What it shows
Foods and drinks can stain the surface of teeth, and brushing removes the stain. Stained eggshells stand in for teeth to compare rinsing, brushing with water and brushing with toothpaste.
The science
Dark drinks such as tea stain the surface of teeth. Plaque, a film of food and bacteria, also builds on teeth and can cause decay, so it must be removed by brushing twice a day. Toothpaste contains a mild scrubbing material and a detergent that help lift stains, while fluoride in it helps strengthen enamel. An eggshell is made of calcium carbonate and is softer than tooth enamel, so it is only a rough model.
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.
- 4 clean eggshell halves from boiled eggs
- A cup of strong, cooled tea
- An old soft toothbrush for each test
- Toothpaste
- A timer
- Four cups for rinsing
- A notebook and a white card for comparing colours
Safety
For this project: An adult must pour hot tea and cool it before use. Do not use the eggshells or the brush in the mouth afterwards. Wash hands after touching eggshell.
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.
All the safety rulesStep 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.
Soak all four eggshell halves in cooled strong tea overnight until they are clearly brown.
Take them out, rinse them lightly in water and let them drain, then label them A, B, C and D.
Leave A as the untouched stained control and place it on the white card.
Swish B in a cup of plain water for one minute, without brushing, and set it beside the card.
Brush C with a wet toothbrush and no toothpaste for two minutes using small circles.
Brush D with a pea-sized blob of toothpaste for two minutes in the same way, then rinse it.
Dry them all and compare the colours against A, ranking them from the darkest to the lightest.
Write a conclusion on what removed the most stain and why we brush for two minutes, twice a day.
What you should see
The unbrushed shell is darkest, the swished one only slightly lighter, the water-brushed shell lighter still, and the toothpaste-brushed shell is cleanest.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which shell was the control?
The stained shell that was not cleaned.
2What is plaque?
A sticky film of food and bacteria on teeth.
3Why is an eggshell not exactly like a tooth?
It is softer and made of different material than enamel.
4How long and how often should teeth be brushed?
About two minutes, twice a day.
Take it further
Compare a soft brush with a hard-bristled brush on stained shells and find out which leaves more scratches under a magnifier.
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
Science projects
Science Projects for Class 6 to 8
Working models matched to middle-school science topics
Science projects
Biology Models Guide
Make the heart, lungs, kidney, eye, cell and more, and explain them well