The purpose of this study was two‐fold: (1) to understand the conceptual frameworks that sixth‐grade students use to explain the nature of matter and molecules, and (2) to assess the effectiveness of two alternative curriculum units in promoting students' scientific understanding. The study involved 15 sixth‐grade science classes taught by 12 teachers in each of two successive years. Data were collected through paper‐and‐pencil tests and clinical interviews. The results revealed that students' entering conceptions differed from scientific conceptions in various ways. These differences included molecular conceptions concerning the nature, arrangement, and motion of molecules as well as macroscopic conceptions concerning the nature of matter and its physical changes. The results also showed that the students taught by the revised unit in Year 2 performed significantly better than the students taught by the original commercial curriculum unit in Year 1 for 9 of the 10 conceptual categories. Implications for science teaching and curriculum development are discussed.
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