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Differentiation between physical and chemical change in chemistry teaching is both fundamental and problematic. Teaching approaches often rely on using macroscopic descriptions of change processes, which by their nature cannot differentiate between physical and chemical change. The aim of this study was to develop. implement, and evaluate a teaching sequence concerning students' understanding of physical and chemical change processes, based upon a submicroscopic model of the structure of matter. The teaching sequence was implemented with secondary students aged 15•16 in the target Saudi school. A case study approach was employed (one school). Three year 10 classes from the selected school (90 male students) and two chemistry teachers were recruited. For the purpose of designing the teaching sequence, the Leeds Group model was used. In the implementation and evaluation phases the focus was on measuring what teachers and students were expected to do and what they actually did, as well as evaluating what students were expected to learn compared to what they actually learnt. Furthermore, an interview approach was used to explore teachers' and students' views about the designed teaching sequence. A conceptual analysis was conducted which identified six broad areas (families) of key scientific ideas (FKSO. The results of this study indicated that the implementation of the designed teaching sequence, to a large extent, covered the required conceptual content. The results also showed that the students talked to the teacher (and each other), as well as the teacher talking to students. So, the students could control the meaning-making process by their interventions, rather than placing sole responsibility for this on the teacher. The designed teaching sequence was, to some extent, effective in relation to students' learning of physical and chemical change across all six FKSls. However, what students found most difficult in the understanding of the physical and chemical change, through the particle model (submicroscopic level), was how macroscopic descriptions and the submicroscopic level relate to each other and differ from one another. The results demonstrated increases in the level of conceptual understanding achieved by students following the designed teaching sequence, compared to students following the normal teaching approach.

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Notre contribution porte sur les processus de conceptualisation et de modélisation de la matière et de ses transformations en chimie. Elle s'articule autour de deux parties. Une première partie consistant en une analyse du type sémioépistémologique de ce processus pendant la période fin XVIIIe - début XIXe siècles. Nous mobilisons pour cela la théorie sémiotique de C.S. Peirce comme cadre d'analyse. Elle a permis de resituer l'expérimental de la chimie comme étant un ensemble de signes qui prolonge le théorique en même temps qu'il le fonde. Le signe iconique loin d'être signe d'une pensée primitive apparait comme une véritable néo écriture, un instrument heuristique privilégié utilisé par les chimistes pour sonder la matière à la recherche d'indices sur une structure pouvant rendre compte de son comportement, il leur a permis, après de nombreuses controverses, d'aboutir à une structure particulaire, et de construire les concepts d'atome, la molécule et son atomicité. Une deuxième partie où l'analyse sémio-épistémologique a constitué notre cadre théorique pour une transposition didactique de ce contexte. Notre question de recherche est de savoir si le signe iconique, comme défini au sens peircien, peut être mobilisé de manière spontanée par les élèves de 13-15 ans pour construire un raisonnement en chimie, et s'il peut les aider à accéder à l'infiniment petit sur la base de ce contexte transposé ? Nous avons alors élaboré un corpus de 20 séquences d'enseignement engageant les élèves dans une démarche qui nécessite la construction de modèles évolutifs. Nos résultats permettent de dire que le signe iconique loin d'être un moyen pédagogique de transmission facilité, un auxiliaire didactique est au contraire un instrument heuristique privilégié dans la construction des connaissances par les élèves, comme il l'est chez le chimiste / Our contribution focuses on the process of conceptualization and modeling of matter and its transformations in chemistry. It consists of two parts. A first part consisting of an analysis of the semio-epistemological type of this process during the late 18th - early 19th centuries. We mobilize for this semiotic theory of C.S. Peirce as a framework for analysis. It allowed to relocate the experimental chemistry as being a set of signs that extends the theoretical at the same time that he founded. The iconic sign far from signs of a primitive thought appears as a true neo writing, a privileged heuristic instrument used by chemists to probe the matter for clues on a structure that can make account of his behavior, he allowed them, after much controversy, lead to a particulate structure, and build the concepts of atom, the molecule and its atomicity. A second part where the semio-epistemological analysis was our theoretical framework for a didactic transposition of this context. Our research question is to know if the sign iconic, as defined in the peircien sense, can be mobilized spontaneously by students from ages 13-15 to build reasoning in chemistry, and if he can help in accessing the infinitely small on the basis of this converted context? We then developed a corpus of 20 sequences of teaching engaging students in a process that requires the construction of evolutionary models. Our results to say that the iconic sign far from a pedagogical means of transmission facilitated, a teaching assistant is instead a heuristic instrument in the construction of knowledge by students, as it is at the chemist