University students' conceptions of bonding in melting and dissolving phenomena

Abstract

Undergraduate and graduate students' predictions and submicroscopic level explanations for the melting of four materials (salt, chalk, sugar, and butter), and for the mixing of these solutes in two solvents (water and cooking oil) were collected. Twenty-three undergraduate students and seven graduate students participated in the study, and data were collected through individual semi-structured interviews. The purpose of the study was to investigate students' conceptions of how bonding is involved in both melting and dissolving processes, as well as to investigate students' ideas of specific bonds and forces involved in melting and dissolving. Three types of misconceptions about bond breaking in the process of melting were revealed, as well as three types of misconceptions about bond breaking and three types of misconceptions about bond forming in the process of dissolving. Analysis indicated that students viewed the dissolving process as occurring via a single-step mechanism or via a multi-step mechanism. Analysis also revealed that students viewed bond breaking similarly in melting and dissolving processes, likely resulting in confusion between these two processes.

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Introduction

Chemistry is sometimes viewed as a difficult subject because it requires students to go between macroscopic (observable), submicroscopic, and symbolic views of matter (Johnstone, 1991). Submicroscopic (molecular level) views are particularly challenging because students must think about particles of matter which they cannot see. Many research studies have provided evidence for students' preconceptions and misconceptions of the submicroscopic world of chemistry, including areas such as bonding (Butts and Smith, 1987; Peterson and Treagust, 1989; Taber, 1994; Garnett et al., 1995; Taber, 1995; 1997; Boo, 1998; Robinson, 1998; Taber, 1998; Birk and Kurtz, 1999; Tan and Treagust, 1999; Barker and Millar, 2000; Harrison and Treagust, 2000; Coll and Taylor, 2001a; 2001b; Coll and Treagust, 2001; Niaz, 2001; Nicoll, 2001; Taber, 2001; Coll and Taylor, 2002; Coll and Treagust, 2002; Taber and Coll, 2002; Coll and Treagust, 2003; Özmen, 2004; Ünal et al., 2006), melting (Prieto et al., 1989; Griffiths and Preston, 1992; Lee et al., 1993; Ebenezer and Gaskell, 1995; Ebenezer and Erickson, 1996; Valanides, 2000; Goodwin, 2002; Uzuntiryaki and Geban, 2005; Pierri et al., 2008; Calik et al., 2010; Durmuş and Bayraktar, 2010), and dissolving (Fensham and Fensham, 1987; Prieto et al., 1989; Haidar and Abraham, 1991; Longden et al., 1991; Lee et al., 1993; Ebenezer and Erickson, 1996; Blanco and Prieto, 1997; Ahtee and Varjola, 1998; Ebenezer, 2001; Pınarbaşi and Canpolat, 2003; She, 2004; Çalik, 2005; Çalik and Ayas, 2005; Çalik et al., 2007a; Çalik et al., 2007b; Çalik, 2008).

Several reports have appeared in the literature concerning students' misconceptions on how bonding is involved in the process of melting. Authors have noted that some students viewed intramolecular forces or chemical bonds as being broken when a solid melts (Garnett et al., 1995; Othman et al., 2008; Pierri et al., 2008). Othman et al. (2008) also reported that some students conceptualized ionic compounds as containing atoms that became ions when the compound melted. Paik et al. (2004) pointed out that many studies on students' understanding of phase changes focus on evaporation and condensation, while few focus on melting. They proposed that the invisibility of gaseous particles leads to students having widely varying conceptions of evaporation and condensation, resulting in a wealth of research studies. Conversely, they also noted that studies on melting might be few because students may not have such varied conceptions of melting.

Articles on students' misconceptions on how bonding is involved in the process of dissolving have also appeared in the literature. Özmen et al. (2009) reported that some students viewed covalent bonds as being broken during the dissolving process, while Butts and Smith (1987) reported that students viewed sodium chloride as existing as NaCl molecules, which formed ions when the salt dissolved. Prieto et al. (1989) found that some students indicated that a solute breaks up when it dissolves, as has been reported by other researchers (Selley, 2000; Valanides, 2000; She, 2004). However, Selley (2000) pointed out that when students indicate that a solute breaks down, they may mean that a chunk of solute breaks down into smaller chunks, as opposed to individual particles. Haidar and Abraham (1991) also found that when considering sugar dissolving in water, some students viewed the initial step in dissolving as the sugar molecules separating from one another, which indicated a step-wise view of dissolving.

Several authors have also reported student misconceptions on bond formation in the process of dissolving. Studies have reported that some students consider a new substance to be formed after a solid dissolves (Prieto et al., 1989; Ebenezer and Erickson, 1996; Uzuntiryaki and Geban, 2005), such as a sugar–water substance formed when sugar dissolves in water. Other studies have reported that some students do not consider interactions between the solute and solvent when a substance dissolves (Prieto et al., 1989; Haidar and Abraham, 1991; Valanides, 2000), which led to students being unable to explain why some compounds dissolve and some do not. In addition, Goedhart and van Duin (1999) reported that some students, in deciding whether or not organic compounds would dissolve in water, did not recognize functional groups (such as OH-groups or NH-groups) but instead focused in on specific elements, such as oxygen, carbon, or hydrogen, in deciding about solubility.

