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Chemical Properties of Water

High dielectric constant and strong solvating power make water an excellent solvent. In its reactions, water behaves as an acid, base, oxidant, reductant and versatile ligand for metal ions. Because of its high negative heat of formation, water is highly stable thermally. It dissociates less than 0.02% at 1500 K.

Pure water ionizes a little as shown below:
2H2O(l) H3O+(aq) + OH-(aq)
Kw = 1.008 10-14 (mol dm-3) 2, at 298 K

This shows that water can act both as an acid and a base (H3O+ and OH- ions are formed). It acts as an acid in the presence of a base stronger than itself. Thus,

It acts as a base in the presence of an acid stronger than itself. Thus,

In addition to acid-base reactions, several other types of reactions such as oxidation-reduction, hydrolysis and hydrate formation, are also important in the chemistry of water.

Water is reduced to dihydrogen by metals having reduction potential E° less than -0.41 V. Thus, sodium (E° = -2.71 V), magnesium (E° = -2.37 V), iron (E° = -0.44 V), etc. all reduce water to dihydrogen.
2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)
Mg(s) + H2O(l) MgO(s) + H2 (g)
3Fe(s) + 4H2O (l) Fe3O4(s) + 4H2 (g)

Water can be oxidized to oxygen by strong oxidizing halogens.

Dioxygen is generated by green-plant photosynthesis from water


Compounds like calcium hydride, aluminium nitride, calcium phosphide, calcium carbide, silicon halide, etc., undergo hydrolysis with water.
CaH2 + 2H2O Ca(OH) 2 + 2H2
AlN + 3H2O Al(OH) 3 + NH3
Ca3P2 + 6H2O 3Ca(OH) 2 + 2PH3
CaC2 + 2H2O Ca(OH) 2 + C2H2
SiCl4 + 4H2O Si(OH) 4 + 4HCl

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