An internal chamber incorporated with the cap of a container, for example, a disposable beverage bottle. The internal chamber has an internal bore designed to hold a powder or liquid that can be released into the container. The chamber is inserted into a bottle or similar container when the cap is placed over the opening. A push-button on the top of the cap advances an actuator within the internal bore to push a diaphragm out of the distal end outlet port in the chamber to release the ingredient(s) within the internal chamber into the container. The diaphragm also has two or more peripheral guide rails to ensure that it remains aligned with the chamber housing during those movements. Alternative embodiments utilize a dome cover over the push-button. In use, the cap of the container can be removed, which will simultaneously remove the chamber, allowing the liquid in the container to be consumed or decanted.