Liquid dispenser having the following features: a. the liquid dispenser has a liquid reservoir which is filled with a main liquid and which - is designed as a pressure reservoir and has a switching valve by means of which the main liquid can be conveyed from the liquid reservoir to a reservoir outlet, or - is designed as a pressureless liquid reservoir and has a conveying device by means of which the main liquid can be conveyed from the liquid reservoir to a reservoir outlet, and b. the liquid dispenser has a dispensing head which has a dispensing opening and a dispensing duct which connects the reservoir outlet to the dispensing opening, c. the dispensing head can be pressed down in an actuating direction in the direction of the liquid reservoir, with the switching valve or the conveying device being actuated as a result, and d. the liquid dispenser has an interchangeable exchange unit which, in a functional state, forms the dispensing duct at least in certain portions, and e. the exchange unit has a mixing chamber in which an additional medium is arranged, and f. the mixing chamber has an inlet and an outlet separated therefrom, wherein the exchange unit is part of the dispensing duct in such a way that the inlet is communicatively connected to the reservoir outlet and the outlet is communicatively connected to the dispensing opening, characterized by the following features: g. the exchange unit has closures (90, 92, 94) at the inlet and at the outlet by means of which the mixing chamber is closed in a liquidtight manner in a storage state, and h. the liquid dispenser has a dispensing head which has a receiving space which is designed to receive the exchange unit, and i. a feed duct portion of the dispensing duct, which forms part of the dispensing head, opens into the receiving space, and j. the exchange unit is designed in such a way that, upon insertion into the receiving space, it assumes a desired position in which the mouth of the feed duct portion comes into communicative connection with the inlet of the mixing chamber of the exchange unit. a. the liquid dispenser has a liquid reservoir which is filled with a main liquid and which - is designed as a pressure reservoir and has a switching valve by means of which the main liquid can be conveyed from the liquid reservoir to a reservoir outlet, or - is designed as a pressureless liquid reservoir and has a conveying device by means of which the main liquid can be conveyed from the liquid reservoir to a reservoir outlet, and - is designed as a pressure reservoir and has a switching valve by means of which the main liquid can be conveyed from the liquid reservoir to a reservoir outlet, or - is designed as a pressureless liquid reservoir and has a conveying device by means of which the main liquid can be conveyed from the liquid reservoir to a reservoir outlet, and b. the liquid dispenser has a dispensing head which has a dispensing opening and a dispensing duct which connects the reservoir outlet to the dispensing opening, c. the dispensing head can be pressed down in an actuating direction in the direction of the liquid reservoir, with the switching valve or the conveying device being actuated as a result, and d. the liquid dispenser has an interchangeable exchange unit which, in a functional state, forms the dispensing duct at least in certain portions, and e. the exchange unit has a mixing chamber in which an additional medium is arranged, and f. the mixing chamber has an inlet and an outlet separated therefrom, wherein the exchange unit is part of the dispensing duct in such a way that the inlet is communicatively connected to the reservoir outlet and the outlet is communicatively connected to the dispensing opening, g. the exchange unit has closures (90, 92, 94) at the inlet and at the outlet by means of which the mixing chamber is closed in a liquidtight manner in a storage state, and h. the liquid dispenser has a dispensing head which has a receiving space which is designed to receive the exchange unit, and i. a feed duct portion of the dispensing duct, which forms part of the dispensing head, opens into the receiving space, and j. the exchange unit is designed in such a way that, upon insertion into the receiving space, it assumes a desired position in which the mouth of the feed duct portion comes into communicative connection with the inlet of the mixing chamber of the exchange unit.