Teacher Notes

Egg Geodes

Student Laboratory Kit

Materials Included In Kit

Aluminum potassium sulfate (alum), AlK(SO4)2•12H2O, 1800 g
Paint brushes, 15 (minimum)
Plastic cups, 15
Plastic eggs, 8
White glue

Additional Materials Required

Beaker, 500-mL
Beaker tongs or heat-resistant gloves
Food dyes (optional, see Lab Hints)
Heat-resistant pad
Hot plate
Paper towels
Stirring rod
Water, distilled or deionized, 300-mL
Weighing dishes, 3

Safety Precautions

Avoid handling crystals with bare hands. Use caution when handling hot glassware. Use beaker tongs or wear heat-resistant gloves. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.


Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state, and local regulation that may apply before proceeding. The crystals eggs may be saved or disposed of in the regular trash after completion of this activity. It is not recommended that students be allowed to take the crystals home. Dispose of alum crystals and leftover solutions according to Flinn Suggested Disposal Method #26a and #26b, respectively.

Lab Hints

  • Enough materials are provided in this kit for 30 students working in pairs or for 15 student groups.
  • Day 1 of the lab can be done at the beginning or end of a class. It will take approximately 15 minutes to complete. Day 2 of the lab can be completed in one 50-minute period.
  • Have several weighing and dispensing stations around the lab to ease congestion. Three should be sufficient for 15 groups of students.
  • Each group will need a piece of clay approximately the size of a marble.
  • To make colorful crystal eggs, add 10–15 drops of food coloring to the alum solution before heating.

Teacher Tips

  • This is a great integrated activity that combines science and art and may be used as a component for a unit on crystallization, solutions and solubility, or mineral crystal shapes.
  • Use the egg geodes to decorate your classroom or lab. It is not recommended to allow students to take chemicals home.
  • If time permits, turn this into an inquiry activity. Allow students to design and carry out an experiment to test the variable of concentration or temperature of the aluminum potassium sulfate solution and observe the effect on the formation of the crystals.
  • Alum crystals are cubic, and may display an octahedral habit. Have students look at the alum crystals under magnification to observe their crystal shape.

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Asking questions and defining problems
Planning and carrying out investigations
Engaging in argument from evidence
Obtaining, evaluation, and communicating information

Disciplinary Core Ideas

MS-PS1.B: Chemical Reactions
HS-PS1.B: Chemical Reactions

Crosscutting Concepts

Cause and effect
Scale, proportion, and quantity

Performance Expectations

MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
HS-PS1-2. Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

Answers to Prelab Questions

  1. What is a crystal?

    A crystal is a solid substance consisting of an organized arrangement of atoms, molecules or ions.

  2. Briefly describe two ways minerals are formed.

    Minerals can form from the cooling of magma. Minerals also form from elements left behind as liquid evaporates from solution.

  3. What determines whether or not the crystal structure of a mineral is visible to the eye?

    If the unit cells of a crystal overlap, the crystal structure will not be visible to the eye.

Answers to Questions

  1. Why was it necessary to heat the solution once the alum was added to the water?

    Heating the solution allows the excess aluminum potassium sulfate crystals to dissolve.

  2. Explain why a supersaturated solution was needed for crystals to deposit on the egg.

    A supersaturated solution is very unstable. The addition of the egg with seed crystals along with the cooling of the solution caused the excess aluminum potassium sulfate to precipitate out of the solution until saturation was reached.

  3. Predict what would happen if you were to put your crystallized egg back into a saturated alum solution and an unsaturated alum solution, respectively.

    If the crystallized egg is placed into a saturated alum solution, there would be no change. The solute concentration in the solution is equal to the solubility. However, if the crystallized egg is placed into an unsaturated solution, it is likely that the crystals will dissolve back into the solution until saturation is reached.

  4. Sugar can also be used to grow crystals, known as rock candy.
    1. Explain the basic procedure you would use to make rock candy.
      1. Warm a sugar solution and add more sugar until no more will dissolve to make a supersaturated solution.
      2. Remove from heat and let cool to room temperature.
      3. Add a pinch of sugar to the solution to act as a lattice for excess solute to precipitate onto.
      4. Let it sit until all the excess sugar has precipitated out of solution onto the bottom of the jar—this will take a few days. Once all the excess has fallen out of solution, the solution is saturated.
    2. Would rock candy be considered a mineral? Explain your reasoning.

      Rock candy would not be considered a mineral because it is an organic substance. Rock candy is also man-made—it is not found in nature.


Baggaley, K. “Where do geodes come from?” Scienceline. http://scienceline.org/2012/11/where-do-geodes-come-from/ (Accessed August, 2015).

Holden, A.; Morrison, P. Crystals and Crystal Growing; MIT: Cambridge, MA, 1995.

Student Pages

Egg Geodes


Put a spin on egg decorating—add some crystals! Experience the science and beauty of crystal formation while growing your very own egg geodes.


  • Crystal formation
  • Solubility
  • Crystal structure
  • Saturation
  • Minerals


What is a Crystal?

A crystal is a solid substance made up of an organized arrangement of building blocks, such as atoms and molecules. The macroscopic regularity in the shapes of ice crystals, snowflakes, crystalline salts and gemstones suggests that crystals possess some sort of atomic-level regularity. This regularity is referred to as a crystal lattice and every crystal is built upon one. A crystal lattice is an orderly, repeating arrangement of atoms, molecules or ions. The smallest repeating pattern that reflects the macroscopic shape of the crystal is called a unit cell. In general, crystals are extended networks, constructed by repeating this unit cell pattern in all three dimensions.

