This paper presents results from a longitudinell study of students in compulsary school in Sweden. The project started in 1997 when the students were about six years old, and finished in 2006 when they left compulsary school. The aim of the study is to follow individual students’ development of conceptions about evaporation during their years in school. Teaching episodes focused on water, and it´s changes of state connected to everyday phenomena have been taught. Early in the study a simplified particle model was introduced in order to see if this would help the students in their explanations. The students were interviewed before and after the teaching interventions and the explanations were analysed in order to find a comprehensive description of each student. Here we describe the use of the early introduced particle concept and how it effects the students’ explanations of evaporation and their uses of explanatory models.