What it shows
Eggshells soaked in cola and fruit juice soften, stain and lose material within days, while shells in milk and water stay hard. Eggshell, like tooth enamel, is a hard mineral that acids slowly dissolve, so sugary and acidic drinks harm teeth.
The science
Eggshell is mostly calcium carbonate, and tooth enamel is mostly a calcium phosphate mineral; both are attacked by acids. Cola and juices are acidic, and the sugar in them also feeds mouth bacteria that make more acid. Over time the acid dissolves the mineral, leaving the shell soft and stained. Milk is not acidic and supplies calcium, and water is neutral, so those shells stay hard. Brushing removes the stain and the sugary film before the acid can do much.
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, dry halves of eggshell (from eggs used for cooking, washed) or 4 boiled eggs
- 4 cups with cola, packaged fruit juice, milk and water
- A toothbrush and a little toothpaste
- Cling film to cover the cups
- A kitchen scale (optional)
Safety
For this project: Wash the shells in hot soapy water with an adult before use. Do not drink from the test cups. Wash hands after handling.
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.
Weigh each shell half if a scale is available and write the mass down.
Put one shell in each cup and pour in enough liquid to cover it, then cover the cups with cling film.
Label the cups and keep them on a shelf away from sunlight.
Each day lift the shells out with a spoon, look at the colour and press gently with a fingernail to test hardness.
Change the liquids every two days so they stay fresh.
On day 7 rinse the shells, dry them and weigh them again.
Brush the stained shells with toothpaste and note how much of the stain comes off.
Record colour, hardness and mass change in a table.
What you should see
The cola shell is dark brown and soft enough to dent; the juice shell is stained and softened; the milk and water shells look unchanged. Brushing removes some stain but not the lost hardness.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is eggshell used instead of a real tooth?
Both are hard minerals containing calcium that acid can dissolve, and eggshells are easy to get.
2What in cola harms teeth?
Its acid, and its sugar which bacteria turn into more acid.
3Why did the milk shell stay hard?
Milk is not acidic and contains calcium.
4How can we protect our teeth after a sweet drink?
Rinse with water, drink through a straw, and brush twice a day.
Take it further
Coat one shell with fluoride toothpaste for a day before soaking it in cola and see whether it resists better.
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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