Computational simulation tools have become critical components for supporting learning, as well as research, in both science and engineering. One objective of these tools is to help students develop deeper conceptual understanding of difficult concepts. This explanatory sequential mix-method design study characterizes the influence of the use of a computational tool called Gibbs, on the students' understanding and representation of core concepts in thermodynamics. Participants included 48 sophomore-engineering students from a large Midwestern University who were enrolled in the course entitled "Thermodynamics of Materials." In the first phase, quantitative results examined students' conceptual understanding before and after being exposed to the computational tool as part of their laboratory session. In the second phase a qualitative evaluation characterized the nature of students' representational forms and the way they used them. Results for the quantitative study revealed positive gains in students' conceptual understanding for the majority of the concepts tested. In addition, insight gained from the qualitative analysis identified how students' representational systems were used to support their conceptual understanding in thermodynamics of materials concepts.