What change happens when you mix baking soda and vinegar?

Sodium bicarbonate (baking soda) and Acetic acid (about 5% of vinegar) recombine to produce carbon dioxide, water, and sodium ethanoate. The most apparent change is the release of large volumes of carbon dioxide gas. In cooking this effect (with a variety of acids) is why baking soda is used: it produces many bubbles of gas which become trapped in a batter or dough, and thus make the pancakes or the quick bread less dense (called leavening). Yeast has the same purpose, but takes longer.