A longitudinal study of 16 - 18 year old students' understanding of basic chemical ideas.

The understanding 16 - 18 year old chemistry students have of basic chemical ideas was probed in a longitudinal study using a test paper comprising twenty-three diagnostic questions. The students were attending schools and colleges in the UK and had chosen to study chemistry beyond the age of 16. They responded to questions investigating their ideas about the differences between elements, compounds and mixtures, the conservation of mass in chemical reactions, chemical changes and chemical bonding. Aspects of stoichiometry, thermodynamics, equilibria and rates of reaction were also featured. Students' responses were collected three times: at the beginning, in the middle and towards the end of their Advanced ('A') level courses. The written data was supported by interviews with selected students carried out after the first and second surveys. The first survey (of 399 students) established a baseline against which students' progress could be gauged. Some had poorly developed particle ideas. Others did not conserve mass in chemical reactions, or confused mass and density. Many showed poor understanding about acids, combustion and dissolving. Although respondents knew about single and double bonds, many seemed unaware that covalent bonds involve electrons being shared. They found ionic bonds difficult to describe. Students did not know that energy is released when bonds form. Two further surveys of 320 students were carried out. 250 students followed the Salters' Advanced Chemistry (SAC) course which adopts a context-led approach. Their understanding of most chemical ideas probed changed by the third stage, although some weaknesses were still apparent. The changes in responses observed in 70 non-Salters and a sub-sample of 70 SAC students suggest that different approaches produce some similar effects. The findings indicate that A level courses should include strategies for teaching basic chemical ideas and highlights areas for further development of SAC.