Testing the pH of oxides

In this experiment, students test samples of a range of oxides in water with universal indicator solution, observing that their pH varies. They establish the idea that the soluble oxides of metals are alkaline and the oxides of non-metals are acidic.

The experiment itself is quite short, but there should be adequate discussion time before and after the practical work.

Equipment

Apparatus

• Eye protection
• Test tubes, x6
• Test tube rack
• pH colour chart

Chemicals

Access to solutions as follows:

• Nitric(V) acid (labelled ‘Nitrogen oxide and water’), 0.2 M (IRRITANT)
• Phosphoric(V) acid (labelled ‘Phosphorus(V) oxide and water’), 0.2 M
• Sulfuric(VI) acid (labelled ‘Sulfur dioxide and water’), 0.2 M
• Potassium hydroxide (labelled ‘Potassium oxide and water’), 0.2 M (IRRITANT)
• Sodium hydroxide (labelled ‘Sodium oxide and water’), 0.2 M (IRRITANT)
• Limewater (labelled ‘Calcium oxide and water’)
• Universal indicator solution (FLAMMABLE), full range, in a dropper bottle (or bottle with dropper pipette)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Nitric(V) acid, HNO₃(aq), (IRRITANT) – see CLEAPSS Hazcard HC067 and CLEAPSS Recipe Book RB061.
• Phosphoric(V) acid, H₃PO₄(aq) – see CLEAPSS Hazcard HC072 and CLEAPSS Recipe Book RB065. 
• Sulfuric(VI) acid, H₂SO₄(aq)  – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB098. 
• Potassium hydroxide solution, KOH(aq), (IRRITANT) – see CLEAPSS Hazcard HC091b and CLEAPSS Recipe Book RB071. 
• Sodium hydroxide solution, NaOH(aq), (IRRITANT) – see CLEAPSS Hazcard HC091a and CLEAPSS Recipe Book RB085. 
• Limewater, Ca(OH)₂(aq), (treat as IRRITANT) – see CLEAPSS Hazcard HC018 and CLEAPSS Recipe Book RB020. 
• Universal indicator solution (HIGHLY FLAMMABLE) – see CLEAPSS Hazcard HC032 and CLEAPSS Recipe Book RB000. 
• The solutions could be provided in small labelled dropping bottles or in small conical flasks with a dropping pipette. If time is short, the solutions could be provided in labelled test tubes.
• The concentrations of the solutions are not critical (0.1 or 0.2 M is appropriate). They should, however, be less than 0.5 M. At the suggested concentrations, the phosphoric acid and sulfuric acid solutions present a minimal hazard. It may be advisable, however, to label the solutions as IRRITANT.

Procedure

1. Place 2 cm³ samples of each ‘oxide and water’ solution into separate test tubes.
2. To each sample, add 3 drops of universal indicator solution. Observe the colour of the indicator in each sample.
3. Record the results in a suitable table showing the name of the oxide, the colour of the universal indicator, the pH, and whether the oxide is acidic, alkaline or neutral in water.

Teaching notes

The solutions are not referred to or labelled as ‘oxide solution’ since the oxides do not dissolve – they react with water. The ‘oxide and water’ approach allows the focus to remain on the main teaching point without the need to cover another set of reactions (although these could be covered with an appropriate group of students).

In general, metal oxides are basic and non-metal oxides are acidic. Some metal oxides react with water to form alkaline solutions. It is important to point out that some metal oxides do not react with water. They test neutral in water because they are insoluble but they are still bases because they react with acids. Non-metal oxides react with water to form acids.