This study was designed to identify the levels of difficulty encountered by high school chemistry students on topics related to proportional thinking, and then to discover to what extent student performance on these selected chemistry topics was dependent on performance on selected cognitive development tasks.;Two hundred and fifteen high school chemistry students responded to a 40-item chemistry retention test following a year of chemistry classroom instruction. A series of Piagetian-like classification measures, including the Test of Logical Thinking and a battery of three Piagetian-like puzzles was also administered to each subject. Hierarchies of difficulty were constructed based on test items clustered by chemistry topic and expected cognitive skill. These hierarchies were developed using the Ordering-Theorethic Method with the McNemar Test as a measure of statistical significance. Empirically-derived hierarchies were compared to Piagetian classification and to theoretical hierarchies developed by separate panels with expertise in Piagetian theory and chemistry content.;Results of hierarchical analysis indicated that item difficulty was more dependent on cognitive skills requirements than on chemistry topic. Cognitive skills, such as inverse proportional reasoning and direct proportional reasoning with numbers other than small whole numbers, were dependent on first-order direct proportional reasoning as measured by subtests of the classification measures, but not on an overall formal operational classification.;Results confirmed that most high school chemistry students operate at the concrete operational level. Test items that contained small whole number ratios did not require formal operational skills. Some formal operational items were answered correctly by using alternative strategies such as the use of algorithms or the factor-label method.;Implications for teachers are that chemistry instruction can be improved by providing more concrete referents and properly designed demonstrations and laboratory work. Curriculum changes, including a spiral approach in which formal operational topics are visited often during the year in a pattern of ever increasing complexity is desirable. A proposed curriculum based on the findings is presented.
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