Colorful Stalactites and Stalagmites

Crystals turn to stalactites and stalagmites.

Scientific name: The series of double displacement chemical reactions proceed with the formation of insoluble products.


Colorful Stalactites and Stalagmites

by FlinnScientific


by PubliosValesios

Cobalt hydroxide precipitate

by FHS Chemistry

Precipitation Of Cobalt(II) Hydroxide

by Cychr


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Reaction formula

Cu2+ + 2OH- → Cu(OH)2 (blue-green precipitate)

Cu(OH)2 → CuO (black precipitate) + H2O

Co2+ + OH- + NO3- → Co(OH)NO3 (blue precipitate)

Co(OH)NO3 + OH- → Co(OH)2 (pink precipitate) + NO3-

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

Step-by-step instruction

  1. Prepare two beakers and two pieces of wire mesh.
  2. Fold the wire mesh into a cone shape. The cone diameter should be close to the size of the beaker.
  3. Fill the beaker with sodium hydroxide solution.
  4. Put the wire cones into the beaker top.
  5. Place the copper sulfate crystals into the first cone.
  6. Observe the formation of blue stalactites and stalagmites in the first beaker.
  7. Leave the beaker for a few hours. Stalactites and stalagmites will change color.
  8. Place the cobalt nitrate crystals into the second cone.
  9. Observe the formation of pink stalactites and stalagmites in the second beaker.

Scientific background

In this experiment the observed formation of stalactites and stalagmites is the result of a series of double displacement chemical reactions. Interaction of copper sulfate with sodium hydroxide leads to the formation of an insoluble product - copper (II) hydroxide. Copper (II) hydroxide has a blue-green color. Over time copper hydroxide passes to the copper oxide. The outer black crust is formed by this substance. Cobalt nitrate also undergoes a number of chemical transformations with the formation of precipitates. Initially the blue complex compound is produced. The excess of hydroxide ions turns blue basic salt to the pink cobalt (II) hydroxide. Reaction of cobalt hydroxide with water and dissolved oxygen leads to the formation of brownish-black cobalt (II) hydroxide. This process is slow and takes some time to proceed.


Published on 14 May 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst