Under Pressure

• Carbonation keeps your soda fizzy. The process also means that the can of Coke you hold in your hand is under a lot more pressure inside than outside, where the normal atmospheric pressure is at work. Atmospheric pressure is the force exerted on surfaces by the weight of air. In the relatively small space inside a can or bottle of soda, a much higher level of pressure than normal forces a chemical process called molecular bonding. This bonding, which does not happen under normal atmospheric pressure, keeps CO2 gas dissolved in the drink.
  
  

Gases and Liquids

• Carbon dioxide gas is lighter in weight than the liquid drinks into which it is mixed during the carbonation process. Without the extra pressure added to cans and bottles of soda during the packaging process, the light gas would separate completely from the liquid and float up to the top of the can.
  
  

Bubbling Up

• As long as a can or bottle of soda remains sealed, the relatively higher pressure inside keeps the carbon dioxide gas from escaping the liquid. Once a Coke can is opened and the pressure is released, however, there is an immediate rush of gas out of the liquid. The gas, which has been forced to dissolve in the liquid under pressure, expands and floats out of the liquid.
  
  

Tiny Bubbles

• What happens, though, to form those fountain sprays of bubbles after you shake a can of soda? If a bottle or can of a carbonated beverage is left to sit undisturbed, there is relatively little fizz when it is popped open. But if the mixture of liquid and gas in the can or bottle is disturbed while it is sealed, the CO2 immediately tries to break away from the liquid in the form of small bubbles.
  
  

Fountains of Fizz

• With nowhere to go in the sealed, shaken can or bottle, the tiny bubbles of gas may settle back into the Coke after some time. But if the container is opened before these bubbles have a chance to redissolve, the sudden drop in pressure when the top is popped open will cause a messy overflow as the small, stirred-up bubbles suddenly expand in size. The reduced pressure allows the bubbles caused by the shaking to grow and spill out.