9.6.4 Chemical Properties of Water

Water reacts with a large number of substances. Some of the important reactions are given below.

Amphoteric Nature: It has the ability to act as an acid as well as a base i.e., it behaves as an amphoteric substance. In the Brönsted sense it acts as an acid with NH3 and a base with H2S

 

H2O + NH3 (aq) → OH- (aq) + NH4 + (aq)

H2O + H2S → H3O + (aq) + HS- (aq)

 

The auto-protolysis (self-ionization) of water takes place as follows :

H2O (l)+ H2O (l) → H2O+ (aq) + OH- (aq)

Redox Reactions Involving Water: Water can be easily reduced to dihydrogen by highly electropositive metals.

2H2O (l)+ 2Na (s)→ 2NaOH (aq) + H2(g)

 

Thus, it is a great source of dihydrogen.

Water is oxidised to O2 during photosynthesis.

6CO2 (g) + 12H2O (l) → C6H12O6 (aq) + 6H2O (l) + 6O2 (g)

 

With fluorine also it is oxidised to O2

2F2 (g) + 2H2O (l) → 4H+ (aq) + 4F- (aq)+ O2(g)

 

Hydrolysis Reaction: Due to high dielectric constant, it has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent and some ionic compounds are hydrolysed in water.

 

P4O10 (s) + 6H2O → 4H3PO4 (aq)

SiCl4 (l) + 2H2O (l) → SiO2 (s) + 4HCL (aq)

N3- (s) + 3H2O (l) → NH3 (g) + 3OH- (aq)

 

Hydrates Formation: From aqueous solutions many salts can be crystallised as hydrated salts. Such an association of water is of different types viz.,

  1. coordinated water e.g., [Cr(H2O6)]3+ 3Cl-
  1. interstitial water e.g., BaCl2, 2H2O
  2. hydrogen-bonded water [Cu(H2O4)]2+ SO42-.H2O in CuSO4.5H2O

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