An apparatus for mixing a liquid and a substance, including a housing with a liquid inlet and a substance inlet to a mixing chamber, an outlet for a mixture of liquid and substance, and a flow restrictor arranged in the housing, between the liquid inlet and the mixing chamber. The flow restrictor is controllable to switch between a first position providing a first through flow area configured to create a Venturi effect such that a flow of the liquid draws the substance into the mixing chamber, and a second position providing a second through flow area that is larger than the first through flow area to thereby decrease a pressure drop caused by the flow restrictor.