Tell students we will be making a simplified cake to understand the chemistry underlying making a cake.
Younger primaries to start:
Ask students what ingredients go into a cake, and as they list them, mix these ingredients to a well of a muffin tin: about 3 Tablespoons of flour, 3 Tablespoons of water, 1 teaspoon of baking powder. Other ingredients that are mentioned will be left out in our simplified cake batter -water will replace the liquid ingredients, though in a real cake they are needed for taste and for binding all the ingredients together.
As discussion continues, the simplified cake batter will rise slowly, and students will start to notice.
Ask them which ingredient made it rise [the baking powder].
Older primaries and up to start:
Students each get a tray and a stir stick, and flour and water are in pots with their own scoop.
Ask students to mix a little flour (about 1/4 teaspoon) and water until they make a cake batter consistency, using their stir stick to mix them together.
Then they should add a small scoop of baking powder, mix it up, then wait and watch.
Their cake batter will slowly rise, and some of them will see bubbles appearing.
All students, continuing:
Tell them that baking powder has three ingredients - read them from the package:
2. sodium bicarbonate, which is baking soda
3. monocalcium phosphate and/or sodium aluminum sulfate, which do the same job as cream of tartar
Ask students to figure out which two of these ingredients mix to make gas, by mixing small scoops of them with water in wells of their tray. If they add the powders to the tray first, the water can be added to mix them together, so the scoops can remain in the pots of powders.
The attached worksheet can help students keep track of what they find.
[Remove flour and baking powder from their tables, and add baking soda, cream of tartar and cornstarch to tables.]
Students should find that baking soda and cream of tartar make bubbles, but the other combinations of two ingredients do not. (Sometimes bubbles arise from the water being squirted in, but these soon pop, and do not count as bubbles being made.) Students may get different results from each other - tell them that this happens to scientists all the time, and they should simply both repeat the experiment until they get consistent results. Sometimes a powder may contaminate a nearby well to give differing results.
Use molecular models to figure out what gas is being made as the baking soda and cream of tartar are mixed and a chemical reaction happens to make gas.
Students are given HCO3 (baking soda) and H (in the cream of tartar). Ask the to rearrange the atoms to make two new molecules. Tell them that one molecule is water (H2O) and ask them to figure out the other one, by joining all the remaining atoms and bonds and filling all the holes in the atoms. They should arrive at CO2, which is carbon dioxide, a gas.
Summarize that when cakes are made with baking powder, the baking soda and cream of tartar (or other acid) in the baking powder mix together in the wet environment of the cake batter. They chemically react to make carbon dioxide gas, which makes bubbles which are trapped within the cake batter. We can see these bubbles as holes in cake. They make the cake light and fluffy.