Several studies have also appeared in the literature reporting on students' conceptions on how stirring and heating affect the process of dissolving. Blanco and Prieto (1997) examined the views of students on dissolving, and found that some students considered stirring and/or heating to be essential requirements for dissolving, as reported in other studies (Prieto et al., 1989; Haidar and Abraham, 1991; Ebenezer and Erickson, 1996). Blanco and Prieto (1997) also reported that some students viewed stirring and/or heat as causing salt to dissolve or disperse temporarily in water, but that the salt would settle out of solution shortly afterwards. The authors proposed that the students considered dissolving as dispersion, or a macroscopic mixture, as suggested earlier by Haidar and Abraham (1991).

Melting and dissolving are two of the fundamental processes discussed in general chemistry, but the processes are often confused (Prieto et al., 1989; Ebenezer and Gaskell, 1995; Ebenezer and Erickson, 1996; Valanides, 2000; Goodwin, 2002; Uzuntiryaki and Geban, 2005; Pierri et al., 2008; Çalik et al., 2010; Durmuş and Bayraktar, 2010). This confusion can be the result of how students view the processes of melting and dissolving on the submicroscopic level.

Theoretical perspective

The theoretical framework employed for this study was constructivism. The main idea behind constructivism is that knowledge is actively constructed by the learner, as opposed to being simply delivered or transmitted by others (Green and Gredler, 2002). Two main forms of constructivism served as the framework: Piagetian constructivism and social constructivism.

Piagetian constructivism followed from Jean Piaget's theory of cognitive development, in which he stated that an individual's cognitive development did not result simply from copying objects in the external world, but rather was constructed through interaction with the external world (Piaget, 1970). Piaget drew comparisons between the development and growth of biological organisms in response to their environments, and the cognitive development of individuals through interaction with their environments, through the processes of assimilation and accommodation. He used the term assimilation to refer to the integration of external elements into an individual's cognitive structures (also called schemes), such as when an individual acquires a new behavior, while accommodation occurred when a scheme or structure assimilated external elements, and was changed or modified by those external elements.

Social constructivism agreed with Piagetian constructivism in stating that knowledge was not transmitted to learners but instead constructed by learners, but it also emphasized the importance of social aspects in constructing knowledge (Driver et al., 1994). Important ideas in social constructivism are the issues of context (Brown et al., 1989), cognitive apprenticeships (Brown et al., 1989; Blumenfeld et al., 1997), and Vygotsky's zone of proximal development (1978); these ideas describe and illustrate social aspects of constructivist learning.

Purpose and research questions

The purpose of this study was to further explore student conceptions of bonding in melting and dissolving processes, and to focus in on students' confusion between melting and dissolving. As seen from the literature cited in the introductory paragraphs, students hold a variety of misconceptions related to the concepts of bonding, melting, and dissolving. These misconceptions are present even when these chemical concepts are taught in alignment with the current scientifically accepted views. Our theoretical perspective of constructivism lends insight into why these misconceptions are still present. Given that learners actively construct their own knowledge (Green and Gredler, 2002), students' conceptions should not be an exact copy of instructional material delivered in class, so some students should display misconceptions. In addition, students' prior experiences and existing cognitive structures result in a variety of ways of reconciling and understanding new material (Piaget, 1970), leading to a spectrum of student views ranging from scientifically accepted views to misconceptions. Furthermore, students' social interactions with peer students, teachers, and family can affect the knowledge and understanding they construct (Brown et al., 1989; Blumenfeld et al., 1997; Driver et al., 1994; Vygotsky, 1978), possibly leading to misconceptions.

Bonding is an important aspect of melting and dissolving processes, and the literature cited in the introductory paragraphs indicates that students hold a variety of misconceptions on how bonding is involved in melting and how bonding is involved in dissolving, and that students often confuse melting and dissolving processes. There is little literature that exists, however, that specifically addresses students' conceptions of how bonding is involved in both melting and dissolving processes, and how these conceptions might impact students' confusion between melting and dissolving processes. In addition, most of the studies in the literature discuss bonding interactions in melting and dissolving in very general terms, such as the break down of solute or solute–solvent interactions. There is little literature that exists that reports specific chemical terms students use to discuss the nature of the bonding interactions (ionic bonds, hydrogen bond, dipole–dipole forces, etc.) involved in the break down of solute, or the nature of bonding interactions (ion–dipole forces, hydrogen bonds, dipole–dipole forces, etc.) involved in solute–solvent interactions. This study sought to further investigate these issues, and as such our research questions were: (a) How do freshman and graduate chemistry students apply their knowledge of bonding theory to explain their predictions of the melting of solids? (b) How do freshman and graduate chemistry students apply their knowledge of bonding theory to explain their predictions of the dissolving of solutes in solvents?

Methods

Participants

This study examined student conceptions of bonding in melting and dissolving processes. Seven freshman undergraduate students (U1) were interviewed in an initial summer session, seven graduate students (G) were interviewed in the following fall semester, and sixteen additional freshman undergraduate students (U2) were interviewed in the following summer session; this resulted in a total of thirty participants with a mixture of undergraduate and graduate students to allow for comparison.

