The purpose of this descriptive study was to investigate whether students regard isomers as only such compounds as:
(1) belong to the same class of compounds, or (2) have the same shape (cross or bar) in graphical representations of their formulas.
A sample of 7, 441 senior high school students of Grades 11-13 completed paper-and-pencil tests. Individual classes were interviewed on video while solving the tests. The results of the study support the hypothesis that students are inclined to restrict their concept of isomerism to compounds belonging to the same class. The study focused on ethers and alcohols. In the study the students' restricted concepts were remarkably stable, even if they were made to check in their chemistry books. There was no evidence that students expected the molecular formulas of isomers to be of the same shape. In their view the isomeric molecules should have branched-chain carbon skeletons. The following hypotheses were used to explain the results: (a) Students' limited concepts are due to a discrepancy between the definition of isomerism and its application in teaching and research; (b) the iso-nomenclature leads students to identify isomers by branched carbon chains. The results of this study apply directly to practice in the classroom. There is a need for continued research in the sciences to trace similar sources of conceptual difficulties.