Materials Included In Kit

Additional Materials Required

Safety Precautions

Copper(II) chloride is highly toxic by ingestion and inhalation. Copper(II) sulfate is moderately toxic by ingestion and inhalation. Iron(III) chloride and aluminum chloride are slightly toxic by ingestion and are body tissue irritants. Calcium chloride and copper(II) carbonate are slightly toxic. Please review current Safety Data Sheets for additional safety, handling and disposal information before beginning this activity.

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 cobalt chloride crystals should be treated according to Flinn Suggested Disposal Method #27f. All other crystals formed may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• This is a super value kit and will easily serve 5 sections of 30 students working in pairs. This laboratory activity can reasonably be completed in one 50-minute class period.
• For added stability, the reaction strip may be taped down to the countertop. To do so, form two tape circles and place one piece on each end on the bottom of the 12-well reaction strip. Then stick the strip to the countertop.
• The solubility of the metal salts are the main factor in whether or not crystals will form in this activity. Consider discussing (or refresh) the topic of solubility with your students before this lab.
• Some of the crystals grow slowly; allow five to ten minutes before recording final results.
• The test tubes need to be cleaned after the experiment. Pipe cleaners can be used to loosen the precipitates from the sides of the culture tube.
• This lab can use a variety of metal salt compounds that you have in your stockroom. It would make a good lab for a student research project for investigating the different solubilities of the metal silicates.

Sample Data

Answers to Questions

1. What chemicals react best with the sodium silicate solution?
   Iron, aluminum, calcium, copper, and cobalt salts. 
2. What salts never made new crystal columns when mixed with sodium silicate solution?
   Potassium, sodium, and insoluble compounds of the other metals.
3. Formulate a hypothesis on why certain chemicals form crystal columns and others don’t. (Hint: It has to do with solubility.)
   Most silicate compounds are insoluble except for those of the alkali metals (sodium and potassium in this activity). Calcium carbonate and copper(II) carbonate have low solubilities and never form enough ions to react with the sodium silicate.
4. What causes the crystals to grow upwards and defy gravity?
   As the silicate crystals form, they form a membrane around the solid in the bottom of the test tube. The membrane is then forced upward by the difference in water pressure from the bottom of the culture tube to the top.

References

Special thanks to John Mauch, Braintree High School, Braintree, MA.

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.

Introduction

In this experiment crystals will be formed by adding different chemicals to a sodium silicate solution. Some generalizations will then be made about metal ions and the different color crystals they produce. Reasons why some chemicals form crystals and why some do not will also be investigated.

Concepts

• Crystals
• Osmosis
• Solubility

Background

When metal salts are added to a sodium silicate solution, the salts start to dissolve and release metal ions. The metal ions combine with the silicate ions to form a membrane of insoluble metal silicates around the various salt crystals.

The crystals grow upward because the membrane that forms is semipermeable. The concentration of metal ions inside the membrane is greater than the concentration outside the membrane; therefore, through a process called osmosis, water enters 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.

Materials

Safety Precautions

Copper(II) chloride is highly toxic by ingestion and inhalation. Copper(II) sulfate is moderately toxic by ingestion and inhalation. Iron(III) chloride and aluminum chloride are slightly toxic by ingestion and are body tissue irritants. Calcium chloride and copper(II) carbonate are slightly toxic. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Place one culture tube in each well, 1–11, of a 12-well reaction strip.
2. Using a pipet, fill each of the culture tubes approximately halfway with the sodium silicate solution.
3. Use a microspatula and add a scoop of aluminum chloride to the first culture tube.
4. Use a clean toothpick to push the aluminum chloride near the bottom of the tube.
5. Observe whether the salt dissolves and if new crystals form. Record the results in the Data Table.
6. Follow steps 3–5 for the remaining ten metal salts (see the Data Table).
7. Observe the results in each culture tube. Record chemical formulas and all data in the Data Table.
8. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

11951_Student1.pdf