Water is essential for the survival of all forms of life. It forms a major part of all living organisms 60% of the human body is made up of water and in some plants goes up to 90%. It covers 71% of the earth’s surface.
Oceans contain 97.33% of water on the earth’s surface. 2% of water on the earth’s surface is in the form of polar ice and glaciers. The remaining <1 % water is present in lakes, rivers, in the atmosphere and as underground water.
Water is a tasteless and odourless liquid at standard temperature and pressure. It freezes into ice at 273K. Its boiling point is 373K and has a density of 1 gm/cc at 298K. Due to the presence of extensive hydrogen bonding, water has a high freezing point and a high boiling point, a high heat of vaporisation and a high heat of fusion in comparison to other compounds of group 16 elements.
Water is an excellent solvent for the transportation of ions and molecules required for animal metabolism.
Structure of water:
In gas phase, water is a bent molecule with a bond angle of 104.5°. In water molecule the oxygen atom has a higher electronegativity than hydrogen. So, it has a slight negative charge and hydrogen atoms are slightly positive, results polarity in water molecule. In the liquid phase, water molecules are associated by hydrogen bonds.
At atmospheric pressure, ice crystallises in hexagonal form. The density of ice is 0.9167 gram /cc cube at 273 K, whereas water has a density of 0.9998 gram /cc at the same temperature. Thus, ice floats on water. In winter, the frozen ice on lakes provides thermal insulation and is important for the survival of sea life.
Water is amphoteric in nature, so that it can act as an acid as well as a base. In the reactions with hydrogen chloride and hydrogen sulphide, water behaves as a base by accepting H⁺ ions and forming H₃O ⁺ions.
Redox reactions involving water:
Water can easily be reduced into hydrogen by highly electropositive metals.
Ex: Sodium metal reacts with water to give sodium hydroxide and hydrogen.
2 H₂O(l) +2Na(s) →2NaOH (aq) + H₂(g)
Water is oxidised to oxygen during photosynthesis.
(g) + 12H2
O(l)→C6H12O6(aq) + 6H2O(l) + 6O2(g)
It also reacts with fluorine gas to give oxygen and ozone.
(g) + 2H2
O(l)→4H⁺ (aq) + 4F⁻(aq) + O2
(g) + 3H2
O(l)→6H⁺(aq) + 6F⁻(aq) + O3
Water hydrolyses the oxides and halides of non-metals to their respective acids.
Ex: Phosphorus pentoxide on hydrolysis to give ortho phosphoric acid.
P₄O₁₀ + 6H₂0 → 4H₃PO₄
It hydrolyses Silicon chloride to hydrochloric acid and silicon dioxide.
SiCl₄ + H₂O → 4HCl + SiO₂
Calcium carbide reacts with water to give calcium hydroxide and ethyne.
CaC₂ + H₂O → Ca(OH)₂ + C₂H₂
Water reacts with aluminium nitride to give aluminium hydroxide and ammonia.
AlN + H₂O → Al(OH)₃ + NH₃
Many salts can be crystallised as hydrated salts from their aqueous solutions.