Oxides Reacting with Water (AQA A Level Chemistry)

Revision Note

Test yourself
Stewart

Author

Stewart

Last updated

Oxides Reacting with Water

Structure, bonding & electronegativity of the Period 3 elements table

Period 3 element Na Mg Al Si P S
Period 3 oxide Na2O MgO Al2O3 SiO2 P4O10 SO2
SO3
Relative melting point High High Very high Very high Low Low
Chemical bonding Ionic Ionic Ionic (with a degree of covalent) Covalent Covalent Covalent
Structure Giant ionic Giant ionic Giant ionic Giant covalent Simple molecular Simple molecular
Element Na Mg Al Si P S
Electronegativity 0.9 1.2 1.5 1.8 2.1 2.5

  • 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

Oxide Chemical equation pH Comments
Na2O Na2O (s) + H2O (l) → 2NaOH (aq) 14
(strongly alkaline)
-
MgO MgO (s) + H2O (l) → Mg(OH)2 (aq)  10
(weakly alkaline)
-
Al2O3 No reaction - Al2O3 is insoluble in water
SiO2 No reaction - SiO2 is insoluble in water
P4O10 P4O10 (s) + 6H2O (l) → 4H3PO4 (aq) 2
(strongly acidic)
Vigorous / violent reaction
SO2
SO3
SO2 (g) + H2O (l) → H2SO3 (aq)
SO3 (g) + H2O (l) → H2SO4 (aq)
1
(strongly acidic)
-

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

Examiner 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)
Period 3 oxide Na2O MgO Al2O3 SiO2 P4O10 SO2
SO3
Acid / base nature Basic Basic Amphoteric Acidic Acidic Acidic

Reactions of the Period 3 oxides with acid/base table

Period 3 oxide Chemical equation Comments
Na2O Na2O (s) + 2HCl (aq) → 2NaCl (aq) + H2O (l) -
MgO MgO (s) + 2HCl (aq) → MgCl2 (aq) + H2O (l) Used in indigestion remedies by neutralising the excess acid in the stomach
Al2O3

Al2O3 (s) + 3H2SO4 (aq) → Al2(SO4)3 (aq) + 3H2O (l)

Reacts with acid to form a salt and water

Al2O3 (s) + 2NaOH (aq) + 3H2O (l) → 2NaAl(OH)4 (aq)

Reacts with hot, concentrated alkali to form a salt

SiO2 SiO2 (s) + 2NaOH (aq) → Na2SiO3 (aq) + H2O (l) Reacts with hot, concentrated alkali to form a salt and water
P4O10 P4O10 (s) + 12NaOH → 4Na3PO4 + 6H2O (l) -

SO2

SO3

SO2 (g) + 2NaOH (aq) → Na2SO3 (aq) + H2O (l)

SO3 (g) + 2NaOH (aq) → Na2SO4 (aq) + H2O (l)

-

Examiner 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!

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.