Fizzy Fun

    Kids, baking soda and/or baking powder are added to cooking batters to produce the gas bubbles that make cakes and muffins rise (this is called "leavening"). It is caused by the action of baking soda plus a liquid acid. Baking soda is sodium bicarbonate (NaHCO3). When mixed with a liquid acid it releases the gas carbon dioxide (CO2). Of course you've seen this - when you mix baking soda with vinegar it fizzes with CO2 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 CO2. Dry acids are certain tartrates, phosphates, or sulfates. Double-acting powders are the most common. The first "action" is the release of CO2 when one of the dry acids dissolves in liquid and reacts with the baking soda. The second "action" is the release of CO2 when the batter is heated in an oven. This relies on a different acid that dissolves only at higher temperatures.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    EXPERIMENT 1. The purpose here is to determine if CO2 is released when baking soda and baking powder are added to water. Dissolve 1 tsp baking soda in one cup of water and 1 tsp baking powder (a double-acting one) in another cup of water. The baking soda should dissolve without fizzing and the baking powder should fizz. Why? Note also that the baking soda dissolves completely but that the baking powder solution is cloudy. Look at the label and you¹ll see cornstarch is an ingredient. One possible explanation is that cornstarch doesn't dissolve. A follow-up experiment with 1/2 tsp cornstarch a glass of water will confirm this.

    EXPERIMENT 2. The purpose here is to determine whether baking soda and baking powder will fizz when acid is added. So, add 1 tsp of vinegar to the solutions from Experiment 1. We expect that when the vinegar (acetic acid) is added, CO2 gas bubbles will be released from the baking soda solution but not from the baking powder or the control cornstarch solution.

    EXPERIMENT 3. The purpose here is to determine whether baking soda and baking powder solutions will fizz when heated. Prepare one cup of water each with 1 tsp baking soda, baking powder, and cornstarch. After the baking powder stops fizzing, ask an adult partner to microwave them all for 1 minute (in microwave safe containers). Only the baking powder solution should fizz upon heating. In this the other dry acid (probably sodium aluminum sulfate, or alum) dissolves as the product is heated. As it dissolves, more is in solution to react with the bicarbonate.

    Think about all these reactions occurring the next time someone bakes you a cake. This is inorganic chemistry in action!


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    March 2001