All the undergraduate students were enrolled in the second course (CHM 116) of the main introductory general chemistry course sequence. In the first course (CHM 115) of the main introductory general chemistry course sequence they had formal classroom exposure to concepts involving bonding theory. The second course of the main introductory general chemistry course sequence began with topics focused on intermolecular forces, solubility, and solutions. The undergraduate students were all interviewed within three weeks after they had formal classroom exposure to intermolecular forces, solubility, and solutions in the second course of the main introductory general chemistry course. All the graduate students had previously served as teaching assistants for the second course (CHM 116) of the main introductory general chemistry course sequence, ensuring that they also had formal exposure to intermolecular forces, solubility, and solutions. These criteria for the undergraduate and graduate participants ensured that they had been exposed to the concepts of bonding, melting, and dissolving, and thus were appropriate participants to interview about their conceptions of bonding in melting and dissolving processes.

Data collection

All participants were given individual semi-structured interviews, which were audiotaped, and which ranged from 38–124 min in length. The interview protocol was pilot tested and several modifications were made and incorporated into this study.

The initial part of the interviews consisted partly of a series of demographic questions to collect information on the students' ages, majors, and chemistry course history. This initial part of the interviews also involved a series of questions probing the students' background understanding of the relevant concepts of “melting”, “dissolving”, “mixture”, “solution”, “matter”, “ionic bonding”, “covalent bonding”, and “intermolecular forces”.

The next part of the interviews consisted of two sets of scenarios related to melting. The first set of scenarios involved having a small sample of four different materials (salt, chalk, sugar, and butter) simply sitting in a dry beaker at room temperature. The participants gave their predicted observations and submicroscopic level explanations on what they thought would happen with each material in turn. The second set of scenarios involved trying to melt a small sample of each of the materials in a dry beaker. The participants gave their suggestions on how to melt the samples, as well as their predicted observations and submicroscopic level explanations on what they thought would happen with each material in turn. The participants then observed actual experiments of each of the scenarios, and gave explanations for any discrepancies between their predictions and their observations.

The following part of the interviews consisted of two sets of scenarios related to dissolving. The first set of scenarios involved placing a small amount of each of four solutes (salt, chalk, sugar, and butter) into beakers containing water at room temperature, without stirring the solutes in the beakers of water. The participants gave their predicted observations and submicroscopic level explanations on what they thought would happen with each mixture in turn; this scenario was then repeated using cooking oil instead of water. The second set of scenarios involved stirring each beaker of water containing a small amount of each of the solutes that had been placed into the beakers of water. The participants gave their predicted observations and submicroscopic level explanations on what they thought would happen with each mixture in turn; this scenario was then repeated using cooking oil instead of water. The participants then observed actual experiments of each of the scenarios, and gave explanations for any discrepancies between their predictions and their observations. The participants also gave their predicted observations and submicroscopic level explanations if hot water and oil had been used instead of room temperature water and oil.

In addition, chemical formulas and structures of the materials used in the scenarios were supplied to the sixteen freshman undergraduate students interviewed in the second summer session. The participants' responses and explanations were analyzed in order to determine how they viewed bonding in the melting and dissolving processes.

Results and discussion

Importance of bonding in melting and dissolving in general

During the initial part of the interviews, the students were asked to discuss their general understanding of the terms “melting” and “dissolving”. In total, 17 of the participants (3 U1, 7 G, 7 U2) indicated that bonds or forces would be broken during melting. In addition, 21 of the participants (4 U1, 5 G, 12 U2) indicated that bonds or forces in the solute would be broken during dissolving. Furthermore, 12 of the participants (4 U1, 2 G, 6 U2) said that bonds or forces would be formed between the solute and solvent during dissolving. These results indicate that even before being asked about melting or dissolving specific compounds, many of the participants generally viewed the breaking and forming of bonds and forces as important aspects of both the melting and dissolving processes.

Misconceptions on bond breaking in melting and dissolving

Throughout the interviews, the participants identified a wide variety of bonds and forces in the compounds (salt, chalk, sugar, and butter) that they predicted would be broken or weakened during melting by the application of heat; a listing of these bonds and forces is available in Table 1. Expected responses included the ionic bond between sodium and chloride ions in salt; the ionic bond between calcium and carbonate ions in chalk, the carbon–oxygen bond in the carbonate ion in chalk, or the ionic bond between calcium and oxygen ions in chalk; hydrogen bonds, dispersion forces, van der Waals forces, or dipole–dipole forces between sugar molecules; dispersion forces or van der Waals forces between butter molecules. The entries in Table 1, and in the rest of the Tables, are a reflection of the students' responses, and capture the range of ideas expressed by the students. These responses revealed several misconceptions, including:

Table 1 Types of bonds and forces predicted to break during melting

Salt	Chalk	Sugar	Butter
a Unacceptable statements/misconceptions.
Ionic bond between Na^+ and Cl^−	Bonds between all atoms in chalk^a	Bonds between all atoms in sugar^a	Bonds between all atoms in butter^a
Bond between NaCl molecules^a	Bond between Ca^2+ and CO3^2^−	Bonds between sugar molecules	Carbon–hydrogen bonds in butter^a
Intermolecular forces between NaCl molecules^a	Bond between chalk molecules^a	Bonds between the rings in sugar molecules^a	Carbon–carbon bonds in butter^a
Dipole–dipole forces between NaCl molecules^a	Intermolecular forces between chalk molecules^a	Covalent bonds within sugar molecules^a	Carbon–oxygen bonds in butter^a
Van der Waals forces between NaCl molecules^a	Dipole–dipole forces between chalk molecules^a	Intermolecular forces between sugar molecules	Carbon–carbon bonds between the chains in butter molecules^a
		Dispersion forces between sugar molecules	Bonds between butter molecules
		Hydrogen bonds between sugar molecules	Intermolecular forces between butter molecules
		Dipole–dipole forces between sugar molecules	Van der Waals forces between butter molecules
			Hydrogen bonds between butter molecules^a
			Dispersion forces between butter molecules
			Dipole-induced dipole forces between butter molecules^a

