When baking soda (sodium bicarbonate, NaHCO3) and vinegar (dilute acetic acid, typically around 4-8% acetic acid) are mixed together, a chemical reaction occurs, resulting in the production of carbon dioxide gas (CO2) along with a white, foamy substance. The white powder that forms is actually a combination of the leftover baking soda and the sodium acetate.
The reaction can be summarized as follows:
Baking Soda (Sodium Bicarbonate) + Vinegar (Acetic Acid) → Sodium Acetate + Water + Carbon Dioxide
The carbon dioxide gas is responsible for the fizzing and bubbling you see when these two ingredients are mixed. The white substance left behind after the reaction is the sodium acetate. Sodium acetate is a type of salt and is the compound responsible for the white powder you see. It is safe to handle, but it is not commonly used as a baking ingredient.
In baking, the reaction between baking soda and acidic ingredients like vinegar or buttermilk is often used as a leavening agent to help baked goods rise. The carbon dioxide gas produced during the reaction creates air pockets in the batter, leading to a lighter and fluffier texture in the final product. However, in most baking recipes, excess baking soda and the by-products of the reaction are used up during the baking process, so you won't typically find white powder residue in the finished product.