Students’ and teachers’ perceptions for composition of ionic compounds

Introduction

Historically, in 1912, Laue had discovered three-dimensional crystal structures by the appearance of interference pattern via the diffraction of X-ray radiation from sodium chloride or salts (Barke et al., 2009). In 1914, Bragg discovered that the interference pattern seen by Laue was, in fact, an aggregate of sodium ions and chloride ions with electrostatic forces in the solid phase. This view was then believed as the latest scientific idea about how ionic salt is structured.

At the school level, however, the previous idea of ionic salts was still used/taught by teachers. Consequently, students’ achievement was still unsatisfactory; for instance, only 40.3% of 11 grade students and 76.7% of the undergraduate students could represent sodium chloride salt in terms of sub-microscopic level correctly (Gkitzia et al., 2020). Here, sub-microscopic is related to an internal representation of ionic salts.

Characteristic of the structures found by Laue in 1912 through X-ray diffraction is seen as a scientific idea, in contrast to students’ assumptions about the ionic salts as molecules. This would hinder students’ understanding of dissolving ionic salts in acid solutions – particularly distinguishing between proton transfer and electron transfer. Numerous studies prove this through the low performance in interpreting ionic salts structures (Agung & Schwartz, 2007; Barke et al., 2009; Gkitzia et al., 2020; Nyachwaya et al., 2014; Taber, 2002).

In chemistry classes, teachers commonly begin teaching chemical bonding, in which the ionic bond is taught by the concept of giving and taking a valence electron. Further, students bring that concept to understand the ionic salts’ structure. This concept is considered a misconception corresponding to the “give and take” notion in salts solution (Taber, 2015), even though the context is correct regarding ion formation from individual atoms. This study, however, highlights ionic salts in the natural structure that are in line with Laue’s idea that explains the ionic lattice without any reaction between sodium atoms and chlorine molecules.

To understand the structure of the ionic salt, students need to develop an internal representation of ionic salts in a solution corresponding to the sub-microscopic level. It is related to students’ mental models that correspond to the nature of particles. In the reaction of ionic compounds in water as a solvent, for example, dissolving sodium chloride in water is often represented by net ionic equations like Na⁺ Cl⁻ ionic lattice (s) → aq → Na⁺ (aq) ions and Cl⁻ (aq) ions. Students should imagine that Na⁺ (aq) ions hydrate by unique numbers of H₂O molecules. In reality, the ionic lattice is composed of positive and negative ions. The attractive force between oppositely charged ions is maximized, and the repulsive force between atoms of the same charge is minimized (Huheey et al., 2009). The hydration of ions by H₂O molecules in solution rarely appears in chemical equations. It does not tell what chemical change is carried out at the particle level. Therefore, the challenge is unfolding the symbolic level of ionic salts associated with chemical equations to the particulate level.

Through the system thinking approach, chemistry learning would be more powerful, connected, and meaningful (Flynn et al., 2019) to address the challenge above. The system thinking skills themselves require thinking holistically, corresponding to a deep understanding of the whole system, such as identifying the individual component (York & Orgill, 2020) like particles from formulas.

The system thinking lens is designed to reveal one’s mental models in identifying species on the ionic salts holistically and analyzing the interaction between species. Nowadays, efforts have been made by many countries to improve student-teachers understanding of conceptual knowledge. For instance, Indonesia is developing an innovative curriculum (Faisal & Martin, 2019) to cope with many misconceptions about ionic bonding (Prodjosantoso, 2019). An effort has been addressed to elevate students’ thinking in Tanzania (Msonde & Van Aalst, 2017). Ethiopia tried to cope with misconceptions about basic concepts (Gurmu, 2018). Understanding different efforts are attempted in these three countries, and this study is eager to know the extent to which students and teachers understand the sub-microscopic level of the concept. Therefore, the research question in this study is “How do chemistry-teacher students and teachers work on the sub-microscopic level in terms of particle concept within ionic salts?”

Ionic compounds in solid (s) and aqueous (aq) state

The famous structure representing ionic compounds is the NaCl-crystal structure. Students know this compound either in the solid (s) state or aqueous (aq) state. The NaCl-crystal facility invented by Laue is expected to influence chemistry learning by introducing appropriate structural models for beginners – despite determining crystal structure by X-ray diffraction being the routine for chemists (Huheey et al., 2009).

The NaCl-crystal structure is a built-in lattice formation with an equal number of cations and anions, and its coordination number is six. Those ions already exist in such a solid ionic compound, and hence, they do not have to be formed anymore (Barke et al., 2012). By dissolving the ionic compounds in water, ions will be separated by H₂O molecules because of the interactions between solute and solvent. But, when students work with chemical symbols and formulae, they only obtain sign (aq) as a subscript of formula, which means in a particulate level, water molecules move around ions. Particle concepts in the sub-microscopic level lay on imagination or as students’ mental models.

