Potassium Permanganate Volcano   Recently updated !


Potassium permanganate volcano near end of eruption
After the potassium permanganate volcano erupts, the dish contains glowing embers, dark ash, and pink sparks. (Adam Rędzikowski)

The potassium permanganate volcano is a classic chemistry demonstration that produces smoke, colored flames and sparks, a glow resembling lava, and colored ash. This type of chemical volcano captivates students while illustrating principles of redox reactions, oxidation, exothermic reactions, the effect of surface area on reaction rate, decomposition reactions, and the flame test. The demonstration is ideal for middle school and high school chemistry students.

Chemical Volcano Materials

The volcano only requires two chemicals and a few basic materials. You can customize the size of the reaction based on the amount of chemicals you want to use and the effect you’re seeking.

  • 2-3 grams of potassium permanganate crystals (KMnO4)
  • 1 milliliter glycerol in a test tube (also known as glycerin or glycerine)
  • Porcelain dish

Note: Fine crystals of potassium permanganate work better than large crystals. You may wish to perform the demonstration with equal masses of large and small crystals (made by grinding large crystals with a mortar and pestle) to illustrate the effect of surface area on rate of reaction. Expect a delayed reaction if larger crystals are used.

Note: Fresh glycerol works better than “old” glycerol. Possibly this is because glycerol absorbs water from air. Gently warming old glycerol before use improve may drive off any water and improve the display.

Procedure

  1. Place the potassium permanganate crystals in a mound in the dish. If you like, you can make a shape resembling the cinder cone of a volcano. You can improve the display by pushing a small indentation into the top of the mound, resembling a volcanic caldera.
  2. Pour the glycerol on top of the potassium permanganate and immediately move back. If you are studying reaction kinetics, start a stopwatch immediately after moistening the potassium permanganate.
  3. Very fine crystals react almost immediately, while coarse crystals may take 30 seconds. Initially, expect the mixture to steam, smoke, and crackle. Next, it will burst into flame. The flame color will display the characteristic pinkish-purple of the flame test for potassium. Finally, the flames will die back, leaving an orange glow and black, green, or brown ash.
  4. If your discussion includes redox reactions and oxidation states, you may allow the residue to cool and then dissolve the ash in water. The resulting green solution indicates the presence of Mn(VI), with solid brown manganese(IV) oxide. Potassium permanganate contains Mn(VII), so the colors of the products indicate manganese was reduced. Alternatively, you can add potassium permanganate to a strong base solution (e.g., sodium hydroxide solution) to yield the purple color and the ash from the volcano to the strong base solution to yield a yellow-green color, thus demonstrating the reactant and product are different.

Note: If the mixture fails to “erupt” it should be discarded by washing it down the drain with a large volume of water. Do not discard the mixture in a trash can as it may eventually ignite at some later time!

The potassium permanganate volcano erupts when glycerol is added. (Pawaret)

How the Potassium Permanganate Volcano Works

Potassium permanganate (KMnO4) is a strong oxidizer, while glycerol (C3H5(OH)3) is an alcohol that is readily oxidized. The reaction between the two chemicals produces steam, carbon dioxide gas, and manganese oxide according to the following reaction:

14KMnO4(s) + 4C3H5(OH)3(l) → 7K2CO3(s) + 7Mn2O3(s) + 5CO2(g) + 16H2O(g)

Like other chemical fire demonstrations, the reaction is spontaneous. As it releases heat, it is exothermic. The purple flame color results from potassium electrons absorbing energy, becoming excited, and releasing light as they return to a lower energy state.

Safety Information

Potassium permanganate and glycerol are considered to be safe chemicals for chemistry demonstrations. Glycerol is safe enough that it is used as a food additive. Potassium permanganate is even used to treat skin condition and to remove the rotten egg smell from hydrogen sulfide in well water. It does not generate toxic by-products. However, potassium permanganate is a strong oxidizer and can stain skin pink to purple. If contact occurs, wash the affected area with water. Also, the reaction releases smoke, heat, and flames. For optimal safety, the reaction should be kept small, using no more the 10 grams of potassium permanganate. The chemical volcano demonstration should be performed wearing gloves, protective clothing, and safety goggles. After the project is complete, the residue may be washed down the drain with water.

References

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