The purpose of this research was to study primary science students' conceptual development as it related to their understanding of materials and their properties: in particular, to determine how and why some students changed their concepts while others did not. The participants were thirty-two Grade 5 (10-11 year old) students. An instructional unit based on the conceptual change perspective was developed and presented. Data were collected through pre- and post-instructional surveys, classroom observations, student work, and student interviews. The results showed the influence of instructional activities that challenged students' preconceptions and encouraged students' conceptual change, indicating the effects of affective, social, and language factors on students' conceptual development.