pH of Oxides


reactions of solid oxides with water, acid, base


Three solids oxides are added to water. The resulting reactions with water and pH changes are observed by testing with universal indicator and compared to an array of known pH.


Phosphorus pentoxide is corrosive and reacts violently with water.

Chemicals and Solutions

  • Aluminum oxide
  • Phosphorus pentoxide
  • Calcium oxide or magnesium oxide


  • Erlenmeyer flask
  • three 250 ml beakers (or alternatively three crystallizing dishes)
  • universal indicator water (tap water)
  • three stirring rods
  • three scoopulas


  1. Add universal indicator to 0.8L of water. Add spiked water into each of the three beakers or crystallizing dishes.
  2. To one beaker, add a small spatula’s worth of phosphorus pentoxide (caution!!!). Test the solution with universal indicator to show that the solution is acidic (pink).
  3. To the next beaker add a small spatula’s worth of calcium oxide universal indicator to show that it is basic (blue).
  4. To the last beaker add aluminum oxide. The aluminum oxide solution is neutral (green) since it is amphoteric and thus universal indicator will show no color change.

Hint: Because the building DI water source is acidic, tap water must be used in this demo.


Calcium oxide reacts with water to give basic calcium hydroxide:

\( \ce{ CaO_{(s)} + H2O_{(l)} -> Ca(OH)2_{(aq)} } \)

Phosphorus pentoxide, on the other hand reacts with water to form phosphoric acid:

\( \ce{ P4O10_{(s)} + 6H2O_{(l)} -> 4H3PO4_{(aq)} } \)

Aluminum oxide is amphoteric, it can behave has either and acid or a base. It does not react with water.