruction? This study begins to address these important questions by identifying a number of common student conceptions about the important phenomena of solubility. Furthermore, we identified several other potential sources of conceptual ambigu- ity which could account for some of the confusion experienced by students like Andrea.

STUDENTS’ CONCEPTIONS RESEARCH

For the last 15 years, student conception research has been pointing to the value of knowing and considering children’s conceptions and their unique ways of expressing them in science teaching. These studies include investigations of conceptions of physical phenomena held before, during, and after instruction. Researchers have reported that students’ ideas are often perceptually dominated, undifferentiated, driven by everyday language, inappropriately applied to situa- tions by using knowledge from previous learning, inconsistent with the principles taught in science class, and resistant to change in spite of instruction. (See Carmichael et al., [ 19901 and Pfundt & Duit [1994] for comprehensive bibliogra-

STUDENTS’ SOLUBILITY CONCEPTIONS 183

phies of the student conception literature; see also Solomon [1994] for a critique of this literature.)

More specific to the focus of the present study on solution chemistry, studies have been done on student understanding of matter (Ben-Zvi et al., 1986a; Brook et al., 1984; Dow et al., 1978; Novick & Nussbaum, 1978; Nussbaum, 1985; Renstrom, 1988), chemical reactions (Cachapuz & Martins, 1987; Stavridou & Solomonidou, 1989), and the process of dissolution (Abraham et al., 1994; Cosgrove & Osborne, 1981; Holding, 1987; Prieto et al., 1989). Within each of these topic areas researchers have given some consideration to student understanding of solubility, but many of the concepts related to solubility such as the solution process, solubility equilibrium, and miscibility of liquids have not been studied at all.

Why is it important to obtain a better understanding of students’ conceptions of solubility concepts? As suggested in the above vignette, the chapter in a grade 11 chemistry textbook on solution chemistry usually includes topics such as the behavior of solutions during phase changes, the concentration of solutions, electrical properties of liquids and solids, and the solubility of ionic compounds. Since the topic area of “solutions” and the conceptual and procedural knowledge associated with it is one of the major ideas in the study of chemistry, it is important that we obtain a better understanding of why so many students experience difficulty with this topic area.

STATEMENT OF THE PROBLEM

The present study was designed to document the conceptions that grade 11 students express about solubility phenomena. These conceptions were elicited by talking to students about several common phenomena where one or more substances are dissolved to create different solutions. From a pedagogical point of view, we think ihat a better understanding of students’ conceptions of solubility will provide important insights for developing more appropriate cumculum materials and instructional strategies.

The primary research questions of the study were:

1.

2. 3.

What are the most common conceptions of solubility found in grade 11 chemistry students prior to formal instruction on solution chemistry? Can these conceptions be grouped into meaningful categories? What are some of the pedagogically significant factors that are influencing the students’ understanding of solubility?

METHODS

The Pilot Study

The tasks and questions used in the interviews for this study were developed from an extensive set of pilot interviews with several different groups of individuals (e.g., two younger students [aged 12 and 151, ten preservice elementary teachers, and nine grade 11 students). In analyzing the results of these pilot interviews, important deci- sions were made about: the most appropriate tasks to use; the nature and sequencing


of the questions; and the time required to obtain reliable and valid responses from the subjects. Furthermore, the preliminary analyses of data from these pilot interviews provided some indication of the potential fruitfulness of the analytical methods that were eventually employed in coming to an understanding of the students’ conceptions in the main study.

The Present Study

Interviews. Thirteen grade 11 students (nine females and four males) in one chemistry class voluntarily participated in a 30-minute, audiotaped clinical interview which took place during the lunch hour and after school. This relatively short interview was designed to fit into the busy schedules of the students plus the tasks and questions had been refined and proved to be effective in eliciting students’ conceptions during the pilot interviews. Furthermore, the students were familiar with the researcher as she had been a frequent visitor to their classroom prior to the interviews.

The Tasks and Interview Questions. The demonstration tasks used to generate discussion in the interview consisted of three chemical systems’.

(a) Sugar/water (System A). (b) Water/alcohol/paint thinner (System B). (c) Salt/water (System C).