Mental models of ionic compounds

Working with the sub-microscopic level is always related to Johnstone’s chemical triangle, which reported that the origin of misconceptions lies in three levels of representation, namely, macroscopic, symbolic, and submicroscopic levels (Johnstone, 2000). Regarding the context of dissolution, learners are asked for fast-shifting within those representations. Yet, understanding how solid dissolves into an aqueous solution and then separates into ions are still problematic to students. Such problematic understanding then leads to misconceptions about dissolution (Derman & Eilks, 2016).

Mental models are described as learners’ internal representation of concepts and ideas (Rapp, 2005). A suitable mental model is, therefore, needed to understand particle interaction on the solution in terms of the sub-microscopic level. In addition, it requires skills to visualize the process of dissolving solid salts crystals into an aqueous solution by investigating the ions migrating in the puddle (Worley et al., 2019).

Aligning system thinking with mental models

A mental model allows students to recognize atoms, molecules, ions as unseen physical objects. Here, atoms and molecules are neutral particles with one and more than two atoms attached, respectively. Ions can be both positively and negatively charged particles. The mental model is expected to show the interaction between species and H₂O molecules holistically. For this, the skill of system thinking is needed. York and Orgill (2020), for example, developed the CHeMIST tool to address students’ system thinking skills that begin from identifying part of the system and its interaction.

The net ionic equation sequence might represent system thinking practice because it could stimulate students to identify the individual components within a system and analyze their interaction at the sub-microscopic level. Further, interpreting chemical equations on the symbolic level requires translation skills of given reaction symbols to involve atoms, ions, or molecules on the sub-microscopic level. In reality, teachers and learners typically move from the macro-level to the symbolic level and memorize rules and count the numbers of atoms on the left and right sides of chemical equations (Kelly & Akaygun, 2016; Romine et al., 2016).

Methodology

This research applied a mixed-methods approach by the explanatory sequential mixed-methods research design (Creswell, 2009). The quantitative data were collected from students’ scores on the test interpreting ions involved in the given chemical equations provided in the questionnaire. This data was then used for gathering qualitative data through semi-structured interviews with students and teachers. The interview was used to explore information on several issues from the test.

Results and discussion

Three chemical equations provided in the test represent the dissolution of sodium carbonate (Na₂CO₃), calcium compounds (CaCO₃), and metal oxides (MgO) in acidic solutions. Regarding chemical reactions at the particulate level, those ions should be separated directly by H₂O molecules to become hydrated ions. The test results show, however, that students still struggle to mention Na⁺ ions, Cl⁻ ions, H₂O molecules, and CO₂ molecules. Table 4 displays that the average scores for the problems related to CaCO₃ and MgO are less than 1–0.67 and 0.86, respectively. Simplifying ionic compounds into formulas tends to be deceptive for the structure of compounds corresponding to the particulate level.

The trend of misconceptions is shown in the interpretation of solid sodium carbonate. Participants should have mentioned Na⁺ ions, Cl⁻ ions, H₂O molecules, CO₂ molecules in the product. Yet, they only recognized “Na₂CO₃” as molecules building up solid sodium carbonate instead of considering Na⁺ ions and CO₃ ²⁻ ions in a 2:1 ratio for solid carbonate. This misinterpretation of the chemical formula was also found in the previous research (Farheen & Lewis, 2021; Kozma & Russell, 1997; Nyachwaya & Wood, 2014).

Students seemed to memorize the formula without knowing the meaning behind chemical equations. In terms of CaCO₃, for example, they directly think at the surface level that CaCO₃ contains three atoms, as seen in the molecular symbol. They assumed that carbonate ions in CaCO₃ decomposed to C ions and O molecules. Here, they could not distinguish between the polyatomic ions and elements like carbon and oxygen. In this situation, an effort is needed to cross the conceptual threshold between the substance and particle levels (Talanquer, 2015).

Regarding metal oxides from alkali earth, MgO should have been mentioned in Mg²⁺ ions and O²⁻ ions. However, students wrote that MgO corresponds to the smallest particles representing the macro level. Their daily language influenced particle concepts such as “smallest part,” which seems like an animistic mode of speech (Barke et al., 2009). Lack of the ability to distinguish between Mg atoms and the existed Mg²⁺ ions in MgO ionic salt caused the students to gain a lower score on the question on MgO.

