This study focuses on the misconceptions of eighth-grade students in the Arab sector compared to the Jewish sector regarding the conservation of mass during combustion in a closed system and in an open system before the subject is taught, and to what extent the misconceptions change after it is taught. Students (N=195) from six heterogeneous classes were asked to fill in a ten-question questionnaire twice: once before the subject was taught and again afterwards. The findings indicated that students’ understanding improved more with respect to closed systems compared with open systems; before the subject had been taught the situation was very similar for both cases. The students’ explanations were based on visual arguments and showed that there was confusion concerning the effect of chemical, physical, and state of matter changes on the conservation of mass during combustion. Regarding the comparison between the conceptions of eighth-grade students in the Arab and Jewish (N=105) sectors, students in the Arab sector had better achievements in closed systems, whereas Jewish students gave more correct answers to questions in open systems. The study’s findings can help middle-school students and their teachers understand that physical and chemical changes do not affect mass in a closed system, and that students should learn to distinguish between conservation of mass in open and in closed systems.