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A coding scheme is presented and used to evaluate solutions of seventeen students working on twenty five stoichiometry problems in a think-aloud protocol. The stoichiometry problems are evaluated as a series of sub-problems (e.g., empirical formulas, mass percent, or balancing chemical equations), and the coding scheme was used to categorize each sub-problem solution as successful, neutral, or unsuccessful, with more detailed codes comprising the neutral and unsuccessful categories, for a total of eight codes. A relatively high frequency of neutral results was found in which students simply did not realize when or how to approach a sub-problem. A lack of conceptual understanding of the mole concept appears to be closely related to students skipping crucial steps in stoichiometry problems, especially the sub-problems stoichiometric ratio and mole concept. Students' failures were also observed to be due to a lack of basic knowledge, such as the names of chemical compounds. The application of the new code system was shown to reveal difficulties that might have otherwise been missed by an analysis that focused on end results only.

This paper reports on a pedagogical approach to the teaching of chemical equations introduced to first year university students with little previous chemical knowledge. During the instruction period students had to interpret and construct diagrams of reactions at the submicro level, and relate them to chemical equations at the symbolic level with the aim of improving their conceptual understanding of chemical equations and stoichiometry. Students received instruction in symbol conventions, practice through graded tutorial tasks, and feedback on their efforts over the semester. Analysis of the student responses to formative test and summative exam items over consecutive years indicates that there was a consistent improvement in the abilities of the various cohorts to answer stoichiometry questions correctly. The responses provide evidence for diagrams of the submicro level being used as tools for reasoning in solving chemical problems, to recognise misconceptions of chemical formulae and to recognise the value of using various multiple representations of chemical reactions connecting the submicro and symbolic levels of representation. The student-generated submicro diagrams serve as a visualisation tool for teaching and learning abstract concepts in solving stoichiometric problems. We argue that the use of diagrams of the submicro level provides a more complete picture of the reaction, rather than a net summary of a chemical equation, leading to a deeper conceptual understanding.

A qualitative case study was conducted to investigate the understanding of the limiting reagent concept and the strategies used by five Year 11 students when solving four reaction stoichiometry problems. Students’ written problem-solving strategies were studied using the think-aloud protocol during problem-solving, and retrospective verbalisations after each activity. Contrary to several findings reported in the research literature, the two high-achieving students in the study tended to rely on the use of a memorised formula to deduce the limiting reagent, by comparing the actual mole ratio of the reactants with the stoichiometric mole ratio. The other three average-achieving students, however, generally deduced the limiting reagent from first principles, using the stoichiometry of the balanced chemical equation. Overall, the students displayed limited confidence during problem-solving to determine the limiting reagent and to perform related computations.

In this work, we examine whether tenth-grade high school students (N = 197, age 15- 16) as well as first-year university chemistry students (N = 77, age 18-19) can make the connection between chemical reactions and chemical phenomena. We used nineteen physical and chemical phenomena, and asked the students at one stage to distinguish physical from chemical phenomena, and at another stage to state in which cases one or more reactions occur. Students can be categorised into two distinct groups. One group includes those who do not always identify chemical phenomena with reaction(s), while the other group includes those who are successful in that distinction. Further, the students of the first group can be divided into two subgroups: (a) those who perform better in identifying the chemical phenomena; (b) those who perform better in identifying the reactions. A differentiation of chemical changes into natural and man-caused processes seems to be operating, at least with Greek students. On the other hand, students may be intuitively viewing chemical reactions as fairly simple processes, which can be expressed by means of chemical equations. Finally, it might be preferable to group (i) changes of physical state and phase, and (ii) solutions, in a separate category (physicochemical changes). Chemistry Education Research and Practice