A closure device (10, 210) for use with a container having a main liquid compartment and an opening with a container neck, the closure device (10, 210) comprising: a spout cap member having a top wall with a spout aperture, a side wall extending below the top wall and adapted to be secured to the opening of the container, and a plug support structure extending from the top wall inside the side wall, a fluid chamber movably held within the spout cap member and having a bottom aperture, the fluid chamber containing an additive liquid and a pressurised propellant, a plug member (80, 164) fixed to the plug support structure and sealingly engageable in the bottom aperture of the fluid chamber, and a spout cover adapted to removably cover the spout aperture, wherein the spout cover includes a strut extending through the spout aperture to an abutment surface on the fluid chamber, wherein the fluid chamber is adapted to move relative to the spout cap member between: a first armed position of the closure device (10, 210), in which the plug member (80, 164) seals the bottom aperture closed, and the spout cover is engaged with the spout cap member, and the pressurised propellant urges the abutment surface on the fluid chamber against the strut of the spout cover, and a second firing position, in which the spout cover is disengaged from the spout cap member, and the pressurised propellant urges the fluid chamber upwards relative to the plug member (80, 164), so that the plug member (80, 164) no longer seals the bottom aperture closed. a spout cap member having a top wall with a spout aperture, a side wall extending below the top wall and adapted to be secured to the opening of the container, and a plug support structure extending from the top wall inside the side wall, a fluid chamber movably held within the spout cap member and having a bottom aperture, the fluid chamber containing an additive liquid and a pressurised propellant, a plug member (80, 164) fixed to the plug support structure and sealingly engageable in the bottom aperture of the fluid chamber, and a spout cover adapted to removably cover the spout aperture, wherein the spout cover includes a strut extending through the spout aperture to an abutment surface on the fluid chamber, wherein the fluid chamber is adapted to move relative to the spout cap member between: a first armed position of the closure device (10, 210), in which the plug member (80, 164) seals the bottom aperture closed, and the spout cover is engaged with the spout cap member, and the pressurised propellant urges the abutment surface on the fluid chamber against the strut of the spout cover, and a second firing position, in which the spout cover is disengaged from the spout cap member, and the pressurised propellant urges the fluid chamber upwards relative to the plug member (80, 164), so that the plug member (80, 164) no longer seals the bottom aperture closed. a first armed position of the closure device (10, 210), in which the plug member (80, 164) seals the bottom aperture closed, and the spout cover is engaged with the spout cap member, and the pressurised propellant urges the abutment surface on the fluid chamber against the strut of the spout cover, and a second firing position, in which the spout cover is disengaged from the spout cap member, and the pressurised propellant urges the fluid chamber upwards relative to the plug member (80, 164), so that the plug member (80, 164) no longer seals the bottom aperture closed.