What it shows
Eggshell is mostly calcium carbonate. When it reacts with vinegar it gives off carbon dioxide, so the loss in mass of the flask tells you how much calcium carbonate was present.
The science
Calcium carbonate reacts with acetic acid: CaCO3 + 2 CH3COOH gives Ca(CH3COO)2 + H2O + CO2. The carbon dioxide escapes, so the mass of the jar and its contents falls by exactly the mass of CO2 given off. One mole of CO2 weighs 44 g and comes from one mole of CaCO3, which weighs 100 g. So every 44 g lost means 100 g of calcium carbonate. Dividing by the mass of the shell gives the percentage.
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.
- 3 to 4 clean dry eggshells, crushed to powder (kitchen waste)
- White vinegar, 250 ml
- A wide jar or conical flask of 500 ml
- A scale reading to 0.1 g or better
- A paper cup for the powder
- A piece of cotton wool or loosely folded foil to cover the mouth
- A spoon and a notebook
Safety
For this project: An adult must supervise. The mixture froths strongly, so use a jar much bigger than the liquid. Wash hands after handling raw eggshell because it can carry germs.
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.
Wash the eggshells, peel the thin inner membrane, dry them in sunlight for a day and crush them to a fine powder.
Weigh about 5 g of powder in a paper cup and note the exact mass.
Pour 150 ml of vinegar into the jar. This is more than the shell needs, so the shell is the limiting reactant. Weigh the jar with the vinegar and the cup of powder together and write this as the starting mass.
Add the powder slowly to the vinegar in small pinches. Wait for the froth to settle each time so it does not overflow.
When the fizzing slows, lay a loose pad of cotton on the mouth and leave the jar for several hours or overnight so every particle reacts.
Weigh the jar, the empty cup and the cotton together as the final mass. A common mistake is to forget to include the empty cup in the final weighing.
Calculate the mass of CO2 lost, then use mass of CaCO3 = loss x 100 / 44 and percentage = mass of CaCO3 / mass of shell x 100.
Repeat with a second sample and take the average. Write a note on the errors: water vapour lost with the gas and the small mass of the membrane.
What you should see
A calculated percentage of calcium carbonate, typically very high, in the 90s. Two trials that agree within a few percent show careful work.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the mass fall during the reaction?
Carbon dioxide gas escapes into the air, taking its mass away from the jar.
2Why use excess vinegar?
So all the calcium carbonate reacts and the loss measures the whole sample.
3What is the mass of CO2 given by 1 g of pure calcium carbonate?
0.44 g, because 100 g gives 44 g.
4What can make your answer a little too high?
Water vapour and vinegar spray leaving along with the gas.
Take it further
Repeat with chalk, marble chips and a sea shell, and compare the percentage of calcium carbonate in each.
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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