Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Place the 1-L Erlenmeyer flask on the hot plate in a fume hood or well-ventilated area.
2. Using the 500-mL graduated cylinder, measure 420 mL of distilled or deionized water and transfer it to the 1-L Erlenmeyer flask.
3. Heat the water to just below boiling (80–90 °C) then remove from heat.
4. Weigh out 350 g of sodium chloride and slowly add the entire amount to the 1-L Erlenmeyer flask. Stir to dissolve. Note: Some of the sodium chloride may not fully dissolve. More will dissolve as the solution is made.
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 bluing solution and transfer it to the 1-L Erlenmeyer flask containing the salt solution. Stir to mix.
7. Using the 100-mL graduated cylinder, measure out 70 mL of household ammonia and add this to the 1-L Erlenmeyer flask containing the salt and bluing solution mixture. Stir to mix.

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 the bluing solution and 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. 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.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations 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.

Lab Hints

• Enough materials are provided in this kit for 30 students. This laboratory activity can reasonably be completed in one 50-minute class period.
• Using hot water helps the salt dissolve faster and also allows for more rapid crystal formation.
• Use proper decanting techniques to prevent spilling and possibly staining by the bluing solution.
• All the sodium chloride may not dissolve. The solubility of sodium chloride near the boiling point of water is 385 g NaCl per 1 L of water. When the sodium chloride is initially added to the water and stirred, there will be solid at the bottom of the flask. This is done in order to make a saturated solution. With the addition of the bluing solution and household ammonia, more salt will dissolve in the greater volume. Any solid at the bottom of the flask is needed to maintain a saturated solution required for crystal formation.
• Because the bluing solution contains undissolved solids, be sure to thoroughly mix the bluing solution before adding it in step 6.
• To reduce congestion at the crystal forming solution flask, separate the solution into four 250-mL beakers.

Answers to Questions

1. Describe what happened in this activity.

   Trees were cut out of blotting paper and placed in a mixture of bluing solution, sodium chloride, and household ammonia. In several hours, crystals started to form on the tips of the trees. Within a couple days the tree branches were covered in thick crystals.

2. The porous paper promoted both capillary action and evaporation. How might this facilitate the growth of crystals?

   The capillary action draws the liquid up throughout the paper. The same porosity that aids capillary action also allows the liquid to evaporate. As the liquid evaporates, crystals start to form. Evaporation dries the paper and allows greater uptake of solution, creating more crystals.

3. The bluing solution contains Prussian blue in two forms, one soluble and one insoluble. The soluble form is KFe^IIIFe^II(CN)₆, and the insoluble form is Fe^III ₄[Fe^II(CN)₆]₃. These react with ammonia to produce KNaFe^IIFe^II(CN)₆ and Na₄Fe^II₄[Fe^II(CN)₆]₃, respectively. Has the Prussian blue been oxidized or reduced by the ammonia? These reactions are shown in the Background section.

   Both forms of Prussian blue have been reduced by the ammonia, since the Fe³⁺ ions in both cases gained electrons and therefore experienced a decrease (reduction) in oxidation state, becoming Fe²⁺ ions.

4. Write three testable questions that can be answered by modifying your current setup and performing an experiment.

   Would wetting the blotting paper tree before placing it in the solution have an effect on the formation of crystals?
   Does the height of the tree make a difference in the formation of crystals?
   Will the crystals form at the very top of the tree? What effect would using a metal chloride salt other than sodium chloride have on the structure and shape of the crystals?
   Would a different shape of blotting paper still result in the same crystal-formation pattern?

References

DeKorte, J. M. Pocket Guide to Chemistry & Chemical Reactivity, 4th ed.; Saunders College: 1999; pp 295–297.

Katz, D. A. Chemistry in the Toy Store, 5th ed.; Community College of Philadelphia: Philadelphia, 1990.

McDuffie, Jr., T. E.; Anderson, J. Chemical Experiments from Daily Life; J. Weston Walch: Portland, 1980.

Introduction

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

Concepts

• Crystal formation
• Capillary action
• Transpiration
• Oxidation–reduction

Background

In this lab, white fluffy crystals will be “grown” on blotting paper trees. To fully understand how these crystals form from a solution, the concepts of capillary action and transpiration need to be addressed.

Porous blotting paper allows for capillary action and facilitates evaporation. When placed in a solution, the liquid is drawn up through the fibers of the paper. This is called capillary action. As the solution is drawn up the tree, evaporation occurs beginning at the top edges and tips of the branches, and crystals start to form. The evaporation of the solvent causes more solution to be pulled up through the tree. This is called transpiration and is seen in plants and trees. As more solution is drawn up and further evaporation occurs, there is a build up of more crystal deposits.

The crystals that form on the paper trees result from a reaction between bluing solution, ammonia (NH₃), and a saturated solution of sodium chloride (NaCl). The bluing solution is actually two forms of the blue pigment Prussian blue. The “soluble” form, KFe^IIIFe^II(CN)₆, is actually a stable colloidal suspension (a solid dispersed in a liquid). The insoluble form, Fe^III₄[Fe^II(CN)₆]₃•xH₂O, settles out of solution upon standing.

In the reaction between the Prussian blue, ammonia and sodium chloride, two forms of iron II ferrocyanide, KFe^IIFe^II(CN)₆ and Na₄Fe^II₄[Fe^II(CN)₆]₃, are made. The reactions are shown. These forms of iron II ferrocyanide along with ammonium chloride are the crystals that “grow” on the paper trees.

Experiment Overview

In this activity, crystals will be grown on paper trees from a solution of sodium chloride, bluing solution, and ammonia.

Materials

Safety Precautions

The crystal growing solution contains ammonia. Household ammonia is slightly toxic by ingestion and inhalation; both liquid and vapor are extremely irritating especially to the eyes. Use caution when handling the crystal growing solution to prevent spilling on clothes. The bluing solution will stain clothes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

1. Using scissors cut out two tree silhouettes. One tree silhouette is marked with a 1½" line at the top and one with 1½" line at the bottom (see Figure 1).
   {13849_Procedure_Figure_1}
2. Cut the 1½" line on each 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).
   {13849_Procedure_Figure_2}
3. (Optional) Add a drop of food coloring to the tips of each branch on the tree. This will add color to the crystals that form.
4. Measure 30 mL of the crystal growing solution using a 50-mL graduated cylinder. Pour this into the weighing dish.
5. Place the weighing dish in a place where it will not be disturbed. Stand the tree up in the weighing dish, making sure the bottom edges are soaking in the solution. The crystals will start to form in 1 to 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 tree once crystals start to form. The crystals are very fragile.
6. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

13849_Student1.pdf