The pattern of the atomic ontology

The pattern of the atomic ontology was gained from interview results concerning participants’ responses to the test items. One issue raised in the interview was related to the articulation of an ion formation. Students articulated an ion formation by giving and taking electrons to gain stability, such as the following excerpt:

Two electrons from Mg will be taken by oxygen to form MgO with electron transfer. (Transcript_A17, P. 2: 113)

for a reason to reach stability, so they should give and take electrons. (Transcript_A4, P. 2: 829)

Code 1 (see Table 2) appears from the student’s quote on the take-give electron principle. This principle is appropriate in terms of the formation of NaCl(s) crystalline from the chemical reaction between elemental sodium metal, Na(s), and elemental chlorine gas Cl₂(g). The process is a complex process that engages several stages that begin from individual Na⁺(g) ions and Cl⁻(g) ions from individual atomic gaseous Na(g) and Cl(g), which lost and gained their electrons reversely. The concept about energy is also involved in this process, i.e., isolated Na⁺(g) ions together with Cl⁻(g) ions are not energetically more stable than an individual Na(g) atom together with an individual Cl(g) atom. Consequently, the achieved crystalline state ensures minimum energy, which is maximum stability for the system. Numerous studies have investigated misconceptions about the take-give electron principle to explain ionic salts Hilbing (2002) that the octet rule might cause a lack of reason why bonding occurs and not recognizing electrostatic force in chemical bonding (Bergqvist et al., 2013).

Surprisingly, participants came up with a language sensible related to code number 2. It was probably caused by inconsistency of language, such as: sometimes, we call atoms, ions, and molecules randomly. (Transcript_A11, P. 2: 568). Data represent that participants’ perceptions only focus on a surface feature such as phase or patterns related to symbolism or structure. Similarly, knowing the meaning behind symbols was challenging (Bongers et al., 2019; Cooper et al., 2016; Nyachwaya & Wood, 2014).

The inclination of perception about the ionic compound structure

The second category was related to capturing ionic structure at solid and solution. The three most common responses from the participants were (1) position as a reactant, (2) sensible language, the “center of an atom,” (3) the connection between phase with the structure of matter. The following excerpts show those reasons in the sequences:

1. Because when Na ₂ CO ₃ positions as the reactant, their form is the same as solid. (Transcript_A1, P. 1: 404)

2. the bonds of Na ₂ CO ₃ is ionic, but its central bonds are covalent because C is the center of an atom that uses electron together. (Transcript_A4, P. 1: 866)

3. First, I saw the phase; there is a solid phase, for example, Na ₂ CO ₃ . I assume that it is a molecular form or tends to be a molecule because it does not dissociate (at solid phase) (Transcript_A5, P. 1: 567)

The regular chemical equation does not show ionic compounds, whether solid or solution (Barke et al., 2009; Brady & Humiston, 1986; Taber, 2002). However, participants’ intuition aligns with the earlier research that solid ionic compounds do not contain ions (Nyachwaya et al., 2014). Yet, ions already exist in solid crystals and salt solutions and do not have to be formed every time (Barke et al., 2012). In addition, a sign of phases such as (s) for solid, (l) for liquid, or (g) for gas also influenced participants’ intuition, as seen in the following quote:

First, I saw the phase; there is a solid phase, for example, Na ₂ CO ₃ . I assume that it is a molecular form or tends to be a molecule because it does not dissociate (at solid phase) (Transcript_A5, P. 1: 567)

Regardless of Code 3 above, almost all participants, mainly from Ethiopia, failed in identifying polyatomic ions (Code 4). Indeed, Tanzanian participants recognized that polyatomic ions contain individual ionic forms by writing CO₃ ²⁻ ions into C⁴⁺, O²⁻ ions. Similarly, an Indonesian participant also wrote C and O for explaining CO₃ ²⁻ ions.

Conclusions and further research

While interpreting chemical equations, one should avoid the take-give electron principle and the ion formation by atoms. Visualizing ionic salts by 3D molecular models, sphere packing, or introducing ionic formula such as (Na²⁺)₂(CO₃ ²⁻)₁ for Na₂CO₃ – regular formula – might be better to acknowledge polyatomic ions. In addition, this study also confirms that memorizing the symbols seems to be mainly applied when dealing with solving the equation problems.

The study’s implication is mainly related to the chemistry learning practice concerning the basic concepts. Showing natural crystals and scientific models might be helpful to introduce particle concepts, mainly ionic salts. Historical ideas might be a potential approach to introduce the existence of ions in salts. This study also implies that teachers should pay attention when delivering the concept of ionic formation. Researchers suggest first introducing the structure of ionic crystals and then ionic bonding without electron transfer by ion formation. The role of the electrolysis experiment of melted salts and ionic oxides and an aqueous solution of salts and acid-base is essential for understanding ionic compounds and ionic bonding. Ultimately, regarding the limitation of this study, further research will continue to the subsequent stage of ChemMIST tools of York and Orgill (2020).