Chemistry is all about how atoms and molecules interact with themselves and each other resulting in properties and phenomena that can be detected and observed at the macroscopic scale. It follows quite closely then, that an important goal of chemistry education is to help students understand how macroscopic phenomena are governed by interactions and energy changes that accompany changes in interactions at the molecular level. While the literature is filled with examples of ways student incorrectly understand energy across a wide range of topics in chemistry, there is a growing trend to characterize students understanding along a continuum rather than catalog incorrect ideas. This research seeks to characterize how students in a reformed general chemistry curriculum, Chemistry, Life, the Universe and Everything (CLUE), understand solutions and the solvation process in the context of the dissolution of an ionic salt to form an aqueous solution. Preliminary research uncovered representative ways students successfully and unsuccessfully described an observable temperature change that accompanied the solvation process in a series of student interviews. These findings, as well as reasoning strategies that were observed to help students make connections between atomic-molecular level interactions and energy and macroscopic level temperature changes, were used to design a formative assessment activity targeting this phenomenon. Specific assessment tasks in which students constructed representations and explanations of the temperature change that accompanied solvation were then operationalized as research tools and administered as formative and summative assessments. Subsequent analysis of student responses resulted in development of robust coding schemes that were used to characterize how students used mechanistic components in their representations and explanations. In spite of extensive literature describing student difficulties representing solutions, the vast majority of students in the CLUE curriculum were observed to be able to represent the fully dissolved solution as being composed of separated ions each individually interacting with solvent molecules. Further revision of the explanation prompt, leveraging principles influenced by the scaffolding and evidence centered design literature, resulted in a medium to large increase in sophistication of students' use of interactions from representation to explanation tasks; these results were replicated in multiple student populations. Finally, implications of this work relevant to enacting curricular support for students to use atomic-molecular scale interactions in a mechanistic way are presented, including the importance of designing assessments capable of eliciting evidence of student understanding.
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