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(a) Salt and chalk are composed of discrete molecules. For example, 1 U1 student and 1 U2 student indicated that intermolecular forces between NaCl molecules would break, while 1 U1 student, 1 G student, and 2 U2 students indicated that the bond between chalk molecules would break. As an example of this type of misconception, one student said that if chalk is heated to melt it “enough energy is added so that um the bonds that hold each molecule together in that structured form uh begin to break”.

(b) Covalent bonds in sugar and butter are broken during melting. For example, 2 U1 students and 1 U2 student indicated that the bonds between all atoms in sugar would break, while 2 U2 students indicated that the carbon–oxygen bonds in butter would break. As an example of this type of misconception, one student said that if sugar is heated “then you'd start seeing carbon–carbon bonds breaking down, the actual sugar molecules breaking apart down to their constituent atoms and then after a while I think you'd just see lots of carbons”.

(c) Misidentification of intermolecular forces between butter molecules. For example, 1 G student indicated that there were hydrogen bonds between butter molecules, saying that when butter begins to melt, the fatty acid chains in butter are “all associated and closely packed together by nature of hydrogen bonding, so I would say that some of them are sort of loosely hydrogen bonded to one another”.

The participants also identified a wide variety of bonds and forces in the solutes (salt, chalk, sugar, and water) that they predicted would be broken during dissolving in the solvents (water and cooking oil); a listing of these bonds and forces is available in Table 2. Expected responses included the ionic bond between sodium and chloride ions in salt; no bonds in chalk; hydrogen bonds, dispersion forces, van der Waals forces, or dipole–dipole forces between sugar molecules; dispersion forces or van der Waals forces between butter molecules. The students' responses also revealed several misconceptions, including:

Table 2 Types of bonds and forces predicted to break in the solutes after stirring in the solvents

Salt in water	Chalk in water	Sugar in water	Butter in water
Ionic bond between Na^+ and Cl^−	Ionic bond between Ca^2+ and CO3^2^−	Intermolecular forces between sugar molecules	Bonds between all atoms in butter^a
Intermolecular forces between salt molecules^a	Ion-induced forces between chalk molecules^a	Hydrogen bonds between sugar molecules	Carbon–carbon bonds between chains in the butter molecules^a
		Bonds within sugar molecules^a	Covalent bonds between butter molecules^a
		Oxygen–hydrogen bonds in sugar^a
		Carbon–hydrogen bonds in sugar^a
		Bonds between all atoms in sugar^a
		Bonds between the rings in sugar molecules^a

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Salt in oil	Chalk in oil	Sugar in oil	Butter in oil
a Unacceptable statements/misconceptions.
Ionic bond between Na^+ and Cl^−	Ionic bond between Ca^2+ and CO3^2^−	Intermolecular forces between sugar molecules	Intermolecular forces between butter molecules
Ion-induced intermolecular forces between salt molecules^a	Ion-induced forces between chalk molecules^a	Bonds between the rings in sugar molecules^a	Carbon–carbon bonds between chains in the butter molecules^a
	Forces between chalk molecules^a		Hydrogen bonds in butter^a
	Bonds between all atoms in chalk^a		Bonds break to form ions^a

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(a) Salt and chalk are composed of discrete molecules. For example, 1 U2 student indicated that the ion-induced intermolecular forces between salt molecules would break when salt dissolved in oil, while 1 U1 student indicated that forces between chalk molecules would break when chalk dissolved in oil, saying that when chalk dissolves in oil “the chalk molecules will break apart and um dissociate within the oil molecules”.

(b) Covalent bonds in chalk, sugar, and butter are broken during the process of dissolving. For example, 1 U2 student indicated that bonds between all atoms in chalk would break when chalk dissolved in oil, while 1 U2 student indicated that the bonds between the rings in sugar molecules would break when sugar dissolved in water, saying that when sugar dissolves in water “the oxygen that's holding the two different rings together… that can be broken I feel”.

(c) Misidentification of intermolecular forces in salt, chalk, and butter. For example, 1 U2 student indicated that there were ion-induced forces between chalk molecules, saying that chalk dissolves in water when it's stirred because in chalk in “the solid state that it's in um the ion-induced molecular forces that are holding that calcium carbonate are not strong enough to withstand the uh the stirring”.

These results revealed that students held similar types of misconceptions when considering bond breaking in the processes of melting and dissolving. This finding indicated that students viewed the bond breaking aspect of melting and dissolving similarly, which may partially explain why some students confuse melting with dissolving. In addition, these results highlighted the importance of students having a solid understanding of bonding theory in order to apply it to processes such as melting and dissolving.

