Growing Crystals in Gels
Publication No. 10086
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.
Acetic acid solution, 1 M, CH3CO2H, 540 mL*
Copper(II) chloride solution, 1 M, CuCl2, 50 mL*
Lead nitrate solution, 1 M, Pb(NO3)2, 35 mL*
Potassium iodide solution, 2 M, KI, 35 mL*
Sodium chloride solution, 1 M, NaCl, 35 mL*
Sodium silicate solution (water glass), 540 mL*
Water, distilled or deionized
Zinc, Zn, 5" x ½" strip*
Graduated cylinders, 25-mL, 15
Paper clips, metal, 15
Parafilm M® (optional)
Pipets or eyedroppers
Rubber stoppers, #2, 45*
Test tubes, 20 x 150 mm, 45*
Test tube racks
*Materials included in kit.
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.
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.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesAsking questions and defining problems
Planning and carrying out investigations
Analyzing and interpreting data
Engaging in argument from evidence
Obtaining, evaluation, and communicating information
Disciplinary Core IdeasMS-PS1.A: Structure and Properties of Matter
MS-PS1.B: Chemical Reactions
HS-PS1.A: Structure and Properties of Matter
HS-PS1.B: Chemical Reactions
Crosscutting ConceptsEnergy and matter
Systems and system models
Cause and effect
MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.
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 CrystalsIn test tube 2 a single displacement, oxidation–reduction reaction also occurs. Zinc is being oxidized and lead is being reduced.
Zn(s) + Pb2+(aq) → Zn2+(aq) + Pb(s) Lead CrystalsIn test tube 3 a combination, precipitation reaction occurs. The lead and iodide ions combine to form the lead iodide solid.
Pb2+(aq) + 2I–(aq) → PbI2(s) Lead Iodide Crystals
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.