Growing Rock

Introduction

Growing rocks are limestone based rocks found in the Great Basin area of the western United States. When placed in a solution of 5% acetic acid (vinegar), CH₃COOH, the rock will grow popcorn-like crystals on its exposed surface. Show students that the new crystals are the result of a chemical reaction.

Concepts

• Crystal growth
• Flame test
• Solubility test
• Ions

Materials

Safety Precautions

White vinegar solution is an acid solution and body tissue irritant. Wear safety goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

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 limestone rock and calcium acetate crystals may be disposed of according to Flinn Suggested Disposal Method #26a.

Procedure

Part 1. Chemical Tests of Reactants

1. Obtain three 100-mL beakers. Label one beaker water and a second one vinegar.
2. Fill the third 100-mL beaker to the 60-mL mark with distilled or deionized water. Place two wooden splints into this beaker.
3. Using the plastic spoon, take about a half a spoonful of the chalk powder (about 1.5 g) and place the powder in the beaker labeled water. Repeat for the beaker labeled vinegar.
4. Tell students that the properties of the rock will be tested using the chalk powder. The rock and the powder are different forms of the same chemical, calcium carbonate, CaCO₃.
5. Add distilled or deionized water to the 80-mL mark of the beaker labeled water. Stir with a stirring rod.
6. Add vinegar solution to the 80-mL mark of the beaker labeled vinegar. Stir with the stirring rod.
7. Have students record their observations of the contents of each beaker on the worksheet.
8. Fill a 250-mL beaker about half full with tap water.
9. Set up and light the Bunsen burner or portable burner.
10. Dip the soaked end of one of the wood splints from step 2 in the chalk powder, then place the end in the tip of the flame.
11. Have students record the color of the flame in the worksheet.
12. Place the splint into the half-full 250-mL beaker of water to extinguish the flame or embers.
13. Repeat steps 10–12 using lithium chloride insteak of chalk. Note: The lithium chloride will give a bright red flame color characteristic of lithium.

Part 2. Growing Crystals

1. Place the limestone rock in a clear glass bowl.
2. Add acetic acid solution to the bowl. Note: Crystals grow best if only ⅔ of the rock is covered.
3. Place the bowl in an area where it will be left undisturbed and has adequate air circulation.
4. Allow the vinegar to slowly evaporate at room temperature. Note: Sufficient amounts of crystals needed for testing will grow in five to seven days. Total evaporation of the vinegar solution may take several weeks.

Part 3. Chemical Tests of Product Crystals

1. After sufficient crystals have grown on the rock, remove the crystals by scraping them into the disposable weighting dish with the spatula.
2. Soak a wooden splint in water (Part 1, step 2). Repeat the flame test (Part 1, steps 8–12) using the product crystals.
3. Have students record the color of the flame on the worksheet.
4. Repeat the solubility test (Part 1, steps 3–6) for the product crystals.
5. Have students record their observations on the worksheet.

Student Worksheet PDF

14136_Student1.pdf

Teacher Tips

• The kit contains enough materials to perform the demonstration as written seven times.
• Be patient—growing rocks may take a month or more for the crystals to completely grow.
• Be careful when moving the growing rock. The crystals are very delicate while growing.
• Use distilled or deionized water for the flame test. Tap water will contain sodium ions in solution. Sodium produces a bright yellow flame that masks the colors produced from the metals contained in the solid samples.
• The acetate ion is written several different ways, depending on the author and/or textbook. It can be written as CH₃COO^–, OAc^–, CH₃CO₂^– or C₂H₃O₂^–.
• Other salts can be used in the flame test. Sodium salts will produce a yellow flame, potassium salts produce a violet flame, copper salts produce a blue-green flame, and strontium salts produce a red-orange flame.

Sample Data

Part 1. Chemical Tests of Reactants

Solubility Test Observations
Chalk in Water: Milky, white liquid results immediately after stirring. After stirring—white solid settles. Chalk not soluble in water.
Chalk in Vinegar: Chalk reacts with vinegar. Gas is produced. Solution bubbles rapidly when stirred. Chalk “disappears.”

Flame Test Observations
Chalk: Yellow/orange flame
Lithium Chloride: Red flame

Part 3. Chemical Tests of Product

Solubility Test Observations
Product Crystals in Water: Dissolves in water.
Product Crystals in Vinegar: Dissolves in vinegar.

Flame Test Observations
Product Crystals: Yellow/orange flame

Answers to Questions

Conclusions

1. From the flame test, what can be said about the chalk and the product crystal?

   Both contain the same metal element in their formulas.

2. Do the solid reactant and product have the same solubility properties? What can be concluded about the chalk compared to the final crystal?

   The two solids have different solubility properties. If any two compounds have different properties, they must be different compounds.

Discussion

Vinegar is a dilute solution of acetic acid, CH₃CO₂H, dissolved in water. Vinegar is an acidic solution because some of the acetic acid molecules break apart or dissociate into cations and anions (Equation 1).

The hydronium ion, H₃O⁺, is the ion that produces acidic properties.

The limestone rock is made up of calcite, a crystalline form of calcium carbonate, CaCO₃. Calcium carbonate reacts with hydronium ions in vinegar to form Ca²⁺ ions in solution, carbon dioxide gas, CO₂, and water (Equation 2). As water slowly evaporates from the vinegar solution, crystals of calcium acetate, Ca(CH₃CO₂)₂, start to precipitate out of solution on the surface of the rock. If the evaporation is slow enough, the crystals that precipitate have a popcorn-like appearance. Crystal growth continues until all the water has evaporated. Once dry and hard, the crystals will last for years.

The formation of these crystals is the result of the chemical reaction of calcium carbonate with acetic acid. In a chemical reaction, the starting materials, or reactants, are changed to form the products, or new chemicals. These new chemicals have different properties than the reactants.

Two chemical tests are performed on the reactant and product to determine the composition of the product. The first test is for solubility. The chalk, CaCO₃, is slowly soluble in the vinegar solution and not soluble in water. The product crystals of calcium acetate, Ca(CH₃CO₂)₂, are soluble in both vinegar and water. The second test is called the flame test. If a solid compound containing a metal is placed in a flame, a specific color will be produced for each individual metal. Compounds containing calcium produce an orange-red flame. Lithium chloride, included in the kit, gives off a bright red flame. The reactant and product in this demonstration contain calcium metal and both give off an orange-red color.

The tests show that while both the product and reactant contain calcium, their different solubility properties show them to be different chemical compounds.

References

Special thanks to David A. Katz, Cabrini College, Radnor, PA, for providing Flinn Scientific with the instructions for this activity.