The misconception of ionic compounds being composed of discrete molecules has previously been reported in the literature (Butts and Smith, 1987; Boo, 1998; Barker and Millar, 2000). This study extended the ionic compounds investigated to include chalk. Reports have also appeared in the literature indicating that students hold the misconception that covalent bonds are broken during melting (Garnett et al., 1995; Othman et al., 2008; Pierri et al., 2008) and during dissolving (Ozmen et al., 2009). This study extended the range of compounds investigated in the literature, and focused in on revealing specific bonds within the chemical structures that the students viewed as being broken. In addition, there is little literature that exists that reports specific chemical terms students use to discuss the nature of the bonding interactions (ionic bonds, hydrogen bond, dipole–dipole forces, etc.) involved in the break down of solute particles due to melting and dissolving. This study began to address this void.

Misconceptions on bond forming in dissolving

The participants also identified a wide variety of bonds and forces between the solutes (salt, chalk, sugar, and water) and the solvents (water and cooking oil) that they predicted would be formed during the dissolving process; a listing of these bonds and forces is available in Table 3. Expected responses included ion–dipole forces between sodium ions (from salt) and water as well as ion–dipole forces between chloride ions (from salt) and water; no forces between chalk and the solvents; hydrogen bonds and dipole–dipole forces between sugar molecules and water; dispersion forces between butter molecules and oil. The students' responses also revealed several misconceptions, including:

Table 3 Types of bonds and attractions predicted to form between the solutes and the solvents after stirring

Salt and water	Chalk and water	Sugar and water	Butter and water
Positive/hydrogen end of water and Cl^− from salt	Positive/hydrogen end of water and CO3^2^− from chalk^a	Attraction between oxygen in water and hydrogen in sugar	Bonds between hydrogens in water and oxygens in butter^a
Negative/oxygen end of water and Na^+ from salt	Negative/oxygen end of water and Ca^2+ from chalk^a	Attraction between hydrogens in water and oxygens in sugar	Bonds between water and carbons, hydrogens, oxygens in butter^a
Ion–dipole forces between water and salt ions	Hydrogen from water and carbon of chalk^a	Hydrogen bonds between sugar and water	Hydrogen bonds between oxygens in butter and hydrogens in water^a
Intermolecular forces between water and salt	Oxygen from water and chalk molecules^a	Polar attractions between sugar and water	Hydrogen bonds between hydrogens in butter and oxygens in water^a
	Hydrogen bonds between hydrogen from water and oxygen from chalk^a	Oxygens/hydrogens in water and carbons in sugar^a
	Dipole-induced forces between chalk and water^a
	Ion–dipole forces between hydrogen from water and carbonate from chalk^a
	Ion–dipole forces between oxygen from water and calcium from chalk^a

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Salt and oil	Chalk and oil	Sugar and oil	Butter and oil
a Unacceptable statements/misconceptions.
	Attraction between negative oxygen from chalk and positive hydrogens in oil^a	Nonpolar interactions between sugar and oil^a	Hydrogen bonds between butter and oil^a
	Induced dipole forces between chalk and oil^a	Hydrogen bonds between hydrogens in oil and oxygens in sugar^a	Dispersion forces between butter and oil
	Ionic bonds between chalk ions and oil^a	Hydrogen bonds between oxygens in oil and hydrogens in sugar^a	Non-polar interactions between oil and butter
	Covalent bonds between oxygens in chalk and hydrogens in oil^a		Dipole-induced forces between oil and butter^a
	Covalent bonds between carbons in chalk and carbons in oil^a		Dipole-induced dipole forces between oil and butter^a
	Bonds between all atoms in chalk and all atoms in oil^a		Attraction between double bonds in butter and oil^a
			Bonds between oxygens in butter and oxygens in oil^a
			Bonds between hydrogens in butter and carbons in oil^a
			Bonds between all atoms in butter and all atoms in oil^a

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(a) Covalent bonds are formed between the solute and the solvent during the process of dissolving. For example, one U2 student indicated that the oxygen and hydrogen atoms in water would form bonds with the carbon atoms in sugar, saying that when sugar dissolves in water “the carbons [on the sugar], it can attach, I think it can attach to any of ‘em, like the oxygen or the hydrogen [on the water]”.

(b) Misidentification of intermolecular forces formed between the solutes and solvents. For example, 1 U1 student indicated that nonpolar interactions would occur between sugar and oil, saying that when sugar dissolves in oil “I think that the sugars have non-polar characteristics, they have a lot of carbon to carbon bonds and uh s- yeah fatty acid, the tails are all non-polar so it's possible that there could be interaction there”.

(c) Identifying intermolecular forces through the presence of single atoms. For example, 1 U2 student indicated that hydrogen bonds would form between the hydrogen atoms of butter and the oxygen atoms of water, saying that when butter dissolves in water “definitely the hydrogen and the oxygen part [of butter]… would form the biggest interaction with water obviously because of hydrogen bonding, it just has that potential to hydrogen bond”.