The first task (System A) consisted of introducing a cube of sugar into a beaker containing hot water. After allowing the student to observe the system for about a minute, the following types of questions were asked: (a) What might be happening 10 the sugar? and (b) Can you draw a picture to describe what is happening to the sugar in the water? Depending upon their responses, typical follow-up questions explored their understanding of terms or phrases such as: dissolving, breaking, diffusing, melting, transforming into a liquid, combining, disappearing, decomposing, falling apart, separating, mixing, attaching, reacting, becoming a solution, and particles.

System B consisted of pouring alcohol into a beaker of water, adding a drop of blue food coloring to this mixture for desired effect, and then pouring paint thinner into water/alcohol mixture. The student was allowed to invert the bottle to see if any mixing would take place. The initial questions pertaining to System B were: (a) Why are there two layers? and (b) What do you think is in the upper layer? Follow-up questions often included: Why do you think the paint thinner did not mix with the water/alcohol mixture?

System C consisted of a closed bottle containing a saturated solution of table salt with recrystallized salt settled out in the bottom. The beginning questions used with this system were: (a) Why has salt settled at the bottom? and (b) Is there salt in the liquid in the bottle?

‘The idea for using these chemical systems came from a chemistry teacher from Memphis, Tennessee, in a session at the “Chemical Education ’89” conference in Waterloo, Ontario.


Phenomenography: An Analytical Tool. The analytical framework used in this study is called “phenomenography” and it was developed by Ference Marton and colleagues over the last 20 years at Gothenburg University in Sweden (Marton, 198 1 , 1988, 1989; Marton & Saljo, 1984; Renstrom et al., 1989). Phenomenography is a study of how people experience and make sense of their encounters with the world. It is a research method which provides a way to identify, interpret, systematize, and describe the qualitatively different ways in which individuals experience phenomena (Linder, 1989; Renstrom et al., 1989).

The decision to use this analytical framework was based upon several considera- tions. First, in analyzing the pilot interview data the framework was very effective in generating the type of categories of student understanding that we were seeking; furthermore, this framework had been used successfully in an earlier study in our research group (Linder & Erickson, 1989). Second, the conceptual and methodological components of this approach have been well documented (Marton, 1988; Marton & Saljo, 1984; Renstrom, 1988). And finally, the underlying ontological assumptions made by this approach were considered to be preferable to those of other analytical schemes. For example, student “conceptions” are not considered to re- side “within” the minds of the students (as is the case for most cognitive science frameworks), but they represent one of many potential reltztions between an individual and the situation or context they are seeking to understand. Linder (1993), in making the case for the critical importance of context in the teaching and learning of science concepts, argues that in the student conceptions literature one can distin- guish between “a mental model- based perspective and an experientially-based perspective” (p. 294, emphasis his). He associates the latter with a phenomenographic approach and claims it leads to a more defensible view of conceptual change and of science instruction.

The task of phenomenography, then, is to identify the “qualitatively different ways” (Marton, 1981, 1984, 1986) of experiencing a given phenomena which oc- curs in some physical or cultural context. These qualitatively different ways of relating to phenomena are called “categories of description.” The extensive studies undertaken by Marton and his many colleagues, examining a wide variety of phenomena ranging from questions about economic topics to scientific topics, have always identified more than one such category of description-usually four to six. Marton metaphorically describes these categories as occupying a type of “outcome space” where a given individual can “move about” somewhat freely and can employ varying conceptions, depending upon their assessment of the contextual features for a particular setting. (See Linder & Erickson [ 19891 for examples of this variability.) Thus, one does not classify students as holding a particular conception or occupying a given category of description because there is always the potential for response variability within that student. These conceptual and methodological assumptions are much closer to some of the contemporary writings in the rapidly expanding fields of “social cognition” and “situated cognition” (Bereiter & Scardamalia, 1993; Brown et al., 1989; Hennessy, 1993; Lave & Wenger, 1991; Light & Butterworth, 1992; Wertsch, 1991) than to the more traditional models of cognition to be found in the study of student conceptions. (See the recent set of articles by Bereiter [1994], Cobb [1994], and Driver


et al. [ 1994al in Educational Researcher and Solomon [1994] for a discussion of the differences between “the personal” and “the sociocultural” perspective on knowledge construction.)

