The study is set in a constructivist framework within which\ud children are regarded as actively engaged in constructing and restructuring their knowledge of the 'physical' world. The overall purpose of the research is to describe the development of children's conceptions about the nature of matter as disclosed by their ideas concerning the dissolving process.\ud \ud Information about children's ideas concerning various aspects of the dissolving process was obtained through individual interviews with a representative sample of pupils between the ages of seven and seventeen selected from school year groups 3,5,7,10 and 12. In addition a survey was conducted in which a further representative\ud sample of 588 pupils were given group administered tasks relating to the same phenomena. In both the interview and the survey, pupils were required to make predictions, observations and explanations which were subsequently categorised to reflect recurrent features in their\ud responses. These categories were coded and entered on a computer for further analysis.\ud \ud Aspects of children's conceptions as they related to atomistic ideas and to the conservation of matter, weight and volume formed the focus of the research. Most of the pupils in all five year-group conserved substance but a considerable number did not conserve its weight/mass and/or its volume. A U-shaped trend was found in the development of weight/mass conservation. This is interpreted in terms\ud of the developing complexity of children's conceptions making schema selection and co-ordination more problematic. The development of 'dissolved volume' conservation started with few pupils in the early years and progressed in an almost linear fashion. There is evidence of a complex relationship between the development of volume displacement\ud and the way matter is modelled.\ud \ud The findings about atomism indicated that whereas a major\ud proportion of pupils in each year-group spontaneously imagined an atomistic view of matter, few of them used such a conception to explain weight/mass or volume conservation. It appears that early atomism is based on the view Of matter being broken down into 'bits'. The way this interacts with conservation reasoning is described.\ud \ud Educational implications of the findings are discussed together with suggestions for further research
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