The invention relates to a mixing device for mixing a liquid, such as water, with an instant ingredient to form a beverage. The mixing device comprises a mixing chamber and a rotor. The mixing chamber comprises an inlet chamber, a rotor chamber and a pipe bend. The inlet opening of the pipe bend extends in the horizontal plane. The rotor is provided in the rotor chamber. The internal wall of the pipe bend has, on the side of the outer curve, a planar surface which is defined by a first direction and a second direction. The first direction extends in the horizontal direction and the second direction extends at right angles to the first direction, at an angle to the horizontal plane. The planar surface is situated perpendicularly below the inlet opening of the bend.