How do a material's properties relate to its composition and structure? (the question of Structure-Property Relationships)
Chemical thinking is a powerful tool to explain, predict, and control the properties and behaviors of chemical systems in our surroundings. These goals are often achieved by building submicroscopic models of matter that help us connect the macroscopic properties of a system with the composition and structure of its submicroscopic components. For example, we explain the electrical conductivity of solutions of NaCl by assuming that this compound is comprised of electrically charged ions that get separated when interacting with water molecules. We predict that cooking oil will have a higher boiling point than liquid water because the molecules of the oil interact more strongly with each other than the molecules of water do.
If we want our students to productively use structure-property relationships to make sense of the properties of the materials in their surroundings, we must create multiple opportunities for them to recognize that the construction of structure-property relationships is based on the analysis of three main things: a) the properties of the submicroscopic components of the system of interest (e.g., the composition and structure of the molecules that comprise a molecular compound, the charge and size of the ions that comprise and ionic compound), b) the types and relative strength of the interactions between such submicroscopic components (e.g., existence of hydrogen-bonding interactions between molecules, presence of repulsive or attractive forces between ions), and c) the activities or processes in which the submicroscopic components engage, due to their interactions, which are ultimately responsible for the properties that we observe
There is a wide variety of questions under the umbrella of structure-property relationships that we can pose or that students can suggest. Consider the following example:
|Which substance is the best coolant?|
Central Ideas: Energy is required for the particles that make up a liquid to separate and form a gas phase. The weaker the intermolecular interactions between the particles, the less energy is required to separate them and the faster the transition can occur given a constant supply of energy.
Science Practices: Planning and carrying out investigations; analyzing and interpreting data; constructing explanations and designing solutions
Crosscutting Reasoning: Patterns; systems and system models; energy and matter; structure and function.
Performance Expectation: Students will design an experiment to identify the substance that produces the largest cooling effect upon evaporation and explain their results based on the comparative analysis of the chemical composition and structure of tested substances.
Anchoring instruction on tasks that actively involve students in the comparative analysis of different types of materials with a practical purpose, like the one illustrated above, creates rich opportunities for the development and application of structure-property reasoning. These types of tasks often involve the identification and justification of strategies to separate or extract the main components of a mixture (e.g., extracting fat from food products, separating a mixture of plastics to facilitate recycling), or the evaluation of different types of materials that can be used in the manufacture of a desired consumer product (e.g., selecting best heat insulators for the design of cup holders or best heat conductors to design defrosting trays). The separation of substances or their selection for specific purposes is facilitated when we understand how their properties depend on the composition and structure of their submicroscopic components (e.g., molecules of fat are only slightly polar and will be more easily extracted using nonpolar solvents).
Eliciting and Exploring
Some curricula provide an approach to teaching lessons from the perspective that students do not have preconceived ideas about the properties and behaviors of the submicroscopic components of chemical systems (i.e., atoms, molecules, ions) because they do not have direct experiences with them. However, educational research has shown that most students tend to map their ideas and beliefs about the properties and behaviors of macroscopic objects to the submicroscopic world. Novice learners often believe, for example, that atoms of copper may have a red-brownish color and be malleable like the actual metal, and that molecules of water are like tiny little droplets of liquid. It takes considerable instructional effort and time for students to understand that the observable properties of materials "emerge" from the dynamic interactions of tiny particles with very different properties from those of the macroscopic sample.
Meaningful understanding of structure-property relationships can be fostered by systematically engaging students in the generation of submicroscopic models of matter and in the use of these models to build explanations, to create arguments to support claims, and to make predictions about the properties and behaviors of systems of interest. Students’ ideas and beliefs are more effectively revealed when students are invited to make their mental models public, and when they are challenged to use those models to build chemical rationales. Students’ models improve, and their ideas and beliefs become more sophisticated, as they receive formative feedback on their work.
Advancing and Connecting
Helping students advance in their ability to build and apply structure-property relationships requires careful scaffolding. Becoming familiar with the results of educational research that characterizes how students' ideas tend to progress in this area helps teachers to better understand and guide student thinking. For example, teachers can expect students to struggle to differentiate the properties of different types of submicroscopic components (e.g. distinguishing between electronegativity and polarity, between intermolecular interactions and chemical bonds, or between dipole-dipole interactions and hydrogen-bonding interactions). Students are also likely to build initial models of the composition and structure of substances that are crude, incomplete, or include incorrect elements. Correcting those models by telling students what they are missing or is wrong with their thinking will not be as productive as creating opportunities for students to recognize the limitations of their ideas and to revise them accordingly. Asking students to articulate their initial ideas using their own language can facilitate the construction of bridges between students' colloquial language and important chemical concepts.