DATA ANALYSIS

Considering the three systems together, the outcome space for solubility consisted of six qualitatively different conceptions. Considered individually, two, four, and five qualitatively different categories were identified in the students’ responses to Systems A, B, and C, respectively. These categories of description are presented in Table 1.

Before discussing the significance of the results in the table, the categories of description will be outlined with illustrative interview excerpts. In the excerpts, “R” stands for the researcher and “S” stands for the student. The students are given pseu- donyms. An outline of the categories of description for the concept of solubility fol- lows.

1. Physical Transformation from Solid to Liquid

Many students viewed dissolving as a process of a solid transforming into a liquid form. Some students called this process “melting.” Two conceptions in this category were identified as students spoke about the transformation of a substance from the solid state into its liquid state. These can be characterized by: (a) a continuous view of the “liquid state” and (b) a particle view of the “liquid state.” Consider the following excerpt for the first conception, “a continuous view of the liquid state”:

Shamila [sugar/water] R: I am going to drop a cube of sugar. See what happens. S: Is it hot water?

TABLE 1 Frequency Distribution for Preinstructional Outcome Space of Solubility for System A,B B,b and Cc (n = 13)

Frequency

Categories of Description A B C

4 1

8 3 1 3 1 4 3

- Physical transformation from solid to liquid 10 Chemical transformation of solute 5 Density of solute - Amount of space in solution

Property of solute -

-

- - Size of solute -

aSystem A: sugar/water. bSystem B: water/alcohol/paint thinner. CSystem C: salvwater.


R: Yes, it is. Could you describe what might be happening? S: There are bubbles going up and sugar cube is melting. The sugar is going. It is

R: What do you mean by melting? S: It dissolved. It was a cube. When you dropped it in the water and like you see it,

it is falling apart. I think hot water is making it softer. It will be more stickier. Yeah, it will stick. Sugar melted somewhat like a syrup. That’s what I think.

melting practically. It is no longer a cube.

R: Let me stir this and let us see what happens. What is happening? S: There are no more crystals. Mixed in with hot water. R: What do you mean by saying “mixed in with the hot water?” S: It liquifies like the water. R: Do you think the sugar is in the liquid state? S: Yeah.

The foregoing excerpt is illustrative of students who had the notion that the solute (sugar or salt) when added to water melts and becomes a liquid. Students subscribing to this notion appear to have a “continuous liquid state” conceptualization. Shamila found it useful to think of the process of dissolving in terms of melting-the solid becoming a liquid-because she did not see the solid sugar any longer. She saw a liquid in the beaker. From experience, she knows that the water will taste sweet because of the dissolved sugar. To Shamila it seemed reasonable to state that solid sugar had been converted into its liquid form. Shamila also talked about hot water making solid sugar soft and sugar turning into a syrup, a liquid state. What is interesting here is how Shamila brought her everyday talk to a chemical system. For example, when a piece of candy is sucked, often children say that it is melting in the mouth. The candy becomes syrupy and sticky inside the mouth. Similarly, when sugar was put into hot water, several students stated that the sugar is melting.

When sugar dissolves in water, it is clear that the resulting solution is in liquid form. Often students seemed to be confused between the liquid-solution state and the liquid state of a pure substance, such as the melting of wax or ice. Shamila would seem to be one of these students as there appears to be some confusion in her mind between a true liquid state and a liquid-solution state.

Some students stepped into the microscopic world of particles in order to explain the process of dissolving. As is demonstrated in excerpts from Nila’s interview below, she used terms such as atoms, molecules, and particles to explain the suger/water and salt/water systems:

Nila [sugar/water] R: I have hot water here. I am going to add a cube of sugar. Could you tell me what

is happening? S: It is dissolving. R: What do you mean by dissolving?


S: It’s combining with water. It is mixing. It’s not like it started out with the solid. But the molecules are tightly packed together and when you mix it with the water, it mixes with water. Then when they are moving freely but not really becoming like a gas, really spaced out, it is in a different state. . . . They are all just mixing together in the liquid state.

Nila [salt/water] R: Could you describe what you see here? S: That is salt and water. Salt does not dissolve in water. Oh no, salt does. When

R: It was hot. S: What happens is when you put the salt in, it melts. Then when it cools, the

molecules cool down and they re-form in the bottom. So that is just a physical change from a solid to a liquid. Then it came back to the solid and then because (long pause) . . . I don’t know why, I guess some of it escaped and they came down.

you started with this, was the water hot or cold?

