Students usually learn about gases and the particulate theory of matter in the seventh grade. The objective of this research was to study the knowledge students possess regarding the concept gas prior to and following instruction. The findings show that students do not develop spontaneously a general idea of gas prior to instruction. After instruction, they first acquire knowledge about the substantial properties of gases, then they refer to gas as a state of matter and only thereafter do they apply the particulate theory of matter in order to explain the term gas. In addition, it was found that students’ reactions to essentially identical scientific problems are different. Educational implications are discussed.