For decades, the case has been made for inquiry-based activities that enable teaching and learning of science concepts through the process of doing science. Laboratory courses provide a unique setting with opportunities for students to learn to ask questions, plan and carry out investigations, analyze data, and construct scientific arguments. Yet, most postsecondary laboratory courses still rely heavily on confirmatory laboratory activities. One of the problems regarding the implementation of inquiry-type laboratory experiments is the issue of assessing students' achievement to demonstrate pedagogical effectiveness. The design and scoring consistency of the Investigation Design, Explanation, and Argument Assessment for the first semester of General Chemistry Laboratory (IDEAA-GC1) has been described previously. The IDEAA-GC1 is a practical laboratory assessment for General Chemistry I that measures student ability to design and conduct an investigation, analyze and interpret data, and construct an argument. This study examines the impact of an inquiry-based laboratory course in developing student proficiency with two scientific practices: (1) plan and carry out an investigation and (2) generate a scientific argument. This manuscript presents the analysis of student responses (N = 325) to the assessment which indicated proficiency with both scientific practices to varying degrees as well as the limitations based on student content knowledge.