Why are there bubbles in champagne ?

During the second fermentation, the addition of sugars and yeast causes a chemical reaction:

 

            SUGAR → ALCOHOL + CARBON DIOXIDE GAS

            C₆H₁₂O₆ → 2C₂H₆O + 2CO₂

 

In the un-opened bottle, the CO₂ is partly dissolved in the alcohol and partly found between the cork and the champagne. The concentration of gas in the liquid and the pressure of the gas are proportional according to Henry's law:

 

            P = H.C           P in bars, C in g.L^-1 and H constant ~  0.5 bars/ (g.L^-1)

 

Before the opening of the champagne bottle, the internal pressure is of 6.0 bars.

 

            P = H.C or C = P/H                therefore C = 6.0/0.5 = 12 g.L^-1

 

When the bottle is opened, the pressure decreases sharply and the champagne becomes over-saturated in CO₂, which was trapped inside escapes from the liquid: this phenomenon is called effervescence. However, this phenomenon only represents 15 to 20% of the degassing. The remaining 80 to 85% escape by direct contact with the air/champagne surface. This is called degassing due to free dissemination.