The main purpose of this study was to describe the development and use of a flowchart for preservice chemistry teachers' problem solving on the First Law of Thermodynamics. Twenty-seven undergraduate preservice chemistry teachers attending a Physical Chemistry course participated in this study. The responses of the participants to the set of test problems associated with the reversible isothermal, isochoric, isobaric, adiabatic, and cyclic processes, with and without the flowchart, and the views of the participants about the flowchart were used as data sources. The results showed that the flowchart developed in this study could influence the ability of preservice chemistry teachers to solve the problems associated with the First Law of Thermodynamics and decrease the errors while solving these problems. Moreover, it was found that almost all of the participants were successful in correctly solving the problems related to reversible isothermal and isochoric processes with the flowchart. The views of the participants about the flowchart indicated that using the flowchart is an effective way to solve the problems.