This study focuses on designing a laboratory learning program for a high school chemistry course in which students could discover the fundamental principles of Hess's law via stepwise inquiry. By exploring the chemical properties of common desiccants, mainly those that were reactive in water, students were introduced to the exothermic reaction of CaO(s) with H[subscript 2]O(l) as a heating agent. Due to the difficulties associated with directly measuring the heat of reaction for CaO(s) with H[subscript 2]O(l), the heats of reaction for CaO(s) with HCl(aq) and Ca(OH)[subscript 2](s) with HCl(aq) were measured. Students could find the most appropriate experimental procedures through discussion in each group and subsequently establish a protocol in the class. The heats of reaction determined by students' experiments closely corresponded to those calculated using the thermodynamic database. Using the experimental values obtained for the reactions of CaO(s) with HCl(aq) and Ca(OH)[subscript 2](s) with HCl(aq), students could discover the relationship that existed among the heats of reaction for CaO(s) with H[subscript 2]O(l), CaO(s) with HCl(aq), and Ca(OH)[subscript 2](s) with HCl(aq) by drawing an energy diagram and making the relevant thermochemical calculations.