Most of the materials children encounter in everyday life are mixtures which, from a science viewpoint, cannot be regarded as pure. Con - sequently, confl ict of meaning may arise when these materials are labelled ‘pure’. 1 2 3 Not only pupils but also student teachers suffer confusion. 4 Although about 60 to 70 per cent of a sample population of Belgian and Dutch pupils (aged 13) were found to understand that ‘pure substance’ meant ‘not a mixture’, for some 13 to 17 per cent the meaning was ‘without harmful contents’. When the meaning of the word ‘pure’ itself was explored, just 45 per cent had the idea of unmixed but 48 per cent had ideas that included ‘clean’, ‘bright’, ‘beautiful’, ‘as it should be’ and ‘accurate’. 1

Although in science the term ‘chemical change’ is reserved for processes in which the reacting chemical substances disappear and other (new) substances appear, several studies have found that children often use the term ‘chemical change’ to encompass changes in physical state and other physical transformations, particularly so when the colour of a substance alters. 3 5 6

Some educators do not distinguish between physical and chemical changes on theoretical grounds but others fi nd such a distinction useful for the development of science ideas. 7 8 9 How well pupils make such a distinction depends partly on their conception of ‘substance’. For instance, if they regard ice as a different substance from water, then they are likely to classify the melting of ice as a chemical change. Vogelezang 10 has suggested that substances should be regarded as identical if their properties are identical when compared under the same conditions.