Crystal Growing Rock

Introduction

A great crystal growing activity for students! When a limestone rock is placed in a solution of 10% acetic acid, CH₃CO₂H, the popcorn-like crystals will grow on the exposed rock surface.

Concepts

• Chemical reaction
• Crystal formation
• Properties of acids

Materials

Safety Precautions

Acetic acid solution is a corrosive liquid. Wear safety goggles, a chemical-resistant apron and chemical-resistant gloves. Have students wash hands thoroughly with soap and water before leaving the lab. Please consult 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 rocks and calcium acetate crystals may be disposed in the trash according to Flinn Suggested Disposal Method #26a. Excess acetic acid may be disposed of according to Flinn Suggested Disposal Method #24a.

Procedure

1. Distribute one rock and one plastic weighing dish to each student.
2. Have students place their limestone rock in the plastic weighing dish, then label this dish.
3. Using the 50-mL graduated cylinder, add 30–40 mL of the acetic acid solution to the plastic weighing dish. Crystals grow best if only ⅔ of the rock is covered by the solution.
4. Place the dish in an area where it will be left undisturbed and has adequate ventilation. Note: Complete evaporation may take up to a week.

Teacher Tips

• Be patient—complete crystal formation may take a week or more to complete.
• Use care when moving the rocks. The crystals are very delicate while growing. Don’t touch the crystals until they are hard.
• The rock can be rinsed and reused many times.
• As an extension of this activity, use limestone rock from the area.
• The kit contains enough chemicals and materials for 30 students to perform the activity as written.
• Vinegar solution is typically 5% acetic acid.
• Vinegar will also work, but crystal formation will take more time.

Discussion

Acetic acid solution is composed of acetic acid molecules, CH₃CO₂H, dissolved in water. Once dissolved in water, some, but not all, of the acetic acid molecules break apart or dissociate into cations and anions (Equation 1).

The hydronium ion, H₃O⁺, is the ion that gives the acetic acid solution acidic properties.

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

The formation of these crystals is the result of a chemical reaction—the reaction of calcium carbonate and 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.

The overall chemical reaction is