R: Why do you think some of it escaped? S: I think it was melted. Well, it was a liquid, right? Now that it is cooling, it’s all,

all the molecules are going crazy because they are hot and when they are cooling down, they are slowing down, I think. And when it is really hot some of them could evaporate, because they escape, they move so fast. They get pushed away by all the other molecules. And they get out and it cools off right away. They call it cool air.

R: Do you think there is any salt in this part of the jar? [points to liquid in the jar] S: No.

R: You think all the salt has come down? S: I think it has gone back to the original state. Some of it out of the water. Some of

it in the water. R: Is there anything happening between the water and the salt? S: Oh yes, there is water. The water molecules are moving around just like the solid.

They are still moving but so tightly packed, not like moving freely.

Nila distinguished the molecular arrangement in solid, liquid, and gas. She noted that, in a solid, molecules are tightly packed; in a liquid, molecules are moving freely; and, in a gas, the molecules are really spaced out-the standard textbook treatment of molecular motion in three states of matter. Nila considered that molecules are really spaced out. Nila considered that molecules of sugar are in move- ment when sugar is added to water. Therefore, she argued that sugar becomes a liquid.

With regard to the salt/water system, Nila stated that heat melted the salt. She figured that on cooling, salt will re-form. Her idea was that heat energy speeds the process of dissolving. Nila brought her understanding of kinetic molecular theory to bear upon the relationship between heat energy and dissolving. She stated that the molecules are “going crazy” when the salt is in the melted form (liquid), and when the molten salt cools, the molecules would slow down. One might also note that, in


her explanation involving the cooling down of the molecules, Nila used a very common strategy of ascribing macroscopic properties (i.e., temperature) to microscopic entities.

2. Chemical Transformation of Solute

As found in the pilot study, some students had the notion that when sugar is added to water some type of chemical reaction or combination is taking place. Close to half the students generated a conception that dissolving is a process of combining two or more substances. How do students picture this combination? Since they gave different versions for describing the nature of chemical transformation, two excerpts are presented to illustrate this category.

Attachment or Attraction between Components. When Sheila was asked about the sugar- water system, she spontaneously mentioned sugar and tea-apparently a system with which she had more direct experience.

Sheila [sugar/water] R: You said it is mixing, dissolving, it is a solution. What ideas do you have about

these words? S: Um, one thing mixes into another. Like it can be combined. R: What do you mean by combining? S: I don’t know. The sugar dissolving in tea. It mixes throughout. It does not stay

that way. . . . Oh yes, tea and everything like is attaching to another like a molecule.

R: Could you draw what you mean by attaching? S: (Draws). This is the tea, sugar, and it’s all put together. So that’s the way it looks

like in a cup. . . . the heat melted it. Combined. It will react with water and join with it. Going to the molecules of air that are empty.

R: In what way is it combining? S: Not absorbing. By mixing together they are becoming one. Not two anymore. R: Any new substance being formed? S: Yeah, sugar water (laughs). R: Sugar water is different from sugar and the water? S: Yeah, because sugar is no longer solid anymore. It has turned to gas. It’s more

like a liquid. Something like that.

Sheila explained her understanding of a solution with her own example, sugar dissolving in tea. She used words such as mixing, dissolving, and combining to describe the process when sugar was added to hot water. When asked to visualize the relationship between sugar and tea, Sheila immediately stated that the relationship between sugar and tea is like the attachments in a molecule. To show what she meant by attachments in a molecule, Sheila drew two circles (one large, one small) to indicate


Figure 1. Sheila’s drawing of a tea-sugar solution.

that one is for sugar and the other for tea (Fig.1.). She drew these circles attached to each other. Does it mean that Sheila’s notion of combination is the same as a molecule formation?

When probed again for the meaning of combination with sugar/water system, Sheila stated that heat melts sugar and then it combines with water, becoming one. She did not think that sugar and water are separate entities. According to Sheila, the new substance that formed was sugar-water. She emphasized that sugar is no longer a solid but is a liquid. According to Sheila, the change of state is another reason why sugar dissolving in water can be considered a chemical change. Sheila does not asso- ciate retrieval of the original substance as a characteristic of physical change. So Sheila continued to propose that the combination of sugar and water was chemical.

