Page 87 - Chemistry ICSE Class IX

Water 75
Chemical Properties of Water

What are the chemical properties of water
Water shows a versatile chemical behaviour. It acts as an acid, a base, an
oxidising agent, a reducing agent and as a ligand to metal ions. Some typical
chemical properties of water are described below:
1. Stability. Although water is a very stable compound, yet at temperature
above 2000°C, it decomposes slightly (~0.02%) to give hydrogen and
oxygen.
2000°C
2H O(g) + 486 kJ o 2H (g) + O (g)
2
2
2
2. Self-ionisation of water. A water molecule can act both as an acid
and a base. So, water shows self-ionisation in accordance with the
equilibrium.
+
–
H O(l) + H O(l) U H O (aq) + OH (aq)
3
2
2
This reaction is also called autoprotolysis of water.
3. Hydrolysis reactions. Water can hydrolyse salts of weak acids with
strong bases and of weak bases with strong acids. For example,
ƒ Hydrolysis of Na CO 3
2
+
2–
Na CO (s) + H O(excess) o 2Na (aq) + CO (aq)
2
3
2
3
2–
CO (aq) + H O o HCO 3 – + OH –
2
3
solution becomes
basic due to the
–
presence of OH ions
–
The resulting solution is basic due to the presence of OH ions.
ƒ Hydrolysis of FeCl 3
–
3+
FeCl (s) + H O(excess) o Fe (aq) + 3Cl (aq)
2
3
+
3+
2Fe (aq) + 6H O o 2Fe(OH) (s) + 6H (aq)
3
2
solution becomes
acidic due to the
+
presence of H ions
The resulting solution of ferric chloride is acidic due to the presence of
+
H ions.
4. Reaction with oxides. Oxides of metals and nonmetals react with water
to give the corresponding alkalis and acids.
ƒ Oxides of metals give respective alkalis: The oxides of metals are
basic in nature. These react with water to form the respective alkalis.
For example,
CaO(s) + H O(l) o Ca(OH) (aq)
2
2
calcium oxide calcium hydroxide
Na O(s) + H O(l) o 2NaOH(aq)
2
2
sodium oxide sodium hydroxide
ƒ Oxides of nonmetals give respective acids. The oxides of nonmetals
are acidic in nature. These react with water to form the respective
acids. For example,
SO (g) + H O(l) o H SO (aq)
2
3
2
2
sulphur dioxide sulphurous acid
SO (g) + H O(l) o H SO (aq)
4
2
3
2
sulphur trioxide sulphuric acid
P O (g) + 3H O(l) o 2H PO (aq)
5
2
2
4
3
phosphorus pentoxide phosphoric acid
CO (g) + H O(l) o H CO (aq)
2
3
2
2
carbon dioxide carbonic acid