Disappearing breed of hydrogen

Water a very common chemical substance which is essential for the survival of all life forms on this Earth. Volumetric and gravimetric composition of water are 2:1 and 1:8 (H:O respectively). Pure water is a bad conductor of electricity due to very low degree of ionization. Maximum density of water is at 4ºC. Ice floats on water because density is less than that of water which in turn is due to the empty space in its crystal structure. Crystal structure of ice is regular hexagon with empty space at the centre.

Water is one of the most important compounds of hydrogen. It is colourless liquid and possesses a high dielectric constant (81) and therefore, salts are highly ionized when dissolved in water, but not so in other solvents. It is the most convenient solvent. In general, water is a good solvent for ionic compounds but a poor one for covalent compounds. The solubility of ionic compounds in water is due to evolution of a large amount of hydration energy due to interaction between the ions of solute and the polar water molecules. The hydration energy is so high that it easily overpowers the attractive forces between the oppositely charged ions in the crystalline solute and also breaks the hydrogen bonded structure of water.

On the other hand, interaction between covalent compounds and water molecules is so weak that it liberates very little energy which can’t overcome even the weak vander Waal’s forces what to say of breaking of hydrogen bonded structure of water; with the result covalent compounds are generally insoluble in water. Insolubility of certain ionic compounds like Silver Chloride in water, is due to their exceptionally high attractive forces in the lattice. On the other hand, solubility of some organic compounds (covalent) like alcohols, carboxylic acids and carbohydrates in water is because of their capability of forming hydrogen bonding with water.

Water molecule is a bent molecule with a bond angle of 104.5º. In water molecules, the central atom (oxygen) is in sp3 hybridised state. The deviation in bond angle from the normal tetrahedral bond angle is due to presence of two lone pairs of electrons on the oxygen atom of water. Because of high electronegativity of oxygen, several H2O molecules associate through hydrogen bonding. Actually, the proportion of isolated molecules of water in the liquid is very small. Ice is the crystalline form of water. In ice, each oxygen is tetrahedrally surrounded by four other oxygen atoms through hydrogen atoms. Thus each hydrogen atom is linked to two oxygen atoms: one through covalent bond and other through hydrogen bond. Thus in structure of ice, each water molecule is surrounded by only four immediate neighbours.

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