Acid-base Characteristics of Metal and Nonmetal Oxides

Magnesium oxide and phosphorus oxide are prepared by burning the metal and nonmetal in air.  The resulting MgO and P4O10 are then added to aqueous solution containing universal indicator.  The MgO turns the indicator blue while the P4O10 turns the red, demonstrating that metal and nonmetal oxides cause the pH of aqueous solutions to become basic and acidic, respectively.

Equipment

  • ring stand with test tube clamp
  • stir plate
  • stirbar
  • 250 mL beaker
  • 1 L round bottom flask with threaded cap and syringe port
  • propane torch or Bunsen burner
  • ring stand with flask clamp
  • 50 mL plastic syringe

Reagents

  • magnesium strip
  • white phosphorus, P4
  • water
  • universal indicator

Presentation

  1. The magnesium strip is clamped to the ring stand over a beaker containing water with universal indicator.  
  2. The Mg ribbon is lit on fire with the propane torch leaving a ribbon of MgO hanging from the clamp.
  3. Once the burning is complete, the ribbon is lowered (or just falls) into the aqueous solution that is being stirred at a moderate speed.
  4. The chunk of white phosphorus is removed from its storage (under water) and placed into the round bottom flask.  The threaded cap with syringe port is tightened onto the flask.
  5. Propane torch is used to gently heat the P4.  It does not take much heating to get the phosphorus to just begin to melt.  As soon as it begins to melt, remove the torch heat source and the reaction will proceed on its own.  The generated P4O10 smoke will shoot out of the syringe port.  Once the reaction is complete, the inside walls of the flask will be coated with P4O10.
  6. Water with universal indicator is drawn into the plastic syringe and added to the phosphorus flask once the reaction is complete.  The water is swirled around in the flask to interact with the P4O10 on the flask walls.

Details

Metal oxides react with water to form basic solutions:

MgO (s) + H2O (l)  →  Mg2+(aq) + 2 OH - (aq)

Nonmetal oxides react with water to form acidic solutions:

P4O10 (s) + 6 H2O (l)  →  4 H3PO4 (aq)

In this demonstration, the oxides are readily generated by burning in air.  At the end of the two reactions, the universal indicator shows blue for the MgO solution and red for the P4O10 solution.  The MgO solubility is rather low, so this reaction should be done first so that the MgO has plenty of time to stir around in the water and indicator solution.

Safety

fire

hot

light

smoke

Special Safety Notes

  • white phosphorus is very reactive and pyrophoric and should be stored under water until use.

  • P4O10 is an irritant that generates phosphoric acid in your mucus membranes.  Not a lot is generated in this demo, but there should be adequate ventilation.
  • The phosphorus reaction flask needs to be vented so that it does not build up pressure.
  • Magnesium burns very hot and should be set up over the water beaker to simplify the presentation.
MgO
Magnesium ribbon burning over water in beaker with universal indicator.
p4o10
White phosphorus being heated by propane torch in vented round bottom flask.
mag phos
The final products from both reactions showing that the water is basic with MgO and acidic with phosphorus oxide.