When the coke is opened, there is suddenly a great pressure differential. The initial loud hiss that is heard is this pressure differential equalizing itself. All of the additional pressure found within the bottle pushes gas out of the bottle until the pressure inside the bottle is the same as the pressure outside the bottle. The movement of this gas causes that initial loud hiss.
However, once this occurs, the pressure inside the bottle is much lower and the gas bubbles that had previously been dissolved into the soda have nothing holding them in the liquid anymore so they start rising out of the liquid. As they reach the surface, they pop and force small explosions of soda. These explosions are the source of the popping and hissing that continues while the soda is opened to the outside air. Of course, after a while, the soda will become "flat" when the only gas left dissolved in the liquid will be the gas that is held back by the relatively weak atmospheric pressure (when compared to the original pressure that existed inside the bottle while it was being shipped).
Answered by: Ted Pavlic, B.S., EE Grad Student, Ohio State Univ., Columbus, OH
But an important question still remains: why does the gas rush out of the Coke bottle and what is that gas? The gas you here is actually carbon dioxide (CO_2). When the Coke is made, in order to carbonate it (those bubbles you see floating to the top, the ones that make you burp!) the men and women who work at the Coca-Cola plant pump carbon dioxide into the Coke to carbonate it so that we can enjoy it. OK, so before you open the bottle, there is a whole lot of carbon dioxide dissolved in the solution (as well as other gasses such as oxygen and nitrogen).
So why does it rush out? The reason the carbon dioxide rushes out is because the concentration of dissolved carbon dioxide in the coke and the partial pressure of carbon dioxide above the coke are not in equilibrium. What the heck does that mean? Basically, whenever there a solution (liquid) in the presence of a gas, some of the gas will dissolve into the liquid and some of the gas that is already in the liquid will escape and "join" the other gas. (The reason fish can breathe is because some of the oxygen in air gets dissolved into the water!) So, we have gas entering and leaving the solution, like water entering a sinking ship the is being bailed out by the captain. IF the captain bails fast enough, the boat will continue to float, if he bails too slow, the boat will sink; however, if he bails at just the right rate, the amount of water in the boat will stay constant even though "new" water is entering the boat (through the hole) and "old" water is leaving the boat as the captain scoops it overboard.
In nature, when we "bail" at the speed where the water level remains constant, we are said to be in equilibrium. Initially, when the Coke is opened there a lot of carbon dioxide in the liquid and not very much in the air (by comparison), therefore, all the carbon dioxide rushed out. Even though you cannot here it, carbon dioxide is going into the liquid too.
A neat experiment might be to put a balloon over the top of the bottle and watch is slowly inflate until the system reaches equilibrium. At that point, the balloon will stop growing. (Well, OK, now we have to consider the atmospheric pressure, but it is still a neat experiment) Try shaking the Coke (but don't spill it into the balloon), does the balloon change? Maybe you could put the Coke in the fridge to cool it down, or even make it solid in the freezer!
Even though the balloon will only get so big, eventually your Coke will go "flat". This is when almost all of the carbon dioxide has escaped. Most of the gas escapes because the gas phase is favored in the equilibrium process. That has to do with order and disorder (entropy and changes of state), which is a whole different question.
Answered by: Anthony Fantasia, Ph.D., Physics Professor, University of Rome