In an investigation of 149 pre-service primary teachers' understanding of the terms "pure" and "natural," the participants were asked to provide definitions of the two words, and classify various substances including drugs of abuse, pharmaceuticals, nutriceuticals, and household substances as either natural or not natural, and as pure or not pure. A common scale of "purity" and "naturalness" could be constructed consistent with the Rasch model, suggesting that the participants conflate the two terms. This conflation is consistent with the qualitative data that suggested participants associated both concepts within a common spiritual metaphor, heavily laden with emotive content. Purity was not seen as relating to the composition of a substance, but to its history. The findings may help explain the development of persistent non-scientific chemical taxonomies observed in secondary students. Furthermore, as health educators, the participants may become the source of inappropriate messages about the use of medicines, or the abuse of drugs.