A dispensing cap for a container, the dispensing cap comprising: a generally cylindrically-shaped body portion having a closed top end and an open bottom end, the closed top end comprising a diaphragm portion; a generally cylindrically-shaped inner wall axially extending from the closed top end towards the open bottom end of the body portion and defining a chamber for holding an additive substance, wherein the chamber has an opening towards the open bottom end of the body portion; the body portion and the inner wall defining a receptacle therebetween for receiving a mouth of the container from the open bottom end of the body portion and sealing the mouth of the container; a plunger axially extending from the diaphragm portion towards the opening of the chamber, wherein the diaphragm and the plunger have a closed position and an open position; and a chamber closure member for sealing the chamber's opening attached to the plunger, wherein the chamber closure member is configured for cooperating with the inner wall and seal the chamber when the diaphragm and the plunger are in the closed position, wherein the chamber closure member is a disc-like structure having a sealing surface along its periphery and the inner wall has a corresponding sealing surface that cooperates with the sealing surface of the chamber closure member for sealing the chamber when the diaphragm and the plunger are in the closed position, wherein the chamber closure member has a top side that faces the closed top side of the body portion and a bottom side that faces away from the chamber, and further wherein the body portion, the closed top end, the plunger, and the inner wall are integrally formed, characterised in that the chamber closure member is provided with stress-relieving depressions (162, 163) on the top side, thereby when an activating force is applied to the diaphragm and the plunger is being moved from the closed position to the open position, the stress-relieving depressions enable the chamber closure member to elastically collapse and facilitate unsealing of the chamber closure member from the inner wall a generally cylindrically-shaped body portion having a closed top end and an open bottom end, the closed top end comprising a diaphragm portion; a generally cylindrically-shaped inner wall axially extending from the closed top end towards the open bottom end of the body portion and defining a chamber for holding an additive substance, wherein the chamber has an opening towards the open bottom end of the body portion; the body portion and the inner wall defining a receptacle therebetween for receiving a mouth of the container from the open bottom end of the body portion and sealing the mouth of the container; a plunger axially extending from the diaphragm portion towards the opening of the chamber, wherein the diaphragm and the plunger have a closed position and an open position; and a chamber closure member for sealing the chamber's opening attached to the plunger, wherein the chamber closure member is configured for cooperating with the inner wall and seal the chamber when the diaphragm and the plunger are in the closed position, wherein the chamber closure member is a disc-like structure having a sealing surface along its periphery and the inner wall has a corresponding sealing surface that cooperates with the sealing surface of the chamber closure member for sealing the chamber when the diaphragm and the plunger are in the closed position, wherein the chamber closure member has a top side that faces the closed top side of the body portion and a bottom side that faces away from the chamber, and further wherein the body portion, the closed top end, the plunger, and the inner wall are integrally formed, characterised in that the chamber closure member is provided with stress-relieving depressions (162, 163) on the top side, thereby when an activating force is applied to the diaphragm and the plunger is being moved from the closed position to the open position, the stress-relieving depressions enable the chamber closure member to elastically collapse and facilitate unsealing of the chamber closure member from the inner wall