Dispensers having a liquid reservoir filled with a main liquid and having a dispensing head, with a dispensing opening and a dispensing channel, which connects the reservoir outlet to the dispensing opening. The dispensing head can be pressed down to actuate the switching valve or the conveying device. The liquid dispenser has a replaceable exchange unit, which, in a functional state, forms the dispensing channel at least in some sections. The exchange unit has a mixing chamber, with an addition medium and an inlet and an outlet separated from the inlet. The exchange unit is part of the dispensing channel such that the inlet is communicatively connected to the reservoir outlet and the outlet is communicatively connected to the dispensing opening. Furthermore, the exchange unit has closures at the inlet and at the outlet to close the mixing chamber.