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Water

Properties of Water Molecule

Water (H2O) molecule is formed by the covalent bonding of oxygen and hydrogen atoms with the V-shape structure in chemistry. Water present in our earth environment as crystalline solids, liquid solutions, and gaseous form. Liquid and solid forms of water arise due to extensive hydrogen bonding with molecular weight 18 gm mol-1, melting point 0°C and boiling point 100°C. The availability of hygienically safe and pure water is a big problem all over the world. Sewage disposal, industrial wastage, sulfuric acid and nitric acid fertilizer, insecticides, and detergents are polluted water in recent days.

The pH scale of water decreases due to acid rain and many metallic salts change the hardness or softness of water solution. Several action plans have been initiated for identity and reduce these pollutants but the overall situation is still disappointing. Water is amphiprotic in nature because in the presence of acid it acts as an acid and in the presence of alkaline, it acts as a base.

Shape of Water Molecule

Shape structure properties of water molecule and H2O solutions

In the water molecule, the oxygen atom is the central atom. Hence the valence energy levels oxygen atom distribute as, 2s2 2px2 2py1 2pz1. All these four atomic orbitals hybridize to form four equivalent sp3 tetrahedral hybrid orbital. Two of these four hybridized orbitals contain singly occupied electron particles. Therefore, these two orbitals overlap with two 1s-orbital of the hydrogen atom to form two O-H sigma bonds. Another two orbitals contain two lone pairs.  Due to the presence of lone pairs on the central atom, the shape of the water molecule V-shaped with the H-O-H bond angle 105.5°.

The electronegativity and electron affinity of oxygen is much greater than the hydrogen atom in the periodic table. Therefore, the polarization arises in the pure water molecule by two contributions, the bond polarity due to oxygen-hydrogen electronegativity differences and the presence of the lone pair in the central oxygen atom.

Crystal Structure of Ice

The structures of ice and water are complex due to many respect. At least nine crystal lattice dimensions of ice established and each stable over a certain range of temperature and pressure. The ordinary forms of pure water on freezing point or 0°C and 1 atm pressure consists of puckered hexagons. The structure is similar to the ammonium fluoride.  In this structure, each H2O-unit surrounded tetrahedrally by the oxygen atoms of four other H2O molecules through hydrogen bonds extended over the entire structure. In ice crystal, there are huge void or empty spaces. Each oxygen atom has two near and two distant neighbors. The experimental zero-point entropy of ice 3.4 j mol-1 with very low density. At a very low temperature of about -120°C, cubic lattice of ice observed.

When the number of H2O molecules brought together, the positive end of one dipole attacks the negative end of the other dipole. Therefore, H2O molecules associated together to form the cluster (H2O)n in liquid form. But in the gas molecule shape of water, no such hydrogen bonding or association formed.

Acid Alkali solutions

All the protons or hydrogen ions in H2O are rapid migration from oxygen atom to another. This makes them very low life in individual H3O+ ion. The proton readily transfers to the base by the concerted shift across the solvent molecules. Therefore, the acid-base neutralization process in H2O involves the diffusion of the proton to the base.

The equilibrium reaction is written as, H2O → H+ + OH. Therefore, the ionic product, kw = k [H2O] = [H+] [OH]. The ionization product of H2O is constantly equal to 10-14. Hence the above relation tells us that in aqueous solution the concentration of hydrogen and hydroxyl ion are inversely proportional to each other. Therefore, pure water solutions function as a basic solution towards acid-like nitric acid, sulfuric acid, acetic acid, etc but acidic solution toward the alkali like sodium hydroxide, potassium hydroxide, etc.

Hard and Soft Water Solution

Hard water contains common chemical salt of the elements like calcium and magnesium. These producing insoluble precipitates with shop material.  But in the presence of some soluble hydrogen carbonates of calcium and magnesium like Ca(HCO3)2 and Mg(HCO3)2 made the solutions temporarily hard. If the solutions contain soluble sulfates of chemical elements calcium and magnesium which are not precipitated by boiling are called permanent hard water.

On boiling, the temporary hardness of can remove but not the permanent hardness. Therefore, in learning chemistry, hard water solutions made soft by passing it through beds of insoluble aluminum sodium silicates or ion exchange resin. These are technically called zeolites. Another useful regent likes EDTA or ethylene diamine tetraacetic acid in acid solution most effective for removing the hardness of the water. Softening of water also done also by Calgon or sodium hexametaphosphate.