In this work, we propose a conceptual model for existential events that involve portions of matter. Portions of matter are entities that constitute most of the physical entities, for instance, water, oil, gold, and blood. Existential events are events that create or destroy entities. The proposed conceptual model is intended to support the design of information systems that capture the dynamic changes of portions of matter in natural and industrial settings. First, we propose a taxonomy for collections of objects (the entities that constitute portions of matter) and define the phase transitions that create and terminate the circumstances for a portion of matter to exist (i.e., for a collection of objects to come to constitute a new portion of matter). Second, we define three existential events that affect portions of matter: (i) Forming Stuff, in which a portion of matter is created when a collection of objects becomes connected; (ii) Terminating Stuff, in which a portion of matter is terminated when a collection of granules becomes scattered; (iii) Changing Stuff Type, in which a portion of matter of one kind is created and a portion of matter of another kind is terminated. Lastly, we present an application example that uses the condensation of a portion of water and its freezing into a portion of ice to illustrate how to use the conceptual model in concrete cases.