This paper describes a set of studies carried out as part of a larger project on the development of children's concepts of matter and its changes. In this paper, the impact of visibility and familiarity of the process of dissolving on children's concepts is analyzed. Two different studies with kindergarten and third grade students were carried out. The results of the study show that even children as young as four have some understanding of the conservation of substances during dissolving, although 9-10 year-olds do not have compact understanding of the process. (Contains 19 references.) (MM)