Solute Occupying Air Spaces in Wafer. Gary has a slightly different conception of how sugar “reacts” with water as illustrated below:

Gary [sugar/water] R: This is not water direct from the tap. I am introducing a cube of sugar. Can you

S: It is dissolving in the water. Eventually it will be all gone. R: What do you mean by dissolving? S: It will react with water and join with it. Going to the molecules of air that are

R: Sugar reacts with water and it fills the air spaces. How do you picture that? S: Because there is air in the water and sugar takes this place. R: Could you draw that for me? I’d like to see it. S: (Draws diagram shown in Figure 2). R: You think granules of sugar settle inside the air spaces in water? S: Mmm. R: What would you call this whole thing?

tell me what is happening to the sugar?

empty.


Figure 2. Gary’s drawing of sugar being dissolved in water.

S: Solution. R: Can you give me another example of solution? S: Salt and water. R: Same thing happens to salt and water? S: Basically, yeah. I think. R: Can you give other examples of solution? S: Like water-oxygen and hydrogen.

Gary proposed the notion that there are small pockets in water once occupied by air and sugar drives these out and occupies these empty spaces. This conception seems to be similar to that of Ulrika as described in Renstrom’s (1988) study. Ulrika, a grade 9 student, drew particles of salt with air between the particles when salt was dissolved in water. Using a diagram Gary showed how the combination takes place. Finally, Gary concluded that water is also a solution because oxygen and hydrogen are combined. In some of our earlier work we have observed that other students also consider water to be a solution. When one of these students was asked to explain which is the solvent and which is the solute in water, he suggested that hydrogen is the solvent because water has two parts of hydrogen and oxygen is the solute because water has one part of oxygen, which suggests the view that the solvent must be present in greater quantity.

3. Density of Solute

Difference in density between the two substances was given as a reason why different liquids did not combine (in System B) or why salt settled at the bottom of the jar. In the case of the water/alcohol/paint thinner mixture, no separation was seen when alcohol was added to water. However, when paint thinner was added to the water/alcohol mixture, it was observed to float on the mixture. The following examples reveal the conceptualization that solubility depends on the density of solute:


Tammy [water/alcohol/paint thinner] R: This is paint thinner. S: It went to the top like oil because it is lighter than water . . . R: So that is why you think they don’t mix together? S: They keep coming up to the top whereas salt, they always went down to the bot-

Tammy [salt/water] R: Here I have a jar containing salt and water. Salt has settled down. Can you de-

scribe what this is all about?

S: Salt is at the bottom of the water. Is heavier. Have more mass than the water . . . R: How come some went to the bottom and just a few are still in the water. S: Like some are little bit lighter than the others. It’s broken apart more than the

tom. It is heavier than water.

other one.

According to Tammy, a solute tends to seek the top or the bottom of the jar because of its weight. Tammy figured that the salt that is still in solution is lighter because of its “broken” state.

4. Amount of Space in Solution

Some students argued that substances do not dissolve because they do not find sufficient space in the dissolving medium. Consider the following example of this conceptualization:

Sheila [water/alcohol/paint thinner] R: Alcohol and water are soluble. Why isn’t paint thinner soluble? S: The particles are closer together than the paint thinner. That is why water and al-

cohol are soluble. They may be not attracted to each other [meaning watedalco- hol to paint thinner]. May be both of them are attracted [meaning water to alcohol].

R: Can you draw a picture of what you are telling me? S: There is the paint thinner. Then there is alcohol and water together. This stuff is

insoluble. I don’t know what it means. It is not able to mix. Maybe there is no room in the alcohol and water mixture for the particles to come in. There is no combination. It is not attracted towards them.

According to Sheila, there is no space available in the water/alcohol mixture for the paint thinner to lodge because the attraction or closeness between the particles of water and alcohol prevents another liquid that has no “attraction” toward water from finding space. She conjectured that because of the power of attraction between water and alcohol they are able to mix together; whereas water and paint thinner are not soluble because there is no attractive power between them. Thus, Sheila seems to


Figure 3. Sheila’s drawing of a water-alcohol and paint thinner solution.

have some rudimentary knowledge of the mechanism which a chemist might use in explaining the solution process of two liquids (Fig. 3).

