Question

Why does carbon dioxide still pass through the layer of oil during fermentation

Answer 1

Well, you see, during fermentation, yeast and bacteria are chomping down on sugars and producing carbon dioxide as a byproduct. Now, carbon dioxide is a gas and it can easily pass through the layer of oil that may be present in the fermentation vessel. Plus, that oil layer may not be one solid barrier, so there could be small gaps or channels for the carbon dioxide to sneak through. So, that's why it's able to get through.