These results again revealed the importance of students having a solid understanding of bonding theory in order to apply it to processes such as melting and dissolving. The misconception of covalent bonds being formed between the solute and solvent during the process of dissolving has previously been reported in the literature by studies reporting that some students consider a new substance to be formed after a solid dissolves (Prieto et al., 1989; Ebenezer and Erickson, 1996; Uzuntiryaki and Geban, 2005). This study extended the range of compounds investigated in the literature, and focused in on identifying specific bonds within the chemical structures that the students viewed as being formed. There is little literature that exists that reports specific chemical terms students use to discuss the nature of the bonding interactions (ionic bonds, hydrogen bond, dipole–dipole forces, etc.) involved in the solute–solvent interactions formed in the dissolving process. This study began to address this void. In addition, there is little literature that exists that details students' views of how aspects of chemical structure affect the dissolving process (see for example Goedhart and van Duin, 1999). This study began to address this void as well.

The range of misconceptions on bond breaking in melting and dissolving processes, and bond forming in dissolving processes, is not completely unexpected, given our theoretical perspective of constructivism. Indeed, there is a variety of ways for students to reconcile and understand new material (Piaget, 1970), and this study reinforces the importance of investigating student conceptions, and providing suggestions to alleviate student misconceptions.

How bonds and forces in solutes break during dissolving

Throughout the interviews, the participants gave various mechanisms by which the bonds or forces in the solutes would be broken in the process of dissolving. These mechanisms included:

(a) Bonds and forces in solutes being broken or overcome by the forces in the solvents. As an example, one student said that if salt is placed into water “the forces in the water would uh maybe des-make the salt break apart so it would dissolve… forces are from the water like uh the dipole forces”.

(b) Bonds and forces in solutes being broken by collisions between particles of solutes and solvent molecules. As an example, one student said that if chalk is stirred in water “when you stir it up, the molecules are gonna bump into each other and it's going to separate the Ca from the other, I guess from the CO₃and dissociate into the water and be surrounded by water molecules”.

(c) Bonds and forces in solutes being broken directly by stirring. As an example, one student said that if salt is stirred in water “I think the stirring rod helps the sodium and the chlorine, sodium chloride ions kind of break apart and the water make room for them… and start to dissolve them”.

(d) Bonds and forces in solutes broken directly by heat. As an example, one student said that if chalk is placed into hot water to dissolve it “I think hot water helps because I know hot water, when something when you warm up a reaction it always goes faster, um so I feel maybe hot water would have helped, be a catalyst for the reaction… I think maybe just the warm water would have helped again break the bonds with the, with calcium carbonate”.

These predictions are summarized in Table 4.

Table 4 Mechanisms by which bonds and forces in the solutes break during dissolving

Before Stirring at Room Temperature: Bonds and Forces in Solutes

Broken/overcome by forces in solvents

Broken by collisions between particles of solutes and solvent molecules

After Stirring at Room Temperature: Bonds and Forces in Solutes

Broken/overcome by forces in solvents

Broken by collisions between particles of solutes and solvents caused by stirring

Broken directly by stirring

Using Hot Solvents : Bonds and Forces in Solutes

Broken/overcome by forces in solvents

Broken by collisions between particles of solutes and solvents caused by heat

Broken directly by heat

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These responses indicated that some of the participants viewed the bond breaking process on the particulate level, through the interaction between particles of the solutes and the solvents, whereby the bonds and forces in the solute were either broken or overcome by the forces in the solvents, or the bonds and forces in the solute were broken by collisions between solute and solvent particles. Other participants viewed the bond breaking process on the macroscopic level, with the actual act of stirring or heating causing the bonds in the solutes to be broken.

Several studies in the literature have previously reported that some students considered heating and/or stirring as a requirement for dissolving (Prieto et al., 1989; Haidar and Abraham, 1991; Ebenezer and Erickson, 1996; Blanco and Prieto, 1997). The findings in this study may further explain why some students view heat and/or stirring as a requirement for dissolving—since some students view the bond breaking aspect of melting and dissolving similarly, then actions that cause bonds and forces to break resulting in melting, such as heating, may be viewed as necessary for causing solute bonds and forces to break resulting in dissolving.

When students say “Melt” but really mean “Dissolve”

Throughout the interviews there were 3 graduate student and 12 undergraduate predictions or explanations that were referred to as “melting” by the students but explained or discussed in terms of “dissolving”. We interpreted this mislabeling as due to how the participants viewed the process of dissolving.

During the interviews, the participants identified various ways (as discussed above) in which solute bonds/forces could break in the process of dissolving: (a) bonds and forces in solutes are broken or overcome by the forces in the solvents; (b) bonds and forces in solutes are broken by collisions between particles of solutes and solvent molecules; (c) bonds and forces in solutes are broken directly by stirring; (d) bonds and forces in solutes broken directly by heat. The last three of these mechanisms of solute bond/force breaking involved momentum (i.e., collisions between particles or stirring) and heat energy. These mechanisms focused only on breaking the bonds and forces in the solutes, and did not address the aspect of the dissolving process which involves forming intermolecular forces between solute and solvent particles. As such, these mechanisms indicated a multi-step view of the dissolving process: the first step to break the solute bonds and forces, and possible subsequent steps such as to form solute–solvent intermolecular forces, or disperse the solute in the solvent.

The first mechanism listed above involved bonding interactions between the solute and the solvent. As such, this mechanism indicated a single-step view of the dissolving process, in which the solute bonds and forces were broken and the solute–solvent intermolecular forces were formed simultaneously. As such, these results indicated that students viewed dissolving as occurring in one of two ways: via a single-step mechanism or a multi-step mechanism. Haidar and Abraham (1991) reported that when considering sugar dissolving in water, some students viewed the initial step in dissolving as the sugar molecules separating from one another, which indicated a step-wise view of dissolving; this study extended this step-wise view of dissolving to include a single-step mechanism as well as a multi-step mechanism.