5. Size of Solute

This category. which may be closely related to the one above, was articulated by one student when he argued that the size of the solute particles must be small enough for dissolution to take place. If the solute is broken into tiny bits, then it will dissolve in the solvent. Prior to the excerpt below, Ari argued that the paint thinner will not dissolve because it moved together and it will dissolve only if the paint thinnerJayer is broken up by heating or stirring.

Ari [water/alcohol/paint thinner] R: How does heat help to break things up? S: Heat breaks down the molecules that holds the paint thinner that separates the

R: Molecules are breaking into what? S: Into separate particles. Like, you know, how sticky these particles are when they

paint thinner from water.

are together. When it is heated, it breaks. That’s happens for it to go down.

This is being presented as a separate conception, at present, because it appears that the mechanism of dissolving (physical separation of the particular by heating) is different from the earlier conception. Ari seems to be suggesting that you need to break some substances (like paint thinner) into smaller particles by heating them up before they will dissolve. He also seems to be suggesting that a layer of molecules (whose origin is unknown) “separates” the paint thinner and the water and it is these molecules which must be broken down in order for the paint thinner “to go down” into the water. In either case, his conception is that smaller particles are required for dissolving to occur.


6. Property of Solute

Some students argued that, for a substance to dissolve in another substance, the solute must possess certain properties. In some instances these properties were very vague and their explanation almost seemed to be tautological as in the case of Bruce who invokes the need for a special “element” that mixes well with water:

Bruce [water/alcohol/paint thinner] R: Why do you think paint thinner did not dissolve in water? S: Because it does not have the element that mixes with the water very well. The el-

ement does not dissolve.

Other students proposed the idea that salt does not dissolve in water because of its pure, insoluble crystal state. Consider the following excerpt:

Sarah [salt/water] R: Why has salt settled down at the bottom? S: Maybe it is because it is stuff like elements, can’t break down. They are pure ele-

ments like gold. They can’t break down because they have-I don’t know what they have but they won’t break. This is in the pure state. So it won’t break down. You can’t make anything out of it, just like gold. You know, try heating it, shak- ing it, melting it, freezing it. We can’t make anything out of it. It is one of the pure stuff. You can’t.

R: Is salt a pure substance? S: Yeah. They are crystals. You can’t change.

Sarah referred to the salt that was formed at the bottom of the jar as pure crystals. Obviously, the salt crystals were hard, shining, and clear. Sarah also suggested that salt can be likened to a pure element, for instance, gold, which she associates with a chemical inactivity. In both of these examples, students attribute special properties to the solute to account for their observations of these systems.

Table 1 is a form of representation of Marton’s “outcome space.” As indicated earlier, any one student might invoke a different conception, depending upon the context under consideration. What we can tell from the frequency distribution of conceptions across the three different solution systems is that the sugar-water system (and to a lesser extent the salt-water system) tended to evoke conceptions (notably #1 and #2) which had to do with some type of transformation of a solid into a liquid form. This result, in itself, is not surprising for two reasons: first, similar results have been reported in earlier studies (e.g., Cosgrove & Osbome, 1981; Hess, 1987; Prieto et al., 1989); and second, the strong perceptual clues in this context of the solid sugar granules disappearing in the water readily suggests this type of conception (Driver, 1985). However, what the present study does demonstrate is the inherent variability of these conceptions across different contexts. For instance we might ask: Why did the other two systems elicit quite a different set of conceptions? It seems to us that the combination of the students’ prior “everyday experience” (Cole, 1990; Hatano & Inagaki, 1992; Gergen & Semin, 1990) with solutions and the different perceptual cues in


these tasks would account for much of this variability. So, even though Systems A and C are similar to the extent that both contain a crystalline solid dissolved in water, the visual impact of the salt crystals in the bottom could certainly evoke any one of the four latter conceptions listed in Table 1. The important role played by the context which elicits this type of conceptual variability, or “dispersion” as Linder ( 1993) calls it, has significant pedagogical implications which will be discussed in the next section.

DISCUSSION

These findings on students’ conceptions of solubility contribute to our understanding of some of the difficulties which students experience in their chemistry classes. Some of the pedagogical issues arising from the study can be grouped under three headings: (a) the relationship between student explanations and their experiences; (b) the tendency for students to extend macroscopic properties of matter to the microscopic level; and (c) the differences in meaning between the students’ use of chemical language and that used in their science classrooms. Each of these areas will be discussed below.

