Empowering students with a robust Science, Technology, Engineering, and Mathematics (STEM) education, while equipping their teachers with comprehensive training, fosters critical thinking and problem-solving skills, which are important aspects of innovation, social progress, and advancement. The main objective of this study is to investigate how a STEM-lab activity -- an experimental chemistry laboratory activity integrated with Arduino prototyping -- implemented within a teacher education program can promote the development of professional knowledge that enables the development of activities aligned with the STEM approach among pre-service teachers. Twenty Portuguese pre-service physics and chemistry teachers participated in the study. Pre- and post-activity tests, incorporating closed and open-ended questions, were used to assess participants' perceptions. Data analysis included quantitative assessment of conceptual gains and qualitative content analysis of reflective responses. The results suggest that the activity fosters a reflective process among pre-service chemistry teachers, enabling them to integrate STEM components with a socio-scientific contextual approach and encouraging critical consideration of adapting STEM activities for greater applicability and effectiveness in secondary education. While this approach significantly enhances pedagogical effectiveness, challenges persist in identifying aspects related to engineering and using Arduino prototyping.