Device for the separate conservation of a first substance to be mixed with a second substance contained in a bottle and for closing said bottle comprising • a generally concave plug that comprises a substantially cylindrical portion and an edge portion suitable for making a seal on the mouth of said bottle, • inside said substantially cylindrical portion a cavity is formed, also cylindrical, in which a fracturing element is inserted, which for this purpose is externally shaped so as to match the cavity itself, • a membrane being fixedly connected to the plug so as to close the cavity, • said fracturing element has a fracturing tip or edge suitable for penetrating and perforating the membrane when it moves downwards, the fracturing element is kept inside the cavity so that it cannot fall by gravity through annular projections that engage with as many projections present on the inner surface of the cavity and the membrane is associated with the lower edge of the cavity through thermowelding, the projections and the membrane ensuring the hermetic seal of the cavity in which the first substance resides, characterised in that the fracturing element is hollow inside to contain the first substance, the upper surface of the substantially cylindrical portion is flexible and is convex with respect to the cavity and the top of the fracturing element has substantially the same convexity as the upper surface of the substantially cylindrical portion, so that the two surfaces are in contact with each other, the upper surface of the substantially cylindrical portion being flexible so that when the top of the plug is pressed down, the upper surface bends towards the inside of the plug and exerts a pressure on the top 32 of the fracturing element, the contact between the upper surface with the top of the fracturing element pushes the fracturing element itself downwards. • a generally concave plug that comprises a substantially cylindrical portion and an edge portion suitable for making a seal on the mouth of said bottle, • inside said substantially cylindrical portion a cavity is formed, also cylindrical, in which a fracturing element is inserted, which for this purpose is externally shaped so as to match the cavity itself, • a membrane being fixedly connected to the plug so as to close the cavity, • said fracturing element has a fracturing tip or edge suitable for penetrating and perforating the membrane when it moves downwards, the fracturing element is kept inside the cavity so that it cannot fall by gravity through annular projections that engage with as many projections present on the inner surface of the cavity and the membrane is associated with the lower edge of the cavity through thermowelding, the projections and the membrane ensuring the hermetic seal of the cavity in which the first substance resides, characterised in that the fracturing element is hollow inside to contain the first substance, the upper surface of the substantially cylindrical portion is flexible and is convex with respect to the cavity and the top of the fracturing element has substantially the same convexity as the upper surface of the substantially cylindrical portion, so that the two surfaces are in contact with each other, the upper surface of the substantially cylindrical portion being flexible so that when the top of the plug is pressed down, the upper surface bends towards the inside of the plug and exerts a pressure on the top 32 of the fracturing element, the contact between the upper surface with the top of the fracturing element pushes the fracturing element itself downwards.