In this chapter we introduce the Energy-Interaction Model, which is the primary focus of this course. We will apply this model together with the Three-Phase Model of Pure Substances in order to unders...In this chapter we introduce the Energy-Interaction Model, which is the primary focus of this course. We will apply this model together with the Three-Phase Model of Pure Substances in order to understand various physical phenomena. We will analyze pure substances going through temperature and phase changes and chemical reactions.