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• IMPORTANCE OF WATER

  Water is an extremely important molecule

  It actively participates in many chemical reactions,

  particularly those involving aqueous processes

  • Furthermore, the behavior or properties of biomolecules

  maybe influenced by the presence of water.
• PHYSICAL AND CHEMICAL

  PROPERTIES OF WATER

  ✓ an odorless,tasteless, and colorless substance

  ✓ a water molecule can form hydrogen bonds with

  many other molecules

  ✓ water molecule : POLAR (due to the lone pair in the

  oxygen atom, therefore, the electron density of the

  molecule is concentrated on the oxygen atoms
• PHYSICAL AND CHEMICAL

  PROPERTIES OF WATER

  • Each water molecule is surrounded by four other

  neighboring water molecules through hydrogen bonding,

  thus liquid water is composed of a three-dimensional

  network of water molecules that are bonded by hydrogen

  • Water expands on freezing, and at 277 K (40C)

  • Liquid water has a density of 1.00 gram per milliliter

  (g/mL)
• WHY WATER IS POLAR?

  The unequal sharing

  of electrons gives the

  water molecule a

  slight negative charge

  near its oxygen atom

  and a slight positive

  charge near its hydrogen

  atoms.
• Water molecules forming a

  hydrogenbond(dottedline)
• Water may act both as an acid and as a base, enabling it to

  undergo autoprotolysis to give hydronium (H30+) and OH ions
• Acid (Arrhenius definition) – is a molecule that contains a

  hydrogen proton (H+)

  HCl > H+ + Cl-
• Base (Arrhenius definition) – is a molecule that contains a

  hydroxide (OH-)

  NaOH > Na+ + OH-
• H20 + H20 > H3O+ + OH-
• AUTOPROTOLYSIS

  Also known as self-

  ionisation

  Involves the transfer of protons from and to

  the same identical molecule, as given in the

  previous equation

  Pure water gives a neutral pH concentration

  of 7
• pH = measure of how acidic or basic water is.

  pH below 7 = acidic

  pH above 7 = basic
• AUTOPROTOLYSIS

  This imposes that the strongest acid in the

  solution is the hydronium ion, whereas the

  strongest base in the solution is the

  hydroxide ion
• AUTOPROTOLYSIS

  Acid-base reactions involving water

  impose a limit on the relative strength of

  the acid and base components in solution

  Leveling effect of water
• Water exhibits high heat capacity

  Water’s heat capacity, the amount of heat energy required to

  increase its temperature, is relatively high.

  Large bodies of water, such as oceans and lakes, can absorb

  large amounts of heat with only small changes in temperature.

  This protects organisms living within from drastic changes in

  temperature.

  Water’s specific heat is more than 4

  times that of most metals.
• BUFFER

  Is the ability to resist

  extreme pH changes

  The buffering capacity

  of water is so important

  from an environmental

  perspective

  • Any accidental spill of

  substances with extreme

  pH such as strong bases

  and acids (pH > 7 or pH

  < 7, respectively) will not

  immediately induce

  significant pH changes in

  natural bodies of water