What it shows
Alum dissolves easily in hot water and much less in cold water. A saturated alum solution left in half an eggshell slowly deposits sparkling crystals on the shell wall, making a model of a geode, the hollow rocks lined with crystals that form in the same way underground.
The science
Like sugar, alum becomes much more soluble as water gets hotter. The hot solution is saturated; as it cools it cannot hold all the alum and the extra must come out as solid. Crystals form fastest on rough surfaces that are already dusted with alum, so the glued, dusted shell wall becomes covered with pointed octahedral crystals. In nature, mineral-rich groundwater seeping into a hollow rock does the same over thousands of years.
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 tablespoons alum (fitkari) from a general store
- Half a cup of hot water (adult handles)
- Two clean, dry eggshell halves
- White glue and a small brush
- A little extra powdered alum for dusting
- Food colour, optional
- An egg carton to hold the shells level
Safety
For this project: An adult must handle the hot water and pour the solution. Alum is a kitchen and water-purifying chemical but must not be eaten; wash hands afterwards.
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 eggshell halves, remove the inner skin and let them dry completely.
Brush the inside of each shell with glue and sprinkle powdered alum over the wet glue; shake off the loose powder and let it dry.
Ask an adult to dissolve alum in the hot water, a spoon at a time, until a few grains stay undissolved at the bottom.
Add a drop of food colour if you want a coloured geode, and let the solution cool for five minutes.
Set the shells in the egg carton and have the adult fill each one to the brim with the solution.
Leave the shells undisturbed in a warm, dust-free place for two to three days.
Pour off the leftover liquid when the crystals are large enough and let the geode dry on a tissue.
Compare the crystal shapes under a magnifying glass with the sugar or salt crystals you have seen before.
What you should see
The inside of each shell is lined with glittering, pointed alum crystals, often a few millimetres across, that look like a real geode when the shell is cracked open for display.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why dust the shell with alum first?
The grains give the crystals a starting point, so they grow on the shell rather than at the bottom.
2What is a saturated solution?
One that holds as much dissolved solid as it can at that temperature.
3How do real geodes form?
Mineral-rich water seeps into a hollow in rock and slowly deposits crystals as it evaporates or cools.
4What is alum used for in homes?
To settle mud out of water and as an after-shave because it kills some germs.
Take it further
Grow a second geode with sugar or with Epsom salt and compare the crystal shapes and growing speed.
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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