Intellectual demands of learning outcomes in the intended curriculum have always been a concern across the field of science education. In particular, the representation of those learning outcomes stipulated by the intended curriculum in science textbooks has become a big issue for both science curriculum studies and science teaching practice. To address this issue, the concepts of semantic gravity (SG) and semantic density (SD), as part of the dimension of Semantics from Legitimation Code Theory (LCT), were employed in this study with the purpose of examining the degrees of abstraction and complexity of chemical knowledge under the topic "common substances" in four series of chemistry textbooks, which were compiled in compliance with the national chemistry curriculum of the compulsory education (Grades 1-9) in China. Based on the principles of LCT (Semantics), a new scheme for differentiating the strengths of SG and SD was developed in the current study to analyze the representation of 34 knowledge points in the four series of chemistry textbooks. Results show that these knowledge points are embodied with less complexity and avoid more abstraction in the four series of chemistry textbooks. Specifically, it was found that the overwhelming majority of the knowledge points are represented with strong semantic gravity and weak semantic density. Uniqueness was also identified with individual series of chemistry textbooks. The implications of the results of this study are discussed for the effective representation of science (chemistry) knowledge in textbooks.