A mixing apparatus for mixing at least two components comprises a mixing chamber having a first component and at least one component chamber having a further component. The component chamber is connected to the mixing chamber by a feed line. A negative pressure device generates a negative pressure in the mixing chamber in the starting position. By activating the negative pressure device, the pressure is equalized, whereby the further component is transferred into the mixing chamber. A display apparatus is provided at the component chamber to display a pressure change in the component chamber. The display apparatus is preferably formed by elastic bellows provided at one end of the component chamber, which is disposed opposite of the end of the component chamber following the feed line.