Student Explanations and Their Experiences

While the students’ conceptions identified in this study suggest that their explanations of solutions are based upon a combination of their everyday experiences with solutions and their school-science experiences, they tend to rely more on the former. Thus, their inclination is to focus on the seen and not on the unseen. For example, since the mixing of sugar and water yields a sugar solution which appears to be in a liquid state, several of the students proposed the idea that solid sugar is converted into liquid sugar. This conversion was compared to the process of melting; in this instance, the heat energy was supplied by the hot water. This conception is a reasonable one because students have no doubt had the experience of seeing substances such as wax and ice melting from their solid state to liquid state in the presence of heat. This finding is consistent with a finding of the Cosgrove and Osbome (1981) study which stated that over 25% of students used the words “melt” and “dissolve” synony- mously.

A second example is Sheila’s claim that the dissolving of sugar was a form of chemical combination of sugar and water to produce “sugar-water’’ in which the product is different from its constituents in physical appearance and also in taste. This claim is similar to the findings of Stavridou and Solomonidou (1989), and Prieto et al. (1989).

Such speculations about chemical reactions are extrapolations from students’ experiences with solutions. For most students, the visible characteristics of these chemical changes seem to guide their reasoning (Cosgrove & Osborne, 1981; Driver, 1985; Hess, 1987; Piaget & Inhelder, 1974). These studies also indicate that there are commonalties in students’ conceptions across age and sociocultural background. In school, students are expected to abandon their perceptually sensible models in favor of the more abstract models developed by scientists. For in-


stance, a particle model often contradicts one’s sensory perceptions of matter and therein lies many of the instructional difficulties (Novick & Nussbaum, 1978; Nussbaum, 1985).

Ben-Zvi et al. (1986b) have pointed out that the difficulties students have in adopt- ing the particulate model should not be surprising since it took scientists hundreds of years to develop our current model of matter. Furthermore, Butts and Smith’s (1987) results suggest that the interpretations of macroscopic observations in terms of atomic and molecular properties may be more difficult for students than teachers realize. The 16 and 17 year olds in this study attempted to explain the transformation of solid sugar to liquid sugar at the microscopic level but their explanations and dia- grammatic representations most often corresponded with what they could actually see rather than using a molecular or atomic model.

Extensions of Macroscopic Properties to the Microscopic Level

While this study did not focus explicitly on the students’ reasoning which generated the conceptions identified above, the interview data often provided some rich insights into the sources of these students’ conceptions. Because we think these insights may be pedagogically useful we include them in this study. Perhaps one of the most powerful influences on students’ reasoning in this domain is their tendency to use a strategy of extending the properties and behavior of substances at the macroscopic level to explaining phenomena at the microscopic level (Lijnse et al., 1990). Studies suggest that, for most students, a particle is a very small piece of matter; therefore, the properties of the particles are those of the matter (Ben-Zvi et al., 198Sa; Hess, 1987).

This study identified several examples in discussing solutions where students attributed macroscopic properties to the microscopic world. For example, one student explained the crystallization of salt at the bottom of the bottle of water by stating: “The molecules cool down and they [salt crystals] re-form in the bottom.” Here the student describes an energy change of the overall system in terms of individual molecules. This is similar to the kind of reasoning by students in Haggerty’s study (1986) concerning the melting of ice: “Ice melts because the particles become warmer and melt”; or “Warm air rises because the particles become lighter.”

These students’ conceptions of matter bear some resemblance to those held by some early Greek scholars which were essentially based on a continuous model of matter. They posited that even though matter could be divided into smaller and smaller pieces, these pieces, no matter how small, will retain the properties of the bulk material (Toulmin & Goodfield, 1962). Similar ideas held by the present chemistry students should not surprise us nor when we hear comments like: “all the molecules are going crazy because they are hot.” Given their extensive, previous linguistic and personal experiences with the behavior of physical objects, attributing the property of “hotness” to a molecule is perfectly reasonable. However, the interplay and distinction between macroscopic and microscopic properties is an important characteristic of chemistry and crucial for success in understanding contemporary chemical concepts.


