A storage cap for use with a container includes a first structure having a first wall at least partially defining an inner storage chamber operable for storing a first substance and defining a first annular opening having a first annular convex surface facing toward the storage chamber. The storage cap includes a second structure coupled to the first structure and has a plunger element located within the storage chamber having a domed-end with a convex surface facing to the annular opening and a radius greater than one-half of the diameter of the first opening. The second structure is moveable to make and break contact with the annular opening. When the domed end is in contact with the first annular convex surface, a seal is formed between two convex surfaces along an annular path.