The Coke is pumped with CO2 gas for
carbonation process. Coke sealed under pressure condition. Partial gas gets
dissolved in the liquid as bubbles and the remaining occupy the volume at the surface
of the liquid and sealed cap. When the bottle is opened due to pressure
difference, there is rush of gas at liquid surface to outside the bottle followed
by dissolved gas rushing out from liquid making small explosions of liquid at
surface leading to hissing sound and bubbles.