Of course you've seen this - when you mix baking soda with vinegar it fizzes with CO 2 bubbles. Recipes that use baking soda for leavening always have an acid somewhere, like vinegar, lemon juice, or buttermilk. Less obvious acids are those in honey and molasses. Baking powder is a combination of baking soda plus a dry acid. When baking powder is mixed in a batter, the dry acid and the baking soda can react together and release CO 2 .
Dry acids are certain tartrates, phosphates, or sulfates.
Double-acting powders are the most common. This relies on a different acid that dissolves only at higher temperatures. The purpose here is to determine if CO 2 is released when baking soda and baking powder are added to water. The baking soda should dissolve without fizzing and the baking powder should fizz. Note also that the baking soda dissolves completely but that the baking powder solution is cloudy.