This study consists of a categorisation of the vocabulary of chemistry, focusing on the terminological difficulties students face when learning this subject. Therefore, this classification intends to provide chemistry teachers an awareness of the language barriers students must overcome in their understanding of this scientific discipline. Two main groups of words have been considered: technical (i.e. chemistry-specific) and non-technical terms (i.e. general academic words). The first category is mainly centred on the analysis of chemical concepts. Among the main issues of technical terms that have been examined are the following: a) new coined specific terms of chemistry; b) historical evolution of metaphors and different science context meanings; c) daily & scientific meanings; d) nominalisation; e) mathematical terms; e) dual concepts. The second category is not restricted to chemistry as it is common to other scientific related areas. The four cases of non-technical terms analysed in this work are the following: a) general non-technical words in science school settings; b) meta-representational verbs; c) linking items; d) teleological or intentional terminology.