Water :

Water is the oxide of hydrogen. It is an important component of animal and vegetable matter. Water constitutes about 65% of our body. It is the principal constituent of earth’s surface.

(1) Structure : Due to the presence of lone pairs, the geometry of water is distorted and the H–O–H bond angle is 104.5°, which is less than the normal tetrahedral angle (109.5°). The geometry of the molecule is regarded as angular or bent. In water, each O–H bond is polar because of the high electronegativity of oxygen (3.5) in comparison to that of hydrogen (2.1). The resultant dipole moment of water molecule is 1.84D.

In ice, each oxygen atom is tetrahedrally surrounded by four hydrogen atoms; two by covalent bonds and two by hydrogen bonds. The resulting structure of ice is open structure having a number of vacant spaces. Therefore, the density of ice is less than that of water and ice floats over water. It may be noted that water has maximum density (1 gcm^–3) at 4°C (277 K).

(2) Heavy water: Chemically heavy water is deuterium oxide (D₂O). It was discovered by Urey.

It is obtained as a by-product in some industries where H₂ is produced by the electrolysis of water.

Heavy water (D₂O) is used (a) as a moderator and coolant in nuclear reactors (b) in the study of mechanism of chemical reactions (c) as a starting material for the preparation of a number of deuterium compounds, e.g.,

SO₃ + D₂O → D₂SO₄
                          ^{Deuteriosulphuric acid}

Al₄C₃ + 12D₂O → 3CD₄ + 4Al(OD)₃
                                    ^{Deuteromet hane}

CaC₂ + 2D₂O → C₂D₂ + Ca(OD)₂
                                ^{Deuterioacetylene}

(3) Physical properties: Water is colourless, odourless and tasteless liquid at ordinary temperature.

At 273K water is in equilibrium with ice and vapour this point is known triple point.

Table 17.3 Some physical constants of H₂O and D₂O at 298 K

(4) Chemical properties : Water shows a versatile chemical behaviour. It behaves as an acid, a base, an oxidant, a reductant and as ligand to metals.

K_w = 1.0 × 10^–14 mol² L² at 298K

(ii) Amphoteric nature :Water can act both as an acid and a base and is said to be amphoteric. However, water is neutral towards litmus and its pH is 7.

(iii) Oxidising and reducing nature : Water can act both as an oxidising and a reducing agent in its chemical reactions. e.g.

2Na + 2H₂O → 2NaOH + H₂
           ^{Oxidising agent}

2F₂ + 2H₂O → 4HF + O₂
           ^{Reducing agent}
(iv) Hydrolytic reactions : Water can hydrolyse many oxides, halides, hydrides, carbides, nitrides, phosphides, carbonates etc. to give an acid or a base or both as shown below:

SO₂ + H₂O → H₂SO₃

Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃

CaH₂ + 2H₂O → Ca(OH)₂ + 2H₂

CaO + H₂O → Ca(OH)₂

Na₂CO₃ + 2H₂O → 2NaOH + H₂CO₃

SiCl₄ + 4H₂O → Si(OH)₄ + 4HCl

Ca₃P₂ + 6H₂O → 3Ca(OH)₂ + 2PH₃

CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂

(v) Water forms hydrates with metal salts : There are three main types of hydrates.

(a) Compounds in which water molecule are co-ordinated to the metal ion (complex compounds) [Ni(OH₂)][(NO₃)₂, Fe(OH₂)₆]Cl₃ etc.

(b) Compound in which water molecule may be hydrogen bonded to oxygen to form oxo-anion. For example in CuSO₄.5H₂O , 4 molecules of water are co-ordinated to Cu²⁺ while the fifth molecule is hydrogen bonded to SO₄^2– ion.

(c) In some compounds, water molecule occupies, interstitial sites in the crystal lattice e.g., BaCl₂.2H₂O.