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Crystal Growing Lab

Introduce your students to crystal growing with this economical and engaging activity. Using low-cost materials, students grow crystals and learn about crystal formation and structure, solutions, precipitation, and safe lab practices.

Grade level: Although designed for 4th through 9th graders working in groups, this activity works well as a whole-class demonstration for younger children. High school students can use the extensions to explore different types of crystal structures.

National Standards: Grades K–4 Properties of Objects and Materials, Grades 5-8 Properties and Changes of Properties in Matter, and Grades 9-12 Structure and Properties of Matter and Interaction of Energy and Matter.

Background

Crystals are solids that form when molecules join together in a regular repeating pattern. In this activity, you dissolve crystallizing molecules in water and your students create a setup to observe the ordered structure of these crystallizing molecules as they solidify.

Materials needed

  • 250-mL beaker or 9-oz plastic cup (for each group)
  • 500-mL beaker
  • Coffee stirrer or pencil (for each group)
  • Hand lens (for each group)
  • Hotplate
  • Lab apron
  • Lab marker (for each group)
  • Pair of forceps
  • Piece of nylon or cotton string 15 to 20 cm long (for each group)
  • Shallow container or petri dish
  • Small piece of masking tape (for each group)
  • Rubber gloves
  • Safety goggles
  • Salt (at least 100 mL)
  • Water

Preparation (instructor)

Caution: Review lab safety with your students before they begin work or before you demonstrate the activity. Use caution around hotplates and hot water. When demonstrating this activity, ensure students maintain a safe distance from the work area to avoid injury from accidental splashes or spills.

  1. Put on the gloves, apron, and goggles.
  2. Add about 300 mL water to the 500-mL beaker.
  3. Place the beaker on the hotplate. Adjust the heat to medium and heat the water until it is very hot, but do not let it boil.
  4. Gradually add salt to the beaker and stir to ensure it all dissolves in the water. Add at least 100 mL salt to the water or as much as will go into solution.
  5. Use the forceps to remove the beaker from the hotplate, and then pour a small amount of saltwater solution into the shallow container or petri dish. Allow the solution to sit undisturbed for about one hour. Small salt crystals will form in the container. These small crystals will be used as the "seed" crystals for each group.
  6. Divide your class into groups of 3 to 4 students each.
  7. Make sure each group has the following materials: a 250-mL beaker or 9-oz plastic cup, a coffee stirrer or pencil, a piece of nylon or cotton string 15 to 20 cm long, and a hand lens.

Procedure (students)

  1. Obtain a "seed" crystal from your instructor and tie it to one end of the piece of string.
  2. Tie the other end of the string to the coffee stirrer or pencil.
  3. Label the beaker or cup with your group's name using the piece of masking tape and lab marker.
  4. Place the end of the string with the seed crystal into the salt solution and lay the coffee stirrer or pencil across the mouth of the beaker or cup.
  5. Place your group's beaker or cup in a place designated by your instructor. Allow it to sit undisturbed for at least 24 hours.
  6. After the 24 hours have passed, remove the string from the solution. Use the hand lens to examine the crystalline structures that have formed on the string.

Discussion

Heating the water allows it to hold much more salt in solution than it could at room temperature. As the water cools, its ability to hold the salt in solution diminishes. This causes the excess salt in the solution to precipitate (form a solid) in crystal form on the string.

Extensions

  1. Examine the effects of cooling on crystal growth. Prepare 2 crystal-growing containers as detailed above. Place one in a refrigerator or container of ice and leave the other exposed to room temperature air. Let them remain undisturbed for 24 hours, and then observe and compare the crystals grown at different temperatures.
  2. Prepare 2 different crystal-growing solutions using 2 of the chemicals listed below. Use the salt solution preparation procedure, substituting the chemicals in the list for salt. Observe and compare the different types of crystals grown.

Caution: Some of the chemicals listed below may be harmful if swallowed or inhaled. Do not eat or inhale any of the chemicals used in this activity. Do not lick the crystals or put them in your mouth. Wash your hands after handling chemicals and crystals.

  • Copper sulfate
  • Magnesium sulfate heptahydrate (Epsom salt)
  • Nickel sulfate hexahydrate
  • Potassium aluminum sulfate dodecahydrate (alum)
  • Potassium ferricyanide
  • Potassium sodium tartrate tetrahydrate (Rochelle salt)
  • Sodium nitrate
  • Sucrose (sugar) (Procedure note: Make sure the water is boiling when you add the sugar.)

Additional resources

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