Colorful Silicate Garden

Introduction

In a matter of seconds, columns of various colors sprout up from the bottom of a beaker containing a clear liquid. The crystals form like stalagmites in a cave and will continue to grow for several days. This beautiful and colorful crystal garden is fascinating to watch.

Concepts

• Crystals/Insoluble silicates
• Osmosis

Materials

Safety Precautions

Do not use anhydrous aluminum chloride; it is violently reactive with water and corrosive to body tissue. Use only aluminum chloride hexahydrate (AlCl₃•6H₂O); it is a body tissue irritant and slightly toxic by ingestion. Iron(III) chloride is a skin and tissue irritant, corrosive and slightly toxic by ingestion. Copper(II) chloride is highly toxic both by ingestion and inhalation. Cobalt(II) nitrate is an oxidizer, a fire risk in contact with organic material, moderately toxic and a possible carcinogen. Sodium silicate solution is a body tissue irritant. Wear chemical-resistant gloves, a chemical-resistant apron and chemical splash goggles. 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. The colorful silicate garden may be flushed down the drain with water according to Flinn Suggested Disposal Method #26b. The sand should be rinsed off and disposed of in the trash.

Procedure

1. Optional: Cover the bottom of a 1-L beaker with sand. The sand will prevent the crystals from sticking to the bottom of the beaker and will make cleanup much easier (see Tips).
2. Using a 500-mL graduated cylinder, measure out 375 mL of water and transfer it to the beaker. Transfer it slowly so you don’t stir up the sand.
3. Using a 500-mL graduated cylinder, measure out 225 mL of sodium silicate solution and slowly transfer it to the beaker. Gently stir the solution with a glass stirring rod.
4. With the use of spatulas, forceps, or teaspoons, sprinkle about 2–3 grams of each of the metallic salts (aluminum chloride, Iron(III) chloride, copper(II) chloride and cobalt(II) nitrate) into the beaker. Sprinkle the crystals evenly and use only small crystals. Cover the beaker with plastic wrap.
5. Observe the colorful silicate crystals. They will start to grow in seconds and will continue for several days. Notice that the crystals formed by aluminum chloride are white, iron(III) chloride are brown, copper(II) chloride are light blue-green and cobalt(II) nitrate are dark blue.
6. After several days, the sodium silicate solution may become cloudy. You may carefully replace the solution with tap water to preserve the colorful silicate garden, providing you did not use sand in step 1.

Student Worksheet PDF

12592_Student1.pdf

Teacher Tips

• The sand used must be clean. Dirty sand will cause the crystal garden to be cloudy. The sand will give the garden some topography, as well as a more rugged terrain. The crystal garden will be much more fragile if sand is used. If sand is not used, the sodium silicate solution may be replaced with water after the crystals have stopped growing; the garden may be enjoyed for a longer period of time.
• When solid salts are added to the solution, they tend to float and may need to be pushed down with the spatula.

Answers to Questions

1. Describe how the crystal garden was prepared.

A sodium silicate solution was prepared by mixing 375 mL of water with 225 mL of sodium silicate solution. Then a few grams of aluminum chloride, iron(III) chloride, copper(II) chloride, and cobalt(II) nitrate were all added. After a few seconds, crystals had already started to form. These crystals grew larger and taller over the course of several days.

2. What color were the crystals formed by each of the salts?

The aluminum chloride crystals are white, iron(III) chloride crystals are brown, copper(II) chloride crystals are light bluegreen and cobalt(II) nitrate crystals are dark blue.

3. The metallic ions from the salts combined with the silicate ions to form an insoluble, semipermeable membrane around the salt crystals. Knowing this, what do you think caused the crystals to grow upward?

The concentration of salt inside the membrane is much higher than the concentration outside the membrane. Thus, via osmosis, water enters the membrane to stabilize the concentrations. Because of the pressure of water now inside the crystal, the membrane breaks upward.

Discussion

Various salts are added to the sodium silicate solution. The salts begin to dissolve releasing metallic ions. The metallic ions combine with the silicate ions to form a membrane of insoluble silicates around the various salt crystals.

The crystals grow upward because the membrane that forms is semi-permeable. The concentration inside the membrane is greater than the concentration outside the membrane. A process called osmosis allows the water to enter the membrane to equalize the concentrations. The membrane breaks upward due to the increased pressure of water on the inside walls of the membrane. The break in the membrane causes more salt to be exposed to the silicate solution and thus the membrane continues to grow.

References

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin Press: Madison, WI, 1989; Vol. 3, pp 379–380.

Summerlin, Lee R. and James L. Ealy, Jr. Chemical Demonstrations: A Sourcebook for Teachers, Volume 1. Washington, DC: American Chemical Society, 1988.