Water
the importance of organic ions:
dissociation is the splitting of a molecule into smaller molecules, atoms or ions, especially by
a reversible process.
Important anions:-
Nitrate ions (NO3-)- needed in plants to make DNA and amino acids, and therefore
proteins from the products of photosynthesis.
Phosphate ions (PO4^3-)- needed in all living organisms to make ATP and ADP as well
as DNA and RNA.
Chloride ions (CL-)- needed in nerve impulses, sweating and many secretory systems
in animals.
Hydrogencarbonate ions (HCO3-)- needed to buffer blood pH to prevent it becoming
too acidic.
Important cations:-
Sodium ions (Na+)- needed in nerve impulses, sweating and many secretory systems
in animals.
Calcium ions (Ca^2+)- needed for the formation of calcium pectate for the middle
lamella between two cell walls in plants, and for bone formation and muscle
contraction in animals.
Hydrogen ions (H+)- needed in cellular respiration and photosynthesis, and in
numerous pumps and systems as well as pH balance.
Magnesium Ions (Mg^2+)- needed for the production of chlorophyll in plants.
The chemistry of water
The chemical formula of water is H2O, two atoms of hydrogen joined to one atom of
oxygen. All held by strong covalent bonds.
In the case of water the molecule is slightly polarized: the electrons in the covalent bonds
are not quite evenly shared, consequently, the molecule has a part that is slightly negative
and another that is slightly positive. This separation of charge is called dipole
In water the oxygen is more electronegative than hydrogen, the oxygen atom in H2O pulls
electron toward itself, resulting in a partial negative charge on the oxygen atom and partial
positive charges on the hydrogen atoms.
And the molecule is described as a polar molecule.
-dipole is the separation of charge in a molecule when the electrons in the covalent bonds
are not evenly shared.
-a polar molecule is a molecule containing a dipole.
Effect of the polarity of water:
the most major effect is that water molecules form hydrogen bonds.
The slightly negative oxygen atom of one water molecule will attract the slightly
positive hydrogen atoms of other water molecules in a weak electrostatic attraction
called a hydrogen bond.
- Each individual hydrogen bond is weak but there are many of them so the
molecules of water ‘stick together’. This is why water has a relatively high
melting and boiling points compared with other substances with molecules of
the similar size; a lot of energy is needed to break all the hydrogen bonds that
hold the molecules together.
Hence hydrogen bonds are weak electrostatic intermolecular bonds formed between polar
molecules containing at least one hydrogen atom.
The importance of water
› Water molecules are non-linear and dipolar due to different sizes of atoms and uneven distribution
of charges.
› Water has a dipolar nature due to uneven distribution of charges. This allows the formation of
hydrogen bonds with all polar particles. Positive ions bond with the negative end of water while
negative ions bond with the positive end of water. So all polar structures dissolve in water. This
makes water an excellent solvent.
(in ionic substances; the compound separates into -ve and +ve ions (dissociation). The cations are
attracted to the negative charges on the oxygen atom, and the anions are attracted to the positive
charges oh the hydrogen atoms)
› Moreover, water molecules can form hydrogen bonds with each other (cohesion) as well as with
nearby surfaces (adhesion). This allows water to move as one unbroken column.
› An excellent solvent that moves as one unbroken column is definitely a perfect transport
medium.
1. Excellent Solvent.
2. Amphoteric:
Keeps constant pH as it acts as a proton donor or a proton acceptor. So, it acts as a buffer during
metabolic reactions.
3. Transparent:
Allows marine life to exist due to passage of sunlight and photosynthesis.
4. Low Density:
Ice floats on the surface insulating the lower water which allows for the continuity of marine life.
5. High specific heat capacity (SHC):
Keeps water temperature within narrow limits. This allows for proper enzyme activity of aquatic
organisms.
