I'm going to attempt to articulate my question with appropriate lucidity, but please let me know if I fail to do so. I am a vintner (a wine maker), so achieving a palatable batch of wine necessarily elicits some required understanding of chemistry and physics. I understand that the wine yeasts I use ferment by breaking down the sugar into ethyl alcohol and carbon dioxide gas. I also understand that the interaction of oxygen with the resultant alcohol produces acetaldehyde and acetic acid. During the first phase of the fermentation process the yeast are usually fervent (depending on temperature), and as a result there is a sufficient blanket of CO2 on the top of the wine to keep air out. When the fermentation slows down we usually put it in a container with little room for oxygen and attach an airlock. The CO2 is released by the airlock, but oxygen is not allowed in. Ok, that was just to provide the background for my question. What precisely causes the CO2 to bubble up through the lighter air in the container and then through the airlock? Is it the heat from the exothermic reaction freeing some CO2 molecules thereby decreasing the density and creating a convection current? Is it simply more pressure of more CO2 molecules from the fermentation? Is there any point in really distinguishing between heat and pressure in this context? I hope this makes sense. I have a friend who feels that temperature has nothing to do with it, and that if the CO2 were somehow able to keep constant with the temperature above, the increase in CO2 molecules would still cause some of the CO2 to percolate through the air in the container and out the airlock. Thanks!