Period 3 Oxides with Water

Period 3 oxides react with water as summarised in the table below.

Sodium Oxide

• Reaction: 

• Type: Strongly basic oxide

• Result: Highly alkaline solution, pH 

Magnesium Oxide

• Reaction: 

• Type: Weak base, slightly soluble

• Result: Mildly alkaline solution, pH

Comparing Basicity

• Sodium oxide and magnesium oxides are basic oxides as the ions combine with ions from water producing (hydroxide ions).

• Magnesium ions are smaller and have a higher charge than sodium ions resulting in stronger electrostatic forces of attraction between the metal ions and the oxide ions in magnesium oxide. The oxide ions in magnesium oxide are therefore less available to combine with the ions making the resultant solution less alkaline than with sodium oxide with lower .

• Magnesium hydroxide is also only sparingly soluble resulting in a lower concentration in solution.

Neither aluminum oxide or silicon dissolve in water. This means that despite any acidic/basic nature, there will be no effect on the of the water when these solids are added to it.

Aluminium Oxide

• Reaction with water: None (insoluble)

• Type: Amphoteric – reacts with both acids and bases, but not water

• Effect: No pH change

Silicon Dioxide

• Reaction with water: None – insoluble

• Type: Weak acidic oxide

• Effect: No pH change, but reacts with strong bases (not needed here)

Phosphorus Pentoxide 

• Reaction: 

• Type: Strong acidic oxide

• Result: Strong acidic solution, pH 

Sulfur Dioxide 

• Reaction: 

• Product: Sulfurous acid, weak acid

• pH: 

Sulfur Trioxide 

• Reaction: 

• Product: Sulfuric acid, strong acid

• pH: 

A common exam question is to suggest a method to distinguish between samples of and . As they produce solutions of different acids strengths, a simple approach is to react both with the water and then test the of the resultant solutions using universal indicator or a meter.

Displayed formulae of Acids

The structures of the acids and can be drawn simply by:

1. Centring the period element.

2. Adding as many groups as there are in the formula.

3. Adding remaining groups connected with double bonds.

General Trend Across the Period

• Oxides become more acidic across Period 3.

• Ionic oxides (left of the period) form alkaline solutions.

• Covalent oxides (right of the period) form acidic solutions.

• Aluminum oxide is amphoteric (no reaction with water, but reacts with both acids and bases).