Growing Crystals in Gels

Introduction

Grow your own beautiful, long-lasting, demonstration-size crystals in gels!

Concepts

  • Crystal formation
  • Ions
  • Gel formation

Background

In this experiment, you will grow crystals of two different substances—lead iodide and potassium hydrogen tartrate. Although results can be seen within hours, the crystals become more enchanting after growing for 2–3 weeks. The crystals make a stunning year-long classroom display, and if not allowed to dry out, will last for decades. Display these gel crystals in a showcase to attract the attention of all.

Materials

Acetic acid solution, CH3COOH, 1 M, 540 mL*
Lead nitrate solution, Pb(NO3)2, 1 M, 35 mL*
Potassium chloride solution, KCl, saturated, 200 mL*
Potassium iodide, KI, 33 g*
Sodium silicate solution, (water glass), Na2Si3O7, 15%, 1 L*
Tartaric acid solution, H2C4H4O6, 3 M, 400 mL*
Bottles with caps, 1-L, 2*
Graduated cylinders, 100- and 500-mL
*Materials included in kit.

Safety Precautions

Lead solutions are toxic by inhalation and ingestion. Avoid contact with eyes, skin and 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. 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. Bottles that contain lead iodide crystals and excess lead nitrate should be disposed of using Flinn Suggested Disposal Method #27f. Potassium hydrogen tartrate crystals may be discarded in the solid waste disposal according to Flinn Suggested Disposal Method #26a.

Prelab Preparation

Add 100 mL of deionized or distilled water to the potassium iodide bottle (AP8751C). Cap and shake the bottle vigorously until all solid is in solution. Potassium iodide solution has a poor shelf life. Prepare this solution a few minutes prior to the lab.

The sodium silicate solution included in the kit is diluted to the proper 15% concentration. If using concentrated sodium silicate solution, or waterglass (Catalog No. S0103), first dilute this concentrated solution down to a 15% solution. To carry out this dilution, dilute 120 mL sodium silicate solution to 800 mL with distilled or deionized water.

Procedure

  1. Obtain the two 1-L plastic bottles.
  2. Place 35 mL of 1 M lead nitrate solution, Pb(NO3)2 into one of the 1-L bottles.
  3. Using a 500-mL graduated cylinder, measure 396 mL of 1 M acetic acid, CH3COOH, and pour it into the first bottle. Rinse the graduated cylinder.
  4. Screw the cap on the first bottle and gently turn it upside down several times to mix the chemicals.
  5. Using the 500-mL graduated cylinder, measure 396 mL of sodium silicate solution, Na2Si3O7, and pour it into the first bottle.
  6. Screw the cap on the bottle and again gently turn the bottle upside down several times to mix the chemicals. Set the bottle aside to allow the gel to form and harden.
  7. Obtain the second 1-L bottle. Using a 500-mL graduated cylinder, measure 360 mL of 3 M tartaric acid, H2C4H4O6, and pour it in the bottle. Rinse the graduated cylinder.
  8. Measure 360 mL of sodium silicate solution, Na2Si3O7, and add it to the second bottle. Screw the cap on the bottle and gently turn the bottle upside down several times to mix the chemicals.
  9. Store the bottles overnight.
  10. The first bottle should harden within 24 hours. The second bottle may need 3–4 days to harden.
  11. Remove the cap from the first bottle after the gel has hardened. Gently add 66 mL of the potassium iodide solution, KI, into the bottle and replace the cap.
  12. After the second bottle has hardened (usually 3–4 days), remove the cap and add 150 mL of saturated potassium chloride solution, KCl. Holding the bottle on an angle, slowly and gently pour the KCl solution into the bottle and replace the cap.
  13. After the crystals have begun to grow, fill both bottles to the brim with water to further preserve the crystals.

Student Worksheet PDF

11974_Student1.pdf

Teacher Tips

  • All crystals will take approximately two weeks to grow, but will continue growing over time.
  • If the tartaric acid is diluted to 1.5 M, crystals will be more elongated in shape.

Correlation to Next Generation Science Standards (NGSS)†

Science & Engineering Practices

Developing and using models

Disciplinary Core Ideas

MS-PS1.A: Structure and Properties of Matter
MS-PS1.B: Chemical Reactions
HS-PS1.B: Chemical Reactions
HS-PS1.A: Structure and Properties of Matter

Crosscutting Concepts

Patterns
Systems and system models
Stability and change

Performance Expectations

MS-PS1-4: Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.

Answers to Questions

  1. Note any observations you made about the growth and appearance of the crystals.

A gel hardened in the first bottle overnight. After potassium iodide was added, yellow, sheet-like crystals began to form at the interface between the gel and the solution. In the second bottle, the gel took three days to harden. When the potassium chloride solution was added, white, cubic crystals formed at the gel-solution interface. The crystals in both bottles grew downward, into the gel.

  1. What chemicals were added to each bottle? What are the crystals in each made out of?
  1. Bottle 1—Lead nitrate solution, acetic acid solution, and sodium silicate solution. After 24 hours potassium iodide was added. The crystals are lead iodide.
  2. Bottle 2—Tartaric acid solution and sodium silicate solution. After 3 days saturated potassium chloride solution was added. The crystals are potassium nitrate.
  1. Write the net ionic equation for the crystals that were produced in each reaction.
  1. Bottle 1 (Lead nitrate and potassium iodide)

    Pb2+(aq) + 2I–(aq) → PbI2(s)

  2. Bottle 2 (Tartaric acid and potassium chloride)

    K+(aq) + HC4H4O6–(aq) → KHC4H4O6(s)

Discussion

Reactions

In the first bottle, 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

In the second bottle, a double replacement precipitation reaction occurs with potassium tartrate forming as the solid.

K+(aq) + HC4H4O6–(aq) → KHC4H4O6(s) Potassium Hydrogen Tartrate 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.

†Next Generation Science Standards and NGSS are registered trademarks of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.