In terms of students saying “melting” when they were really referring to dissolving, since bond and force breaking was an important aspect of both melting and dissolving, when participants said “melting” they were actually likely referring to bond and force breaking within the solute as a first step in the dissolving process. As such, these students would have viewed the process of dissolving as essentially the process of melting followed by possible subsequent steps such as to form solute–solvent intermolecular forces, or disperse the solute in the solvent. This explanation could provide insight into why students confuse the terms melting and dissolving, as previously reported in the literature (Prieto et al., 1989; Ebenezer and Gaskell, 1995; Ebenezer and Erickson, 1996; Valanides, 2000; Goodwin, 2002; Uzuntiryaki and Geban, 2005; Pierri et al., 2008; Calik et al., 2010; Durmuş and Bayraktar, 2010).

Comparison of U1 Students and U2 Students

The U2 group of students was supplied with the chemical formulas and structures of the materials, whereas the U1 group of students was not supplied with this information. There were no clear trends in differences in the percentages of U1 and U2 students giving the various predictions and explanations throughout the interviews, indicating that the chemical formulas and structures were not a clear factor in this regard. However, the chemical formulas and structures did allow for the U2 students to be detailed and specific in their considerations of which bonds and forces were involved in breaking and forming.

Comparison of undergraduate students and graduate students

The graduate students showed more experience or understanding of dissolving concepts through their responses, as was expected. There were 7 graduate students and 23 undergraduate students involved in the interviews. One aspect of this greater understanding was indicated in one of the beginning segments of the interviews, when the participants were asked to predict what they would observe if samples of the four solids, salt, chalk, sugar, and butter were left simply sitting in clean, dry beakers. Several of the participants predicted that the compounds would begin to dissolve in atmospheric water, or would begin to absorb water from the atmosphere. Nine participants made these predictions for salt (2 U1, 6 G, 1 U2), one participant for chalk (1 G), and five participants for sugar (4 G, 1 U2). Most of these predictions were made by the graduate students, who numbered less than the undergraduate participants. This result indicated that the graduate students tended to consider the effects of unseen atmospheric water more than the undergraduates, suggesting that the graduate students had more experience or understanding of dissolving concepts than the undergraduate students, as expected.

In addition, on average, 12 of the undergraduate students and 3 of the graduate students predicted that the solutes would dissolve in the solvents after stirring, compared to 7 of the undergraduate students and 3 of the graduate students who predicted that the solutes would dissolve in the solvents even without stirring. These numbers indicated that a much greater proportion of undergraduates viewed stirring as being able to cause the compounds to dissolve in the solvents, compared to the graduates.

Similarly, on average, 15 of the undergraduate students and 4 of the graduate students predicted that the solutes would dissolve in the hot solvents, compared to 7 of the undergraduate students and 3 of the graduate students who predicted that the solutes would dissolve at room temperature without stirring. These numbers also indicated that a greater proportion of undergraduates viewed heat as being able to cause the compounds to dissolve in the solvents, compared to the graduates.

These data indicated that the undergraduate students lent more importance to the external acts of stirring and heating than did the graduate students. Once again, this difference indicated that the graduate students had more experience or understanding of the nature of solubility and factors affecting solubility, as expected.

Conversely, throughout the interviews the graduate students did exhibit various bonding misconceptions, some of which were shared by the undergraduate students. This result highlights the tenacity with which some of the misconceptions are held.

Conclusions and implications for teaching

Bonding aspects of melting

This study revealed three types of misconceptions about aspects of bonding when considering melting. The students viewed ionic compounds as being composed of discrete molecules, they considered various covalent bonds as the bonds being broken during melting, and they misidentified intermolecular forces disrupted during melting. The bonding misconceptions identified here can be used to help instructors become more aware of the types of ideas students hold, and to try to address these ideas.

In order to address these misconceptions, instructors can initiate interventions both on the submicroscopic level and the macroscopic level, in order to help students make the connections between the different levels. In order to address the misconception of ionic compounds being composed of discrete molecules, for instance, for the submicroscopic level instructors can use model kits and animations to illustrate that when ionic compounds melt, the crystal lattice is destroyed and the ions pre-existing in the solid crystal become mobile, versus intact discrete molecules. On the macroscopic level instructors can use demonstrations (or videos of demonstrations) of an ionic liquid—an ionic compound with a relatively low melting point (perhaps ∼100 °C)—both in the solid state and being melted. The conductivity of the ionic liquid can be demonstrated to show that indeed, when an ionic compound melts, mobile ions result, versus intact discrete molecules.

In order to address the misconception of covalent bonds being broken during melting, for example, for the submicroscopic level instructors can again use model kits and animations to show that the molecules in covalent compounds remain intact when the compounds melt, and that the intermolecular forces between the molecules are broken, as opposed to the covalent bonds within the molecules. On the macroscopic level instructors can use a demonstration in which menthol is gently heated to melt it, and it is allowed to cool. The instructors can then show students that the menthol went through the reversible physical change of melting. Instructors can present an argument to students that if covalent bonds were indeed broken during melting, then free radicals would presumably have been formed, which would be very reactive and would react with nearby species to produce new compounds, meaning that one should not get menthol back in the end.

