The Crystal Forest

Introduction

Put a new twist on crystal growing. In this class participation demonstration, students cut out and assemble miniature trees and place them in solutions of various ionic compounds. Overnight, the trees transform into a forest of snow-covered firs.

Concepts

• Crystal formation
• Transpiration

Materials

Safety Precautions

Household ammonia is slightly toxic by ingestion and inhalation; both liquid and vapor are extremely irritating especially to the eyes. Use caution when handling both bluing solution and the crystal growing solution to prevent spilling on clothes. The bluing solution will stain clothing. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

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 trees and crystals and sodium chloride may be disposed of according to Flinn Suggested Disposal Method 26a. The bluing and ammonia solutions may be disposed of according to Flinn Suggested Disposal Method 26b.

Prelab Preparation

1. Distribute six sheets of blotting paper with printed trees and have students cut out the 60 tree silhouettes.
2. Each student should have one set of tree silhouettes, one tree silhouette with a marked 1½" line at the top and one with 1½" line at the bottom (see Figure 1).
   {13850_Preparation_Figure_1}
3. Cut the 1½" line on each tree silhouette. Make a tree by sliding the tree silhouette with a bottom cut over the tree with a top cut to form a three-dimensional tree with four sides at 90° angles (see Figure 2).
   {13850_Preparation_Figure_2}
4. (Optional) Add a drop of food coloring to the tips of each branch on the trees. This will add color to the crystals that form.

Procedure

1. Place the 1-L Erlenmeyer flask on the hot plate in a fume hood or a well-ventilated area.
2. Using the 500-mL graduated cylinder, measure out 420 mL of distilled or deionized water and transfer it to the 1-L Erlenmeyer flask.
3. Heat the water to just below boiling (80 °C–90 °C).
4. Weigh out 500 g of sodium chloride and slowly add the entire amount to the 1-L Erlenmeyer flask. Stir to dissolve.
5. Thoroughly mix the bluing solution by shaking the bottle with the cap securely fastened.
6. Using the 500-mL graduated cylinder, measure out 420 mL of the bluing solution and transfer it to the 1-L Erlenmeyer flask. Stir to mix.
7. Using the 100-mL graduated cylinder, measure out 70 mL of the household ammonia and add this to the 1-L Erlenmeyer flask. Stir to mix.
8. Pour 30 mL of the solution into each weighing dish.
9. Place the weighing dishes in a place where they will not be disturbed. Then, stand a tree up in each weighing dish, making sure the bottom edges are soaking in the solution. The crystals will start to form in 1–12 hours, depending on the rate of evaporation. The crystals will continue to be deposited up to 25 to 48 hours or until all the liquid has evaporated. Do not move or touch the trees once the crystals start to form. The crystals are very fragile.

Teacher Tips

• This kit contains enough materials for five classes of 30 students: 2.2 L of bluing solution; 400 mL of ammonia solution; 2700 g of sodium chloride; 30 sheets of blotting paper; and 150 disposable weighing dishes.
• Using hot water helps the salt dissolve faster and also allows for more rapid crystal formation.
• Use proper decanting techniques to prevent spilling and possible staining by the bluing solution.
• All the sodium chloride may not dissolve. The excess can be left in the 1-L beaker.
• Because the bluing solution contains undissolved solids, be sure to thoroughly mix the bluing solution before adding it in step 4.

Discussion

The porous blotting paper allows for both capillary action and evaporation. The solution rises through the blotting paper due to capillary action and the solvent slowly evaporates, beginning at the top edges and tips of the tree branches. As the solvent evaporates, crystals begin to form. Like tree and plant transpiration, evaporation causes more solution to be pulled up through the tree to the edges, where further evaporation leads to a buildup of more crystal deposits.

The bluing solution contains two forms of the blue pigment Prussian blue. The “soluble” form KFe^IIIFe^II(CN)₆ is actually a stable colloidal suspension (a solid dispensed in a liquid), while the insoluble form Fe^III₄[Fe^II(CN)₆]₃•xH₂O settles out of solution upon standing.

The bluing mixture is combined with an ammonia solution (NH₃) and a saturated solution of sodium chloride (NaCl). The fluffy white crystals that form on the branches of the trees are believed to be ammonium chloride (NH₄Cl) and two forms of ferrous ferrocyanide, KNaFe^IIFe^II(CN)₆ and Na₄Fe^II ₄[Fe^II(CN)₆]₃. The latter two compounds are produced from the reduction of the Prussian blue by ammonia in solution.

References

DeKorte, John M., Pocket Guide to Chemistry & Chemical Reactivity, 4th Edition, Saunders College Publishing; 1999; pp. 295–297.

Katz, David A., Chemistry in the Toy Store, 5th Edition, Community College of Philadelphia, Philadelphia, PA, 1990.

McDuffie, Jr., Thomas E., and Anderson, Jacqueline. Chemical Experiments from Daily Life. Portland, ME: J. Weston Walch Publisher, 1980.