Prelab Preparation

Add 100 mL of deionized or distilled water to the bottle of solid potassium iodide. 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), you must 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.

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 may be stored indefinitely. Seal test tubes with Parafilm M^®. Test tubes that contain lead crystals should be treated according to Flinn Suggested Disposal Method #27f. Liesegang rings and all other gel crystals may be discarded according to Flinn Suggested Disposal Method #26a.

Teacher Tips

• The crystals will develop over the coming months. Some crystals will decompose and some will remain unchanged.
• After several months test tube 1 will start producing elemental iron and hydrogen bubbles—slowly losing its beauty.
• Test tube 2 will also produce hydrogen bubbles and will tend to pop its stopper. Seal test tubes with Parafilm M^®.
• The potassium tartrate crystals can be harvested with a metal spatula, and the gel quickly rinsed off the crystals with water.
• All crystals will take approximately two weeks to grow.
• If the tartaric acid is cut to 1.5 M, crystals will be elongated.
• Other organic acids can be tried (e.g., citric, succinic). This gives an opportunity for students to explore other possibilities.

Discussion

The reaction in test tube l can generally be called a single replacement 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.

References

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

Introduction

Grow your own beautiful, long-lasting crystals in gels! Mix a dilute solution of sodium silicate (water glass) with an organic acid (usually acetic acid) in which an ion has been incorporated. A gel forms overnight. Then place another ionic solution or a metal 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 you permit these to grow, the more beautiful they become. In this experiment, you will grow crystals of six different substances—copper metal, lead metal, lead iodide, potassium hydrogen tartrate, copper(II) tartrate, and a copper(II) chromate Liesegang ring system. 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. Students can grow them easily and are quickly motivated. Display these gel crystals in a showcase to attract the attention of all.

Concepts

• Crystal growing
• Saturated solutions
• Chemical reactions

Materials

Safety Precautions

Lead solutions are toxic by inhalation and ingestion. Potassium chromate solution may be corrosive to body tissue. Chromium compounds are alleged carcinogens. Copper(II) sulfate solutions are toxic by 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.

Procedure

Day 1

1. Label six test tubes with your initials and number them 1 to 6. Put labels near the tops of the tubes.
2. Put 2.5 mL of l 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 into tube 3. Rinse the graduated cylinder.
4. Using the graduated cylinder, measure 12 mL of 1 M acetic acid solution, CH₃COOH, and pour it into tube l. Do this again for tubes 2 and 3.
5. Put a stopper on tube 1 and gently turn it upside down several times to mix the chemicals. Then stopper and mix tubes 2 and 3.
6. Rinse your graduated cylinder.
7. Using the graduated cylinder, measure 12 mL of sodium silicate solution, Na₂Si₃O₇, and pour it into each of the three tubes: 1, 2 and 3.
8. Put the stoppers back on each tube and again gently turn them upside down several times to mix the chemicals.
9. Measure 12 mL of 3 M tartaric acid, H₂C₄H₄O₆, and put it into test tube 4. Do the same for test tube 5.
10. Rinse your graduated cylinder.
11. Measure 12 mL of sodium silicate solution, Na₂Si₃O₇, and add it to test tube 4. Add another 12 mL of sodium silicate solution to test tube 5. Stopper and gently turn each test tube upside down several times to mix the chemicals.
12. Measure out 6 mL, of distilled water and put it into test tube 6. Carefully measure out 6 mL of 1 M acetic acid, CH₃COOH, and add it to the test tube to cut the concentration of the acid in half.
13. Put exactly l mL of l M potassium chromate, K₂CrO₄ into test tube 6.
14. Measure out exactly 12 mL of sodium silicate solution, Na₂Si₃O₇, and add it to test tube 6. Stopper and gently turn the test tube upside down several times to mix the chemicals. Work quickly on this step since the gel may harden immediately.
15. Store the test tubes overnight in an area designated by your teacher.

1. Remove the stopper from tube l. 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 l M sodium chloride solution, NaCl, into the tube and replace the stopper.
2. 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 tube and replace the stopper.
3. Remove the stopper from tube 3. Gently add 2 mL of the 2 M potassium iodide solution, KI, into the tube and replace the stopper.
4. Remove the stopper from tube 6. Gently add 7 mL of 1 M copper(II) sulfate solution, CuSO₄, into the tube and replace the stopper.
5. Once tubes 4 and 5 have hardened, remove the stopper from tube 4. Measure 5 mL of saturated potassium chloride solution, KCl. Holding the tube on an angle, slowly and gently pour the KCl solution into the test tube and replace the stopper.
6. Remove the stopper from test tube 5. Add 5 mL of saturated copper(II) sulfate solution, CuSO₄ as described in the last step and replace the stopper. After two weeks, pour out the CuSO₄ solution and replace it with more saturated CuSO₄ solution. This will speed up crystal growth.
7. For permanent storage, test tubes should be sealed with Parafilm M^® instead of stoppers. The stoppers may occasionally pop off as the reactions proceed.