In order to address the misconception of various misidentified intermolecular forces being broken during melting, for instance, for the submicroscopic level instructors can use Lewis structures and model kits to illustrate the types of intermolecular forces existing between molecules. On the macroscopic level, instructors can use Lewis structures discussed in the context of properties such as boiling points and polarities, to illustrate the effects that different types of intermolecular forces have on various properties.

Bonding aspects of dissolving

This study revealed six types of misconceptions about aspects of bonding when considering dissolving. In terms of bond breaking, the students viewed ionic compounds as being composed of discrete molecules, they considered covalent bonds as the bonds being broken during dissolving, and they misidentified intermolecular forces disrupted during dissolving. In terms of bond forming, the students misidentified intermolecular forces formed during dissolving, they considered covalent bonds as the bonds being formed during dissolving, and they narrowly focused on specific atoms in order to identify intermolecular forces. These bonding misconceptions can help educators become aware of the range of ideas held by students about bonding, and can be used to discuss and address faulty ideas.

In order to address these misconceptions, educators can again initiate interventions both on the submicroscopic level and the macroscopic level, in order to help students make the connections between the different levels. The three types of misconceptions on bond breaking are similar to the misconceptions on bond breaking in melting, and the suggestions outlined above can serve here as well.

In order to address the misconception of various misidentified intermolecular forces being formed during dissolving, for instance, for the submicroscopic level educators can use Lewis structures and model kits to illustrate the types of intermolecular forces existing between different types of molecules. On the macroscopic level, educators can use Lewis structures discussed in the context of liquids which dissolve in one another (such as ethanol and water) versus liquids which are not miscible (such as hexane and water), to illustrate the effects that different types of intermolecular forces have on dissolving.

In order to address the misconception of covalent bonds being formed during dissolving, for instance, for the submicroscopic level educators can use model kits and animations to show that the molecules in covalent compounds remain intact when the compounds dissolve in solvents, and that intermolecular forces are formed between the solute and solvent, as opposed to covalent bonds being formed between the solute and solvent. On the macroscopic level educators can use a demonstration in which sugar is dissolved in water, and then the solution is gently heated in order to evaporate the water, leaving the sugar behind. The educators can the show students that the sugar went through the reversible physical change of dissolving. Educators can present an argument to students that if covalent bonds were indeed formed between the sugar and water during dissolving, then a new chemical compound would have been formed, meaning that one should not get sugar back in the end.

In order to address the misconception of narrowly focusing on specific atoms in order to identify intermolecular forces, for instance, for the submicroscopic level educators can use model kits to demonstrate that the entire molecular structure (including lone pairs of electrons) must be considered, as opposed to just specific atoms, in order to decide on aspects such as polarity that will impact intermolecular forces. On the macroscopic level educators can use a demonstration in which sugar is placed into water and oil, and based on a consideration of the structures of sugar, water, and oil, have a discussion with students about reasons behind sugar dissolving in water and not dissolving in oil.

Mechanisms of bond breaking in dissolving

The students described various different ways in which solute bonds/forces could be broken during dissolving. Analysis of these ways indicated that students viewed the dissolving process as occurring in one of two mechanisms: via a single-step mechanism in which solute-solute bonds were broken while solute–solvent bonds were formed simultaneously, or via a multi-step mechanism in which solute-solute bonds/forces were first broken, followed by other possible subsequent steps, such as forming solute–solvent bonds/forces, or dispersing the solute in the solvent. These results highlighted the importance of educators discussing and differentiating aspects of bonding, especially bond/force breaking, in both melting and dissolving.

Educators can use the suggestions outlined above, on both the submicroscopic and macroscopic levels, in order to address these ideas. In addition, in some chemistry textbooks, enthalpy considerations of the dissolving process are discussed in a step-wise fashion in order to determine the overall enthalpy of dissolving. For instance, if a solute dissolves in a solvent, there is an endothermic step presented in which the intermolecular forces in the solute are broken, there is an endothermic step presented in which the intermolecular forces in the solvent are broken, and there is an exothermic step presented in which intermolecular forces are formed between the solute and solvent particles; the overall enthalpy of the dissolving would be the sum of the enthalpies of these individual steps. This step-wise presentation of the enthalpy considerations of the dissolving process could affect how students view the dissolving process, so educators should be careful to let students know that this step-wise consideration of the dissolving process is done only to demonstrate the various enthalpy contributions to the dissolving process.

Comparison of undergraduate students and graduate students

Graduate students were more likely than the undergraduate students to consider the effects of the environment (atmospheric water) on the compounds, and were less likely to say that stirring or heating would cause the compounds to dissolve if they had not dissolved before stirring or heating. These factors may have pointed towards the greater amount of experience the graduate students had with chemistry compared to the undergraduates, as expected. In addition, this study showed that even though students might be familiar with real world materials such as chalk, they still might experience difficulty in thinking about these materials in unfamiliar contexts, such as dissolving chalk in water or oil. These results reflect the importance of students having wide and varied demonstration and experimental experiences while studying chemistry, involving the incorporation of real world materials in demonstrations as well as laboratory experiments.

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