Henry's Law states that the solubility of a gas is directly proportional to the gas dissolved in a liquid. In simple terms, the gas pressure in a coke bottle located in the air section at the top of the bottle must equal the dissolved gas pressure of CO 2 in the liquid of the bottle. If it didn't then the gas would bubble out of the liquid until an equilibrium was reestablished. Effervescence occurs when the gas pressure is suddenly release from the bottle by the removal of the bottle cap and the gas pressure above the liquid rapidly escapes which cause the high pressure gas in the liquid to rapidly come out of solution creating a foam sea as the Coke is pour into a glass!