Chemical Leavening - Baking Powder Reaction

Introduction: This activity will illustrate for students in a boldly visual way the release of carbon dioxide when baking powder reacts with moisture. The diving submarine demo animates the scientific explanation of chemical leavening systems. The activity might be used as part of a unit on the science of baking or as an example how baking can illustrate science (in this case, chemical reactions).

This unit builds interest in both baking and chemistry. Teachers should encourage students to bake at home with their parents and siblings, making their own (edible) leavening experiments.

• Materials you will need:
  1 goldfish bowl or other large transparent container, filled with room temperature water to a depth of about 7 inches
  1 baking powder diving sub*
  1 can Clabber Girl Baking Powder
  1 spoon

Background information for the teacher:

What is the function of leavening?
Leavening releases bubbles of carbon dioxide gas. These bubbles form little pockets (cells) in the batter. These cells have thin walls that hold in the gas and give the baked good a lighter, more airy texture. This improves the color of the item and its eating quality. Leavening also gives the item a uniform shape.

What is chemical leavening?
Chemical leavening is the reaction of a leavening base (such as baking soda) with a leavening acid (such as sodium aluminum sulfate) in the presence of moisture and heat to produce carbon dioxide gas.

HX	+ NaHCO3		NaX + H2O + CO2
(Acid Salt)	(Soda)	(Heat & Moisture)	(Neutral Salt)

What is baking powder?
Baking powder is a mixture of leavening agents (baking soda, acid salts) and inert ingredients (cornstarch, calcium carbonate) that react in the presence of moisture and heat to release a gas (carbon dioxide).

The leavening agents are the active ingredients that hold the necessary elements to produce the carbon dioxide gas. The inert ingredients are buffers (separators) that keep the baking soda from mixing with the acid ingredients and that absorb moisture to prevent premature reaction.

What is baking soda?
Baking soda, or sodium bicarbonate, is a base (alkaline in nature) that is formed when sodium carbonate (purified form of a mineral mined in Wyoming) is mixed with a solution of carbonic acid:

Baking soda is the source of CO2 gas in the leavening system. It also neutralizes the leavening acid, adjusting final pH of the baked good.

What is an acid salt?
An acid salt is a dry, granular white crystal that dissolves in water before acting as an acid. The acid salt reacts chemically with the bicarbonate to release CO2 gas. The type of acid salt used in the baking powder can determine the rate of gas release.

The most common acid salts in home baking powders are:

NaAl(SO4)2 + 3 NaHCO3		Al(OH)3 + 2 Na2SO4 + 3 CO2
3 Ca(H2PO4)2 + 8 NaHCO3		Ca3(PO4)2 + 4 Na2HPO4 + 8 CO2

Background to tell students:

Baking powder is normally made of three different parts: an acid, a base and a filler of some sort to keep them apart. All three need to be dry powders that can be mixed together.

You can make your own baking powder. For example, baking soda (a base), cream or tartar (an acid) and cornstarch (the filler) are three common ingredients. But store-bought baking powders are more convenient, have better baking results and work the same way every time (if they’re fresh!).

When you add water to baking powder, the dry acid and base go into solution and start reacting to produce carbon dioxide bubbles.

Single-acting baking powder produces all of its bubbles when it gets wet. Double-acting baking powder produces bubbles again when it gets hot.

If you want to prove to yourself that this is how baking powder works, simply try mixing a teaspoon of baking powder into a cup of warm water. As long as the baking powder is fresh, you will definitely see the reaction!

Many recipes call for baking soda rather than baking powder. Usually these recipes use some kind of acid like buttermilk, yogurt, chocolate, honey, etc. in them to react with the baking soda to produce the bubbles.

The reason why people sometimes prefer baking powder to yeast is because yeast takes so long - usually 2 to 3 hours - to produce its bubbles. Baking powder is instant, so you can mix up a batch of biscuits and eat them 15 minutes later.

Now that you understand how baking powder works, you can understand two things you often see in recipes:

· Many recipes will instruct you to mix all of the dry ingredients together and then add the liquid. They do this because that keeps the baking powder from reacting until the end of the mixing process

· Many recipes will tell you to mix just until the ingredients are moistened. They do this in order to minimize the escape of the gas from the batter. If you stirred a long time, the reaction would end and the stirring would have allowed the bubbles to escape.

The diving sub demonstration

First drop the sub into the bowl to show students that it will sink.

Pull out the periscope-radar assembly on the top of the sub. Dip the sub in water, then shake out excess water leaving moisture only on the screen grid inside the sub.

Fill the bubble chamber completely with a loose charge of baking powder. Wipe off any excess baking powder.

Immediately re-insert the periscope-radar assembly, pressing it in firmly.
Shake the sub vigorously underwater a few times. Let go and the sub will submerge and surface automatically.

Gently stir the water to make the sub cruise around the bowl.

What’s happening?
As water causes the baking powder inside the sub to react, the carbon dioxide gas that is formed makes the sub buoyant enough to rise to the surface.

When baking, wet and dry ingredients combine in your mixing bowl causing the same chemical reaction you see with the submarine. Only the gas bubbles are trapped in the batter, forming little pockets or “cells” that lift the batter. When you put your batter in the oven, more gas is released, lifting the batter even higher. The further heating of the oven causes the batter to dry out and all the little cells to “set.” This creates a firm structure that you recognize as a cake, cookie, donut, etc.

For further investigation…
Advanced students will appreciate further experimentation with chemical leavening reactions. To demonstrate the effect of heat on the baking powder reaction, use the following demonstration. This exercise is most easily accomplished in a classroom laboratory.

Double-Acting Baking Powder Demonstration
1. Discuss the balanced double action of Clabber Girl as a leavening agent. A double acting baking powder is a mixture of phosphates (usually monocalcium phosphate, which reacts at room temperature when moistened, and sodium aluminum sulfate, which reacts to heat) and baking soda. A balanced double acting baking powder forms many small bubbles in the batter when mixing, allowing the product to remain stable if it stands before baking. The next release of carbon dioxide gas occurs during baking, resulting in finer cell structure and proper volume of the final product.
2. Demonstrate the two-stage process of Clabber Girl.
A. Use two 125-ml Erlenmeyer flasks. In one flask, place one tablespoon of baking powder, and in the other, place one tablespoon of baking soda.
B. Add one tablespoon of water to each flask, beginning with the one containing baking soda.
C. Attach a medium balloon to the top of each flask. Note any activity in the flask and balloon.
D. After reaction is complete and gas bubbles have disseminated (they will not completely disperse), remove balloons and swish the flasks to help disseminate the gas. Place each flask on a burner over low heat for no longer than 30 seconds. Using tongs, remove from heat immediately and observe activity.

E. Discuss the results and the differences between the baking soda and baking powder. This experiment can be repeated using vinegar or HCl to show how much more reactive baking soda is in the presence of acid.

*You can purchase diving subs at many locations. Following are links to online stores that have the diving sub:
American Science Surplus - The Vermont Country Store - The Science E-Store