Testing the pH of oxides

Class practical

The aim of this experiment is to help establish the idea that the soluble oxides of metals are alkaline and the oxides of non-metals are acidic. Students test samples of a range of oxides in water with Universal indicator solution.

Lesson organisation

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

Apparatus

Chemicals

Eye protection Each working group will require: Test-tubes, 6 Test-tube rack pH colour chart

Access to solutions as follows (Notes 1 and 2): 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) Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection throughout. 

Nitric(V) acid, HNO₃(aq), (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Phosphoric(V) acid, H₃PO₄(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Sulfuric(VI) acid, H₂SO₄(aq)  - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Potassium hydroxide solution, KOH(aq), (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Sodium hydroxide solution, NaOH(aq), (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Limewater, Ca(OH)₂(aq), (treat as IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Universal indicator solution (HIGHLY FLAMMABLE) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

1 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.

2 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

a Place 2 cm³ samples of each ‘oxide and water’ solution into separate test-tubes.

b To each sample, add 3 drops of Universal indicator solution. Observe the colour of the indicator in each sample.

c 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.

Health & Safety checked, 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015