Growing Crystals in Gels

Introduction

A dilute solution of sodium silicate (water glass) is mixed with an organic acid (usually acetic acid) in which an ion has been incorporated. A gel forms overnight. Another ionic solution or a metal is then placed on top of the hardened gel, and spectacular crystals of a new substance form in the gel as the aqueous ion slowly diffuses downward. The longer these are permitted to grow, the larger and more beautiful they become. In this experiment, you will grow crystals of three different substances, copper metal, lead metal, and lead iodide. Although results can be seen within hours, the crystals become more enchanting after growing for 2 to 3 weeks. They make a stunning yearlong classroom display, and if not allowed to dry out, will last for decades. Students can grow them easily and are quickly motivated. A display of these in a hallway showcase will attract constant attention.

Concepts

• Crystals
• Chemical reactions
• Saturated solutions

Materials

Safety Precautions

Lead solutions are toxic by inhalation and ingestion. Copper(II) chloride solutions are toxic by ingestion and inhalation. Acetic acid solutions are corrosive to skin and tissue and slightly toxic by ingestion. Please review current Safety Data Sheets for additional safety, handling, and disposal information. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

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. Gel crystals can be stored indefinitely. Seal test tubes with Parafilm M^®. Test tubes that contain lead crystals should betreated according to Flinn Suggested Disposal Procedure #27f. The other crystals may be thrown away according to Flinn Suggested Disposal Procedure #26a.

Prelab Preparation

1. Add 55 mL of deionized or distilled water to the bottle of solid potassium iodide.
2. Cap and shake the bottle vigorously until all the solid is in solution. Note: Potassium iodide has a poor shelf life. Prepare this solution a few minutes prior to the lab.

Procedure

Day 1

1. Label three test tubes with your initials and number them 1 to 3. Put labels near the tops of the tubes.
2. Put 2.5 mL of 1 M copper(II) chloride solution, CuCl₂, into tube 1. Rinse the graduated cylinder.
3. Put 17 drops of 1 M lead nitrate solution, Pb(NO₃)₂, into tube 2 and another 17 drops of Pb(NO₃)₂ into tube 3.
4. Using the graduated cylinder, carefully measure 12 mL of 1 M acetic acid solution, CH₃COOH, and pour it into tube 1. Do this again for tubes 2 and 3.
5. Put the stoppers on the tubes and gently turn them upside down several times to mix the chemicals. Carefully remove the stoppers.
6. Rinse the graduated cylinder, and using the clean graduated cylinder, measure 12 mL of sodium silicate solution, Na₂Si₃O₇, into each of the three tubes.
7. Put stoppers back in the tubes and again gently turn them upside down several times to mix the chemicals.
8. Leave the stoppers in and put your tubes in a test tube rack. Allow them to harden overnight.

9. Remove the stopper from tube 1. Using forceps, push the metal paper clip vertically into the surface of the gel. Push it carefully into the gel so it is just covered. Gently put an eyedropper full of 1 M sodium chloride solution, NaCl, into the tube and put the stopper back on.
10. Cut a piece of zinc metal (¼" x ⅝") small enough to fit into the test tube. Remove the stopper from tube 2. Using the forceps, push the piece of zinc metal, vertically, into the surface of the gel. Push it carefully into the gel so it is just covered. Gently put an eyedropper full of water into the vial and put the stopper back on.
11. Remove the stopper from tube 3. Gently put 2 mL of the 2 M potassium iodide solution, KI, into the tube and put the stopper back on.

Lab Hints

• The crystals will develop over the months to come. Some crystals will decompose and some will remain unchanged.
• After several weeks, test tube 1 will slowly lose its beauty as iron(III) ions are produced due to the oxidation of Fe²⁺. In addition, hydrogen bubbles form due to the slow reaction of Fe²⁺ with the acid.
• Test tube 2 will also produce hydrogen bubbles and will tend to pop its stopper. Seal test tubes with Parafilm M^®.
• All crystals will take approximately two weeks to grow.
• If using concentrated sodium silicate solution (Catalog No. S0102), dilute 120 mL sodium silicate solution to 800 mL with deionized or distilled water. Note: Solid sodium meta-silicate cannot be dissolved to make sodium silicate solution. This process requires a high pressure and high temperature. However, the sodium silicate solution included int his kit is already diluted.
• Growing Crystals in Gels was developed into two chemistry laboratory kits. The introductory kit (AP8750) allows for the preparation of 45 gel tubes (3 different crystals—15 of each) and the advanced kit (AP8751) allows for the preparation of 90 gel tubes (6 different crystals—15 of each).

Discussion

The reaction in test tube 1 can generally be called a single displacement reaction. It can be understood further if we also call it an oxidation–reduction reaction where iron is being oxidized and copper is being reduced.

Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s) Copper Crystals

Zn(s) + Pb²⁺(aq) → Zn²⁺(aq) + Pb(s) Lead Crystals

Pb²⁺(aq) + 2I^–(aq) → PbI₂(s) Lead Iodide Crystals

References

Special thanks to Tanya Phillips, Piedmont Unified School District, Piedmont, CA, for providing us with this activity. Tanya would like to thank Dr. Earle Scott, Professor Emeritus, Ripon College, WI, for introducing her to these wonderful crystals.