What it shows
Cakes rise because gas bubbles form in the batter. Baking powder contains both an acid and a base and makes gas on its own when wet and heated; baking soda alone needs an acid like curd or lemon in the batter. Baking four small cakes with different raising agents shows which rise and why.
The science
Baking soda is sodium bicarbonate, a base. It releases carbon dioxide only when it meets an acid, so in a plain batter of flour, sugar and milk it does little and leaves a soapy taste. Baking powder is sodium bicarbonate mixed with a dry acid, so adding liquid and heat lets the two react and release gas without any other acid. Curd or lemon supplies the missing acid for baking soda. The bubbles expand in the oven heat and the batter sets around them, giving a light cake.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Four steel cups or a muffin tray, greased with a little oil
- Flour (maida), sugar, cooking oil and milk for a simple batter
- Baking soda and baking powder
- Lemon juice
- A toothpick or thin skewer
- An oven, microwave with convection, or a pressure cooker used as an oven (adult only)
- A ruler
- Labels
Safety
For this project: An adult must handle the oven or cooker and the hot cups. Keep hands away from the hot tray and let the cakes cool before measuring.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
Step by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Make one batter of 1 cup flour, 4 tablespoons sugar, 4 tablespoons oil and about 6 tablespoons milk, stirred until thick but pourable.
Divide the batter equally into four bowls and label them: nothing, soda only, soda plus lemon juice, baking powder.
Add a quarter teaspoon of baking soda to bowl two, a quarter teaspoon of soda and one tablespoon of lemon juice to bowl three, and a quarter teaspoon of baking powder to bowl four.
Stir each bowl gently and pour the batter into its cup to the same height; mark the height on the outside.
Ask an adult to bake all four together at the same temperature until a toothpick comes out clean, about 15 to 20 minutes.
Let them cool, then measure how high each cake rose above its starting mark.
Cut each cake in half and look at the size of the holes inside.
Taste a small piece of each and record texture and any soapy or sour taste in a table.
What you should see
The baking powder cake and the soda plus lemon juice cake rise well with an open, spongy inside; the soda-only cake rises a little with a soapy taste and dark colour; the plain cake stays flat and dense.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why did the plain batter not rise?
Nothing in it made gas, so no bubbles formed.
02What is the difference between baking soda and baking powder?
Baking powder already contains an acid; baking soda needs an acid from the recipe.
03Why add lemon juice with baking soda?
Lemon juice is acidic and reacts with the soda to release carbon dioxide.
04What gas makes the cake spongy?
Carbon dioxide.
Take it further
Try doubling the baking powder in one cup and see whether more is always better, or whether the cake collapses.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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