The research project was presented under the supervision of Clara Lucia Lanza, a professor at the Universidad Tecnologica de Pereira, with the purpose of understanding the occurrence of Didactic Unity in the development of the argument. The study had second-grade children from the Liceo Taller Llinas as subjects from which ensued a quantitative methodology with a quasi-experimental research focus. For the didactic intervention in the function of developing the argument, five sessions for the students were designed with a duration of two hours per session, a conceptual tool of Mixtures and Substances and the methodology of Little Scientists; identifying as the first recourse the previous understanding that the students had with regards to the subject. The main results describe the elements of the argument used for the students during the application of the pretest, and the obtained information yielding a basis for the development of Didactic Unity. Different investigations were undertook at an international and nation level concerning the development of the capacity of the argument and the use of Didactic Unities in the teaching of the Natural Sciences as central themes in the completed investigation. The research project concludes with a few recommendations about the importance of implementing Didactic Unities in the classroom oriented towards developing the processes of the argument.