The topic of acid–base chemistry is one of the oldest in general chemistry courses and it has been almost continuously in academic discussion. The central purpose of documenting the knowledge and beliefs of a group of ten Mexican teachers with experience in teaching acid–base chemistry in high school was to know how they design, prepare and organize their classes inside and outside the classroom, from which a set of teaching–learning sequences will be developed, essentially to train new teachers. We decided to document Pedagogical Content Knowledge (PCK), by means of two methodological tools from Loughran, Mulhall and Berry: Content Representation (CoRe), and Pedagogical and Professional experience Repertoire (PaP-eR). In this article, we relate only the first of these tools. It was also important to document concepts, skills and attitudes, so we have made the analysis based on these three items. The main finding was to construct a Canonical PCK including the central concepts of pH and strength in the topic of acid–base chemistry from the main phrases included in the Content Representation answered by those outstanding Mexican teachers. We chose the topic of “acid–base chemistry” because there are numerous examples of its importance related to sustainability (among them acid rain or acidification of oceans), everyday issues (including stomach acidity, antacids, health troubles with the pH of blood and urine), or problems with the longevity of books related to the acidity of paper. Also, it is of fundamental importance because the students often present many alternative conceptions about it.

<span>This work has as its foundation the use of models and the structuring of a didactic unit with three learning sequences, to introduce the Science-Technology-Society dimension in the teaching of acids and bases in high school level. The introduction includes a general survey on the theoretical foundations of STS dimension, modeling in science teaching and the construction of didactic units. The importance of the topic of acid and bases is argued and the design of a STS sequence on this topic is discussed for its application on a sixth semester high school class in Colegio de Ciencias y Humanidades of Universidad Nacional Autónoma de México.</span>