Water- Structure and Properties

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Structure of Water molecule

In a water molecule, each hydrogen atom shares an electron pair with the oxygen atom. The geometry of the water molecule is dictated by the shapes of the outer electron orbitals of the oxygen atom, which are similar to the bonding orbitals of carbon. These orbitals describe a rough tetrahedron, with a hydrogen atom at each of the two corners and un-shared electrons at the other two.

Structure of Water

The H—O—H bond angle is 104.5°, 5° less than the bond angle of a perfect tetrahedron which is 109.5°; the non-bonding orbitals of the oxygen atom slightly compress the orbitals shared by hydrogen. The oxygen nucleus attracts electrons more strongly than does the hydrogen
nucleus (i.e., the proton); oxygen is more electronegative. The sharing of electrons between H and O is therefore unequal; the electrons are more often in the vicinity of the oxygen atom than of the hydrogen. This unequal electron sharing creates two electric dipoles in the water molecule, one along each of the H—O bonds.

The oxygen atom bears a partial negative charge (δ−), and each hydrogen a partial positive charge (δ+). The resulting electrostatic attraction between the oxygen atom of one water molecule and the hydrogen of another water molecule constitutes a hydrogen bond.

Physical Properties of Water

The physical properties of water differ markedly from those of other solvents. For example, water as a hydride of oxygen (H2O) has a higher melting point, boiling point, heat of vapourization and surface tension than do the comparable hydrides of sulfur (H2S) and nitrogen (NH3) and most other common liquids. These unusual properties are a consequence of strong attractions between adjacent water molecules, which give liquid water great internal cohesion.

Water has some unusual properties of physiological importance as below:

  1. Expansion on freezing
    Most substances decrease in volume (and hence increase in density) as their temperature decreases. But in case of water, there is a temperature at which its density exceeds that at higher or lower temperatures. This temperature is 4 °C. In fact, water just above the freezing point is heavier than water at the freezing point. Therefore, it moves towards the bottom, freezing begins at the surface and the bottom is last to freeze. Organisms living at the bottoms of fresh-water lakes are, hence, protected from freezing.
  2. High surface tension
    Like a stretched membrane, the surface of a liquid tends to contract as much as possible. This phenomenon is called surface tension. Water has the highest surface tension (of 72.8) of any known liquid. And it is the reason why water rises to unusually high levels in narrow capillary tubes. This has great significance in physiology.
  3. High heat capacity
    There occurs a smaller temperature rise in water as compared to most other substances, when a given amount of heat is applied. Thus, water acts as a temperature buffer. It maintains its temperature more successfully than most other substances. We may, thus, say that has a high heat capacity (1,000 cal/g).
  4. High solvent power
    Water is a solvent for a great number of molecules which form ionized solutions in water. It may, thus, be called a universal solvent which facilitates chemical reactions both outside of and within biological systems.

WEAK INTERACTIONS IN AQUEOUS SOLUTIONS

  1. Hydrogen Bonding Between Water Molecules
  2. Hydrogen Bonding Between Water and Solute Molecules
  3. Interaction Between Water and Charged Solutes
  4. Interaction Between Water and Nonpolar Gases
  5. Interaction Between Water and Nonpolar Compounds
  6. Van der Waals Interactions