Colorful Stalactites and Stalagmites

Introduction

Introduce stalactite and stalagmite formation by performing this colorful, easy-to-follow demonstration.

Concepts

• Stalactites
• Precipitates
• Stalagmites
• Double replacement reactions

Materials

Safety Precautions

Cobalt nitrate is an oxidizer and a fire risk in contact with organic material. It is moderately toxic and a possible carcinogen as a fume or dust. Copper(II) sulfate is moderately toxic by ingestion and inhalation. The sodium hydroxide solution is a corrosive and irritating solution. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Instruct students viewing the demonstration not to touch or handle the reaction mixtures or beakers. 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 copper-containing waste solution may be treated according to Flinn Suggested Disposal Method 26b. The cobalt containing the waste solutions may be disposed of according to Flinn Suggested Disposal Method 27f.

Prelab Preparation

1. Place the wire screen over the top of the beaker.
2. Shape the wire to form a trough that will extend down into the beaker approximately 1" (see Figure 1). This trough will hold the copper(II) sulfate and cobalt nitrate crystals.
   {13932_Preparation_Figure_1}
3. The sodium hydroxide solution should be diluted before use. The sodium hydroxide given in this kit is 1 M. To make a 0.25 M solution, simply dilute the 1 M sodium hydroxide in a 1-to-3 ratio with distilled water. For example, use 250 mL of 1 M sodium hydroxide and 750-mL of distilled or deionized water to make 1 L of 0.25 M sodium hydroxide solution.

Procedure

1. Place the beaker on a white paper towel on a level surface. Fill it nearly full with 0.25 M sodium hydroxide solution, NaOH.
2. Place the preshaped wire gauze trough on top of the beaker (if it is not on top of the beaker already). The wire gauze should hang down about 1" into the sodium hydroxide solution.
3. Place approximately 10 g of copper(II) sulfate crystals in the wire gauze trough.
4. Within seconds, observe the double-replacement reaction that produces the colorful stalactites and stalagmites.
5. Repeat steps 1–4 in a clean beaker, substituting cobalt nitrate for the copper(II) sulfate.

Teacher Tips

• This kit contains enough chemicals to perform the demonstration seven times: 2 L of 1.0 M sodium hydroxide solution, 100 g of cobalt nitrate and 150 grams of copper(II) sulfate.
• Hydrometer cylinders may also be used in place of beakers. This will allow longer stalactites to form.
• The crystals react chemically with the sodium hydroxide solution to form insoluble precipitates. The beaker must be left completely undisturbed in order for some of the strands to completely reach the bottom.
• The wire gauze may be trimmed to fit your beakers, if desired.

Discussion

As groundwater drips through caves, elaborate formations develop. These cave formations and deposits are called dripstone and can be classified into two unique groups: stalactites and stalagmites. Stalactites are icicle-like formations that hang from the ceilings of caves (“c” for ceiling). When groundwater drips through and reaches the air in the cave, some of the dissolved carbon dioxide escapes from the drop and calcite (CaCO₃) begins to precipitate. The calcite is deposited as a ring around the water drop. Drop after drop a very small amount of calcite is left behind. Eventually a hollow tube made of limestone is created. Water continues to move through this tube and continues to add calcite to the stalactite. The stalactite can be appropriately described as a soda straw at this point. The soda straw becomes plugged and water is forced to flow outside the stalactite. Calcite deposits are then left on the outside of the stalactite and the familiar conical or icicle shape forms.

Formations that occur on the floor of a cavern are known as stalagmites (“g” for ground). The water that supplies calcite to the growing stalactite splatters on the floor of the cave. As a result, stalagmites are formed and grow upwards towards the ceiling of the cave. Stalagmites are often not uniform in shape and are more massive than stalactites because of this splattering.

The precipitates seen in this demonstration are produced by a series of double-replacement reactions. Each of the chemical reactions results in the formation of a precipitate. The reactions can be used to illustrate a variety of chemical concepts ranging from accelerated stalactite and stalagmite formation to the production of precipitates via double replacement reactions.

Reaction in Beaker 1
Initially a blue-green salt is formed:

Cu²⁺ + 2OH^– → Cu(OH)₂ blue-green ppt

An outer crust of black copper(II) oxide eventually forms:

Cu(OH)₂ → CuO black ppt + H₂O

Reaction in Beaker 2
Initially the blue basic salt is formed:

Co²⁺ + OH^– + NO₃^– → Co(OH)NO₃ (blue ppt)

Upon addition of excess reagent, the basic salt is converted into pink cobalt (II) hydroxide:

Co(OH)NO₃ + OH^– → Co(OH)₂ (pink ppt) + NO₃

Some of the precipitate passes into solution. The insoluble cobalt(II) hydroxide is slowly transformed into the brownish-black cobalt(III) hydroxide:

4Co(OH)₂ + O₂ + 2H₂O → 4Co(OH)₃ (black ppt)

References

Flinn Scientific would like to thank Mike Shaw, West Stokes High School, King, North Carolina, for providing us with the instructions for this activity.