We propose a new instructional theory, the Domain of Validity (DoV) Framework, which offers a new way forward for designing teaching for conceptual change, while also resolving conflicts between existing theories related to common, difficult-to-change conceptions students have about particular scientific topics. We propose that knowledge consists of two connected elements: a model and a domain of validity (or DoV). Foregrounding the notion of DoV for given models allows us to reconceptualise and diagnose many problematic preconceptions as examples of an oversized DoV. Mapping the different elements of knowledge -- both the model and its domain of validity -- allows teachers to pinpoint precisely the cognitive conflict that students need to confront in a conceptual change approach to teaching. We highlight the instructional implications related to these scientific learning difficulties and conclude by proposing particular teaching strategies based on this new framework, emphasising the domains of validity of particular scientific models.