Why are there often holes in slices of bread? The holes are created by bubbles of carbon dioxide gas (CO2) that form in the bread dough before and during baking. These bubbles form when single-celled organisms called yeast break down glucose (C6H12O6), a sugar in flour. In addition to carbon dioxide gas, a substance called ethanol (C2H6O) forms during this process. In the chemical reaction described in the passage, which of the following are reactants?