What it shows
A bath bomb is a pressed mix of baking soda and citric acid held together by a little oil. Dropped in water it fizzes as carbon dioxide forms. Timing identical bombs in cold and warm water shows that the same reaction runs faster when the water is warmer.
The science
Citric acid and sodium bicarbonate stay unreacted while dry, and the oil coats the grains and keeps moisture out so the bomb can be stored. In water the two dissolve and react, giving carbon dioxide, water and sodium citrate. Warm water has faster-moving molecules, so the solids dissolve more quickly and the dissolved particles collide more often and with more energy, which makes the reaction, and the fizz, finish sooner.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Baking soda, 4 tablespoons
- Food-grade citric acid, 2 tablespoons
- Coconut oil, 2 teaspoons, slightly softened
- Food colour and a drop of perfume, optional
- Bottle caps or a small spoon as moulds
- Two bowls of water, one cold and one warm (adult pours warm)
- A timer
Safety
For this project: An adult must pour the warm water, which should be bearable to touch and not hot. Keep citric acid out of the eyes, wash hands after mixing, and do not eat the mixture.
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.
Mix the baking soda and citric acid in a dry bowl, breaking up any lumps.
Add the colour to the soft oil, then rub the oil into the powder with your fingers until it holds together when squeezed, like damp sand.
Press the mix firmly into bottle caps to make at least six bombs of the same size.
Leave them to dry overnight, then tip them out carefully.
Drop one bomb in the cold water and start the timer; stop when the fizzing ends.
Drop a second bomb in the warm water and time it the same way.
Repeat each test twice more with fresh bombs so you have three times for each temperature, and take the average.
Draw a bar chart of fizz time for cold and warm water and explain the difference.
What you should see
Bombs fizz for noticeably longer in cold water than in warm water, often nearly twice as long, with the warm bowl bubbling more violently; averages of three runs make the trend clear.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why does the bomb not fizz on the shelf?
The ingredients are dry and coated in oil, so they cannot dissolve and react.
02Why is the fizz shorter in warm water?
Warm water speeds up dissolving and the reaction, so the gas is released faster.
03Why repeat each test three times?
Single results vary; an average is more reliable.
04What does the oil do?
It binds the powder and keeps moisture away until the bomb is used.
Take it further
Make bombs with twice the citric acid and see whether they fizz longer or just leave acid behind.
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.
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