The reaction that causes bubbles to form in muffins is a chemical reaction, not a physical reaction. Bubble/gas formation is one sign of a chemical reaction, as is the one-way action of the process--we cannot easily separate the ingredients back into the components.
The bubbles that form when muffins bake, and cause the batter to rise, are caused by a chemical reaction--carbon dioxide gas is released during the reaction between baking powder and the liquid in the recipe. Baking powder itself is composed of baking soda (sodium bicarbonate, NaHCO3) and cream of tartar (KHC4H4O6). In the presence of the water component of the liquid added, these two react to produce KNaC4H4O6, water, and carbon dioxide gas.
These bubbles of gas are produced as soon as the ingredients are mixed, and the muffins must be baked quickly before the bubbles escape the batter. This type of leavening is different from that used in yeast bread. That reaction is the result of yeast, a microorganism, using sugars to produce carbon dioxide; this reaction takes more time than the immediate reaction of baking powder.
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