WATER

• The versatility of water as a solvent is essential to live organisms. Life is believed to have originated in the aqueous solutions of the world’s oceans, and living organisms depend on the aqueous solutions, such as blood, and digestive juices.
• Water, is a substance composed of the chemical elements H and O existing in gaseous, liquid, and solid states. It is one of the most plentiful and essential compounds.
• Tasteless and odorless liquid at room temperature; able to dissolve many substances.
• Water also exists on other planets and moons both within and beyond the solar system. In small quantities, water appears colorless, but water has an intrinsic blue color by slight absorption of light and red wavelength.
• Water occurs as a liquid on the surface of the earth under normal conditions.
• Water is readily changed to a vapor (gas) allows it to be transported through the atmosphere from the oceans to inland areas where it condenses and, as rain nourishes plant and animal life.
• Aristotle considered water as to be one of four fundamental elements, in addition to earth, air and fire.
• • The 6 valence electrons of oxygen in water molecule are hybridized to four sp3 orbitals that are elongated to thecorners of a somewhat deformed, imaginary tetrahedron.
  • Two hybrid orbitals form O-H Covalent bonds with a bond angle of 105o for H-O-H


  Water Molecule
• The other 2 orbitals hold the nonbonding electron pairs. The O-H covalent bonds, due to the highly electronegative oxygen, have a partial (40%) ionic character.
• Each water molecule is tetrahedrally Coordinated with four other water molecules through hydrogen bonds. The two unshared electron pairs of oxygen act as H-bond acceptor sites and the H-O bonding orbitals acts as hydrogen bond donor sites.
• The O-H distance (bond length) is 95.7 picometers (9.57 x 10-11 m or 3.77 x 10-9 inches)
• Because an oxygen atom has a greater electronegativity, than a hydrogen atom, the O-H bonds in the water molecule are polar, with the O bearing a partial negative charge and H having a partial positive charge.
• Hydrogen atoms in water molecules are attracted to regions of high electron density and can form weak linkages, called H-bonds. This means that the H atoms in one water molecule are attracted to the nonbonding electron pairs of the oxygen atom on an adjacent water molecule.
• Viscosity – resistance of liquid to flow.
• Surface tension – is the energy required to increase the surface area of the liquid by a unit amount.
• Heat of fusion – is the quantity of heat necessary to change 1 g of a solid to a liquid with no temperature change.
• Heat of vaporization – is the amount of heat needed to turn 1 g of a liquid into a vapor, without a rise in the temperature of the liquid.
• Heat of formation – also called standard heat of formation, enthalpy of formation, or standard enthalpy of formation, the amount of heat absorbed or evolved when one mole of a compound is formed from its constituent elements, each substance being in its normal physical state (gas, liquid, or solid)
• Entropy of vaporization – is the increase in entropy upon vaporization of a liquid. The entropy of vaporization is then equal to the heat of vaporization divided by the boiling point. According to Trouton’s rule, the entropy of vaporization (at std pressure) of most liquids is about 85 – 88 J mol-1K-1
• Water is a polar molecule and is attracted to other polar molecules.
• Droplets or beads of water form a nonpolar surface because water molecules adhere together instead of adhering to the surface.
• The arrangements of water molecules in “liquid water” and in ice are still under intensive investigation.
• Due to the pronounced tendency of water molecules to associate through H-bridges, liquid water and ice are highly structured. They differ in the distance between molecules, coordination number and time-range order (duration of stability)
• The coordination number in ice-I is four, the O-H…O (nearest neighbor) distance is 0.276 nm (0oC) and the H-atom between neighboring oxygen is 0.101 nm from the oxygen to which it is bound covalently and 0.175 nm from the oxygen to water molecules forming a tetrahedron, are loosely packed and kept together mostly through H-bridges.
• • Ice (0oC) - 4, 0.276 nm
  • Water (1.5oC) - 4.4, 0.290 nm
  • Water (83oC) - 4.9, 0.305 nm
• Coordination number – also known as ligancy, the number of atoms, ions, or molecules that a central atom or ion holds as its nearest neighbors in a complex or coordination compound or in a crystal.
• When ice melts and the resultant water is heated, both the coordination number and the distance between the nearest neighbors increase.
• An increase coordination number, increases the density whereas an increase in distance between nearest neighbors decreases the density.
• The effect of increasing coordination number is predominant during a temperature increase from 0 to 4oC. As a consequence, water has an unusual property; its density in the liquid state at 0oC (0.998 g/cm3).
• The liquid state of water has a very complex structure, the extensive H-bonding among the molecules in liquid water produces much larger values for properties such as viscosity, surface tension, and boiling point than are expected for a typical liquid containing small molecules.
• The polarity of the water molecule plays a major part in the dissolution of ionic compounds during the formation of aqueous solutions. Earth’s oceans contain vast amount of dissolved salts, which provide a great natural resource.
• Ability of foods to be flavored as they are cooked is made possible by solubility in water of such substances as sugar and salt. Although the solubility of substances in water is an extremely complex process, the interaction between the polar water molecules and the solute plays a major role.
• When an ionic solid dissolves in water, the positive ends of the water molecules are attracted to the anions, while their negative ends are attracted to the cations, this process is called HYDRATION.
• The hydration of its ions tends to cause a salt to break apart (dissolve in the water. In the dissolving process the strong forces present between the positive and negative ions of the solid are replaced by strong water-ion interactions.
• When ionic substances dissolve in water, they break apart into individual cations and anions.
• As NaCl dissolves, four H2O molecules closely associate with the Na+. (The hydration number of Na+ is four).
• Just outside this inner hydration sphere is a region where water molecules are partially ordered by the presence of the [Na (H2O)4]+ hydrated ion. This partially ordered region blends into “regular” (bulk) liquid water. The greater the charge density (the ratio of charge to surface area) of anion, the larger the hydration number will be.
• Nonionic compounds are soluble in water.
• ethanol, the alcoholic component of wine, beer, and distilled water is highly soluble in water.
• Ethanol is soluble in water because of the structure of alcohol molecule. The molecule contains a polar O-H bond like those In water, which allows it to interact effectively with water.