AQA A Level Chemistry复习笔记 6.1.4 Oxides Reacting with Water

Oxides Reacting with Water


Structure, bonding & electronegativity of the Period 3 elements table



  • The oxides of Na and Mg which show purely ionic bonding produce alkaline solutions with water as their oxide ions (O2-) become hydroxide ions (OH-):

O2- (aq) + H2O (l) → 2OH- (aq)

  • The oxides of P and S which show purely covalent bonding produce acidic solutions with water because when these oxides react with water, they form an acid which donates H+ ions to water
    • Eg. SO3 reacts with water as follows:


SO3 (g) + H2O (l) → H2SO4 (aq)

    • The H2SO4 is an acid which will donate a H+ to water:


H2SO4 (aq) + H2O (l) → H3O+ (aq) + HSO4- (aq)


  • Al and Si are insoluble and when they react with hot, concentrated alkaline solution they act as a base and form a salt
    • This behaviour is very typical of a covalently bonded oxide


  • Al can also react with acidic solutions to form a salt and water
    • This behaviour is very typical of an ionic bonded metal oxide


  • This behaviour of Al proves that the chemical bonding in aluminium oxide is not purely ionic nor covalent: therefore it exhibits amphoteric character


Reaction of Period 3 oxides with water table


Behaviour of the Period 3 Oxides with Water

  • Metal oxides (to the left of the periodic table):
    • Sodium oxide, Na2O, and magnesium oxide, MgO, are made up of ions
    • They contain an oxide ion, O2-, which is a strong base and will readily produce hydroxide ions through reaction with water
    • This is why the solutions formed are strongly alkaline
    • Sodium oxide forms a more alkaline solution than magnesium oxide because it is far more soluble in water


  • Oxides in the middle of the periodic table
    • Although ionic, aluminium oxide does not react with water because the oxide ions are held too strongly in the ionic lattice
    • This means the ions cannot be separated
    • Silicon dioxide is a giant covalent molecule - it is the main component of sand
    • It has millions of strong covalent bonds, so it does not react with water


  • Non-metal oxides (to the right of the periodic table):
    • Oxides of phosphorus and sulfur are simple covalent molecules
    • They will react with water to produce acidic solutions


Exam Tip

Key thing to remember: The metal oxides form alkaline solutions in water, the oxides in the middle do not react and the non-metal oxides form acidic solutions.

Acid-Base Reactions of the Oxides

Acid/base Nature of the Period 3 Oxides

  • Aluminium oxide is amphoteric which means that it can act both as a base (and react with an acid such as HCl) and an acid (and react with a base such as NaOH)


Reactions of the Period 3 oxides with acid/base table


Exam Tip

It is crucial that you learn these reactions - make sure that you know the state symbols, the products formed and the full balanced equations!