One of the central goals of modern science and chemistry education is to develop students' abilities to understand complex phenomena, and productively engage in explanation, justification, and argumentation. To accomplish this goal, we should better characterise the types of reasoning that we expect students to master in the different scientific disciplines. This analysis is needed to support the design of instructional tasks, teaching strategies, and assessments that foster those ways of thinking. This essay contributes in this direction by characterising the major types of rationales built and applied by experts when analysing chemical systems and processes. The term 'chemical rationale' is used to refer to any product of reasoning that uses chemical knowledge to build explanations, justifications or arguments. Three main types of rationales are identified and discussed: Phenomenological, Mechanical, and Structural. Difficulties associated with learning how to build each type of rationale are highlighted, and implications for research and practice of this other chemistry 'triplet' are discussed.