Seven types of unit cells occur in nature—cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal and rhombohedral. Variations occur in several of these unit cell types. The unit cell for a given crystal system is made up of the base unit cell plus its variation. The seven types of unit cells, their variations and the associated crystal structures are sketched in Figure 1.

A particular crystal substance will generally exhibit a characteristic shape or group of shapes. This is knows as a crystal habit. The alum crystals grown in this activity have a cubic unit cell. However, the conditions in which the crystal is grown can have a large effect on the overall appearance of the crystal. The specific shape of the crystal that forms is determined by the rates at which its various faces grow. For instance, if a particular crystal face is more exposed to the growing solution, it will grow faster than the other faces. This would lead to a different shape than if all faces are evenly exposed and growing at the same pace. Faster growth can also lead to cloudy and irregularly shaped crystals. This is a result of the improper regularity of the crystal lattice formation. In other words, the molecules or atoms are less likely to line up in the proper orientation due to the rapid growth.

Crystals in Nature

Minerals are naturally occurring inorganic substances that have a regular internal structure and composition. Minerals exist in crystalline form. One way minerals can form is the cooling of magma (liquid rock). As magma cools, the atoms begin to lose heat energy, moving closer together to align into a crystal structure. The types of elements existing in the magma, along with the conditions applied, will determine the resulting mineral formation.

Another way crystals can form is from elements dissolved in liquids. As the liquid evaporates from solution, solids are left behind, forming crystals. Often the unit cells overlap, so the crystal structure is not outwardly visible. However, if the minerals are in an open space, the crystals have room to grow and the characteristic crystal shape becomes evident. Such is the case with geodes. A geode is a rock containing a cavity lined with crystals or other mineral matter. Imagine cutting open a rock to find it is filled with a beautiful array of colorful crystals! Animal burrows or tree roots can serve as hollow areas where geodes form, along with air bubbles in volcanic rock. Throughout time, mineral-rich water trickles into the hollow area. As the minerals hardens, an outer shell if formed and minerals continue to deposit on the inside walls of this shell, growing inwardly. This process can take a couple million years; therefore, many geodes are still somewhat hollow on the inside upon cutting them open.

The largest amount of solute that can be dissolved into a solvent is known as the solubility of that solute. A supersaturated solution exists when the solute concentration is more than the solubility of the solution. More clearly, supersaturation is the state of a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. Supersaturation can be reached by heating the saturated solution and adding more solute. The increase in temperature increases the solubility and allows more solute to be dissolved. As this solution cools, it has more dissolved solute than what normally occurs at that temperature. However, supersaturated solutions are not very stable. If just one crystal of solute is added to this solution, it will act as a lattice upon which the excess solute can precipitate. Precipitation of this solute will continue until the solute concentration is equal to the solubility, or saturation is reached.

Experiment Overview

The purpose of this experiment is to use the concepts of solubility and crystallization to grow your very own crystal geodes.


Aluminum potassium sulfate (alum), AlK(SO4)2•12H2O, 115 g
Food dye (optional)
Water, distilled or deionized
Beaker, 500 mL
Beaker tongs or heat-resistant gloves
Glue, white
Heat-resistant pad
Hot plate
Paint brush
Paper towel
Plastic cup
Plastic egg, one half
Stirring rod
Weighing dishes, 3

Prelab Questions

  1. What is a crystal?
  2. Briefly describe two ways minerals are formed.
  3. What determines whether or not the crystal structure of a mineral is visible to the eye?

Safety Precautions

Avoid handling crystals with bare hands. Use caution when handling hot glassware. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Use beaker tongs or wear heat-resistant gloves. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.


Day 1

  1. Weigh out 15 g of alum powder in a weighing dish.
  2. Obtain one half of a plastic egg and a small piece of clay.
  3. Roll clay into a small ball and attach to the outside bottom of the plastic egg.
  4. Add a quarter-sized amount of white glue to new weighing dish.
  5. Using a paintbrush, coat the inside of the eggshell with white glue. Be sure to also paint glue around the outer rim of the eggshell.
  6. Generously sprinkle alum powder into the eggshell, making sure to thoroughly cover all glued portions of the shell. Shake out any excess powder.
  7. Let dry overnight.
Day 2
  1. Using a weighing dish and balance, weigh out 100 g of alum powder and add it to a 500-mL beaker.
  2. Add 300 mL of distilled or deionized water to the beaker. (Optional) Add 10–15 drops of food coloring to the solution.
  3. Place the beaker on a hot plate and heat the solution until all the solid dissolves. Stir the solution occasionally as it heats to speed up the dissolution process.
  4. Once all of the solid has dissolved, turn off the hot plate. Using tongs or heat-resistant gloves, carefully remove the beaker from the hot plate and let cool for 10 minutes on a heat-resistant pad.
  5. As the solution cools, obtain the plastic cup and prepared egg.
  6. Carefully rest the egg with the clay-side down on the bottom of the cup. Press lightly to adhere the clay and egg to the cup. This will help the alum crystallize evenly.
  7. After the 10-minute cooling period has passed, slowly pour all of the alum solution down the side of the plastic cup. Do not pour directly on top of the egg. If the egg is not adhered strongly to the bottom of the cup, it may begin to float. Use a stirring rod to push the egg down, allowing it to fill with solution. Let the filled egg float at the top of the solution—it will sink as alum crystallizes in the egg.
  8. Set aside overnight.
Day 3
  1. Carefully pour the liquid out of the cup.
  2. Wearing gloves, gently remove the crystallized egg.
  3. Place the eggshell upside down on a paper towel. Let dry for 20 minutes.
  4. Consult your instructor for appropriate disposal of the leftover alum solution.

Student Worksheet PDF


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