The article describes a learning unit project for primary schools, developed using active teaching methodologies, to classify substances as acidic, basic, or neutral and introduce the concept of pH as a measurement scale. The hands-on learning aims to enable all students, each at their own pace, to master the required knowledge and skills. A modular approach involving a sequence of activities designed to be carried out collectively under the guidance of the teacher structures a process of active knowledge construction, using an inductive approach. At the end of each laboratory activity, the key concepts are formalised in the notebook to structure and consolidate them. Ongoing formative evaluation is planned to monitor students’ progress, as well as a summative evaluation at the end of the didactic unit, aimed at providing the teacher with a final judgment on the students’ learning.