A dispensing cap for a container includes a body portion and an integrally formed chamber for holding an additive substance. The body portion includes a closed top end and an inner wall that defines the chamber whose open end is sealed by a chamber closure member. The closed top end includes a diaphragm and a plunger extends axially from the diaphragm within the chamber for unsealing the chamber closure member. The body portion, the closed top end, the plunger, and the inner wall are integrally formed in one-piece.