Students’ Use of Chemical Language

In a chemistry class, students often experience conflicts because the linguistic ex- pressions and metaphors used by the teacher or textbook frequently evoke different meanings in the students. For example, in this study some students used the term “particles” for “granules” of sugar, whereas in the lexicon of the chemistry teacher, the term particle refers to “atoms,” “molecules,” or “ions.” In everyday language the word “particle” refers to a very small, visible piece of solid substance such as a gran- ule of sugar.

For most of their linguistic experiences students use language to communicate the meaning of some event, object, or feeling. What matters most is whether the people involved in the communication act understand what is being conveyed. In school, however, more importance is attached to mastering the meaning of terms and rela- tionships as coded in a specialized disciplinary language. The students are expected to learn this language and be fluent in it almost immediately, as illustrated in the vignette described at the beginning of this article. But for most students, expressing scientific ideas with their everyday vocabulary and thinking seems to be not only sensible but also sufficient and “good enough” (Hess, 1987). In a chemistry class, what students count as good enough may make science learning difficult.

Should the teachers strive to change the students’ everyday talk in their science classes? If so, how successful will the teachers be? Solomon (1983) distinguishes between the symbolic and life-world domains of knowledge and documents the difficulties students have in relating to the two. Similarly, although students’ imprecise use of language can be readily acknowledged, it may be neither realistic nor desirable to expect them to disassociate themselves from the language of everyday experience which is so much a part of their lives when they come into a science classroom. Students should, however, be made aware of the differences in the meanings based upon their everyday talk and the meanings embedded in the chemist’s language. Furthermore, they should learn why it is necessary to learn a distinctive language when studying a subject such as chemistry, and hopefully begin to understand how such a specialized language can eventually provide them with more powerful lenses for viewing their everyday world as well as the world contained in the chemistry classroom. (See Black & Lucas [1993], Linder [1993], and Mitchell [1993] for a more detailed discussion of these issues.)

CONCLUSIONS

This study has identified a number of different conceptions of solutions generated by students in an interview setting and organized these into six categories of description. These results address the first two research questions outlined above regarding the nature of students’ conceptions about solution chemistry prior to formal instruction. A further examination of these data revealed that there was consid- erable variability in the students’ use of these conceptions depending upon the nature of the task context being used in the interviews. These findings have some important implications, not just for the teaching of solution chemistry, but for science teaching in general-the third research question for this study. Three general,


pedagogical issues related to the research findings were identified in the Discussion section. While some of the students’ conceptions we identified have been reported by others, this study adds new conceptions to this literature, but more importantly, it has adopted a different methodological and conceptual perspective from earlier studies in this field. We argued that this phenomenographic framework not only provides a better description of the inherent variability and la- bility of student reasoning, but it also leads to different pedagogical implications. Thus, not only is it important for teachers to try and understand the nature of the prior conceptions that students like Andrea bring with them to the classroom, but they need to realize that these conceptions are strongly determined by the particular phenomenon, or contexts being used in the instructional setting. Furthermore, teachers should be aware that the language and visual representations that they use, as well as those in instructional materials like textbooks, multimedia materials, and so forth, are open to multiple interpretations by their students and a critical classroom activity ought to focus on the clarification and subsequent negotiation of meaning for this specialized language game (Mercer, 1992).

In making these remarks, we are acutely aware that we are adding even more cognitive and pedagogical responsibilities onto the backs of an already overworked classroom teacher. Hence, before making more recommendations to classroom teachers for improving teaching practices, we must ask first what are the responsibilities of the educational research community and the curriculum developers in this regard. Most importantly, we think that researchers need to break down many of the traditional barriers between teachers and researchers and, in so doing, enter into more collaborative teaching and research projects with teachers (Ebenezer, 1991; Erickson, 1991; Erickson et al., 1994). Such an approach should begin to blur the boundaries between teacher and researcher and should result in an enriched set of practices for both. There is also a need to develop better curriculum materials based upon our current understanding of both how students learn and what they have already learned prior to their formal school experiences. (See Driver et al. [1994] for a framework of such an en- deavor and an overview of the germane literature.) While the task of improving student learning of science content is extremely complex (Fensham et al., 1994), we are optimistic that some gradual progress is being made in the right direction.

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Accepted for publication 15 July 1995

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Chemistry students' conceptions of solubility: A phenomenography