| Knowledge Psychogenesis and development in pupils' mind all along are psychological and educational research hotspots. These researches have not only accumulated abundant experiences but also posed diversiform theories. One of the best noticeable theories is Piagetian theory of cognitive development stages. The current educational reform advocates science inquiry-oriented learning that emphasizes construction and growth of knowledge and stress instruction starting from pupils' "where they are". In term of Piagetian theory of cognitive development stages, we study pupils' cognitive bases of learning science knowledge. Carrying out this type of research may give support to science inquiry instruction designs.When junior high school pupils begin to study physics, density is the abstract concept that they met very early. Physics teachers tell that density is the difficult concept for students to understand. Density is an intensive quantity concept that is defined by the ratio of mass and volume. Using the methods of clinic interview and questionnaire, the research carry out "Inquiry Density by Classifying Task" to examine the pupils, ranging from sixth grade to eighth grade, who haven't study density in class, to make sure students' different levels of conceptual development of density, to find its background, that is, three possible cognitive factors: weight conservation, notion of exterior volume, proportional reasoning. The study find that there are three stages of the conceptual development of density: consciously consider mass (weight) without volume can synthetically consider mass (weight) and volume but not separate them to operate be able to separatevolume from mass (weight) and keep one quantity invariable to compare the other quantity. At stage 1, with age going up, numbers of students largely decreases. At stage 2, numbers of students does not make a great change with age going up. In the face of the third stage of classifying task, students in sixth grade always can't deal with or can use adjacent strategy while students in seventh and eighth grades tend to use subtraction strategy and proportional strategy. At stage 3, a half of students in seventh grade keep one quantity invariable to compare the other quantity in action, which seldom shows in sixth and eighth grades. Students using the ratio of mass and volume to solve problem, the number in eighth grade is three times more than it in sixth grade. That is, with students' ages going up, the conceptual development of density makes a great progress, from consider mass (weight) without volume to synthetically consider mass (weight) and volume, to can separate volume from mass (weight) and keep one quantity invariable to compare the other quantity. In the end, primary construction of scientific density concept is achieved.Given students' ages and performance, in the research, weight conversation is not a factor influence to conceptual development of density. The performance of students' using the ratio of mass and volume to solve problem is significant correlation with their notion of exterior volume. Students' having notion of exterior volume is a base that they can separate volume from mass (weight) and operate them respectively. Hence, the notion of exterior volume is a cognitive factor to influence the conceptual development of density.When solving not task-specific proportional questions, students at stage 3 do better than those students who use subtraction strategy in classifying task. Therefore, the ability of solving not task-specific proportional reasoning questions also influences the conceptual development of density. In addition, students' performances of solving task-specific proportional questions are have non-ignorable diversity, which the paper primarily discusses.The study also finds that many students tried to look for the equivalency as the criterion of determining classes. Students believe the same materials must have any equivalent or adjacent aspect. The idea may trace back to the notion of qualitative |
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