The study was conducted at a vocational high school in Ankara during the academic year of 2018-2019 based on the key concepts regarding the units in the chemistry curriculum for 9th grade. The purpose of the study was to investigate whether 9th grade students recognized the key concepts in chemistry units of first and second semester, and how correct their relevant definitions were. In the first unit, 9 students could determine a sufficient number of key concepts; however, in the proceeding units, the number decreased, and none of the students were able to recognize the correct concepts in the last unit. Another important finding of the study was that students included high-level concepts in their dictionaries even though they had difficulty in identifying the basic key concepts offered in the curriculum. It revealed that students attempted to explain unrecognized concepts through an unfamiliar terminology. Accordingly, it was inferred from the data obtained that students entered into chemistry with fundamental conceptual deficiencies. When concept-related definitions in student thesauruses were examined, it was revealed that the majority could make scientific explanations and choose their resources correctly; nonetheless, a small number of students could not make sufficient descriptions or used expressions far from scientific explanations. One of the important aspects of this study is that, teachers need to give the key concepts to students in detail first, and demonstrate in practice how to look up their explanations in the right resources. This constitutes the main scientific background of our study.