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Written by Deb Hodgkin

Looking for a fun and colourful science activity to do? Try making these egg geodes!

Safety: This activity involves a hot saucepan and a microwave oven. Ask an adult to help.
First aid: If you burn yourself, put the burn under cool, running water for 20 minutes.

You will need

• Raw eggs
• Empty egg carton
• Water
• Mixing containers
• Food colouring
• Epsom salts
• Saucepan
• Oven mitts
• Bowls
• Metal spoons
• Slotted spoon
• Ladle
• Rubber gloves

What to do

1. Break the eggs in two over a bowl. You can cook the eggs and eat them, or use them in another recipe. Carefully rinse the inside of the shells to remove the egg white and yolk.
2. Put a few shells into water in a saucepan and boil them for several minutes. Carefully remove the egg shells using the slotted spoon and allow them to cool.
3. Since you’ve been handling raw eggs, remember to wash your hands!
4. The ‘skin’ inside the shell should peel away with some care. After the skin is removed, the shells are fragile, so handle them gently.
5. Put 2 cups of warm water into a microwave-safe bowl. Add 1 cup of Epsom salts into the bowl and stir until as much as possible has dissolved.
6. Put the bowl in the microwave, and heat on a high setting for 3 minutes.
7. Remove the bowl from the microwave using the oven mitts and stir the solution until all the crystals have dissolved. The solution should be clear.
8. Allow the solution to cool for 10 minutes.
9. Carefully ladle the warm solution into different containers, one for each colour. Add a few drops of food colouring to each.
10. Put the egg shells into the containers and allow them to soak for about an hour.
11. Put on some rubber gloves and remove the shells from the solution. The shell should absorb a bit of colour, and you might already see some crystals forming.
12. Put your eggshells into an egg carton to hold them steady.
13. Carefully pour one of the coloured solutions into a shell until it is full.
14. Repeat with the other mixtures in different shells.
15. Put the filled shells somewhere warm and well-ventilated. The water will evaporate and leave crystals. Check them every day to see how your crystals are going.
16. If a crystal skin forms over the top, gently break it so the liquid trapped underneath can evaporate.

What’s happening?

When you dissolve a solid substance in liquid, the solid particles separate and spread out. The substance being dissolved is called a solute and the liquid is called a solvent. When you put in as much solute as the liquid can hold, the mixture is saturated. By heating the water you can dissolve more solute than water at room temperature. When the water cools down it holds more solute than a saturated solution, so it’s said this mixture is supersaturated.

When you leave a saturated or supersaturated solution to dry, the liquid evaporates and the solute becomes solid again. The particles of solute join with one another like tiny building blocks. If the mixture dries slowly, the solute particles have time to line up more precisely, forming crystal shapes.

Different solutes will form different crystals. Salt crystals are cubic and sugar crystals are hexagonal, flat and thin. Epsom salts usually grow long thin crystals.

Real-life science

Your eggs are a model for natural hollows in rocks that contain crystals, called geodes.

Holes in rock are often created by bubbles of gas in hot magma that has since cooled. Water filters through the surrounding rock, dissolving materials as it goes. When it gets into the rocky hollows, water can deposit solute and leave tiny crystals on the inside.

Over millions of years the crystals grow. Sometimes they can fill the entire rock. When this happens it is called a nodule. The most common crystals in natural geodes are quartz and calcite.

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