Unique properties of water

1. Unique properties of Water
Water is a compound with unique physical properties:
• solid form is lighter than liquid form.
• has unusually high boiling point (compared to other group six hydrides) with
large difference between melting and boiling points.
• is a good solvent. Dissolves chemicals → nutrition, toxicity, salinity, water
hardness, acidity etc.
• High specific heat capacity (comfort of aquatic organisms)
• Cohesion- attractions among water molecules → high surface tension.
(insect can walk on water without sinking)
• Adhesion-attraction for other molecules. Adhesion and cohesion → capillary
action (e.g. water rises in a glass tube or stem of a plant )

2. Hydrogen Bonds: responsible for some
unique physical properties of water.
Hydrogen bonds among water molecules (dotted lines)
H H
/
O . .. . H-O H . . . .O
/ /
H H . . . .O H
H
H-O . . H
| |
H . . . O--H
Dimer
• Hydrogen bonds , the strongest of intermolecular forces, increase the
boiling point of water.

3. Boiling and Freezing points of group six hydrides

4. Bonding in Water
• Every oxygen atom is surrounded by four bonds (two
chemical and two hydrogen bonds).
• The four bonds around an oxygen atom form a
tetrahedron.
• The tetrahedral coordination opens up the space between
molecules.
• In liquid water the open spaces are filled with moving
water molecules.

5. Tetrahedral structure of water
• Since the hydrogen bonds are not linear, the real
structure is a little more complicated.

6. Water the Solvent
Water dissolves more substances (ionic compounds and polar
molecules) than any other liquid because of its:
– polarity and ability to form hydrogen bonds (allows polar
molecules to dissolve)
– large dielectric constant (allows ionic substances to
dissolve producing charged species)
– very high molar concentration (55.56M, provides
inexhaustible supply of H+)
– Small dissociation constant (produces small but
biologically significant [H+])
• Most of the chemical reactions important to life take
place in a watery environment inside of cells, and water's
capacity to dissolve a wide variety of chemicals is key in
allowing these chemical reactions to take place.

7. Chemistry of water: Auto-ionisation reaction
(unique chemical property of water)
• H2O(l) + H2O(l)  H3O+ + OH-
• Equilibrium constant [H3O+] [OH-]
K'w = ------------
[H2O]
• Since [H2O] = 1000/18 = 55.56 M, and remains rather
constant under any circumstance, Kw is usually written as
Kw = [H2O]K'w = [H3O+] [OH-] = 10-14
• pKw = -log Kw = 14 (at 298 K)
• For neutral water, [H3O+] = [OH-] = 10-7 (at 298K)
• pH = -log[H3O+] pOH = -log[OH-]
pH = pOH = 7 (at 298 K; in neutral solutions)
• pH + pOH = 14 in any solution(aq).

8. Chemistry of water: Hydrolysis reactions
Metal hydrolysis
• Alkali metals react with water readily.
• Contact of caesium metal with water causes immediate
explosion, and the reactions become slower for potassium,
sodium and lithium.
Alkali metal: 2M + 2H2O → 2MOH + H2
• Reactions with alkaline earth metals (barium, strontium,
calcium) are less well known, but they do react readily.
– However, warm water may be needed to react with calcium metal.
Alkaline earth metal: M + 2H2O → M(OH)2 + H2

9. Chemistry of water: Hydrolysis reactions
Acid – base (Neutralisation) reactions
• Water is amphoteric (can react as a
weak acid or base), due to its ability to
accept or donate a proton. HCl + H2O →
H3O+ + Cl-. (water is base)
NH3 + H2O  NH4
+ + OH-. (water is
acid)
(Hydrolysis reactions with weak acids or
bases are reversible)

10. Chemistry of water: Hydrolysis reactions
Cation Hydrolysis
• Metal-catalyzed Hydrolysis
Al3+ + 3H2O  Al(OH)3 + 3H+.
• For polyvalent cations.
• Transition metal ions (but not alkali metal ions) act as
Lewis acids in metal-ligand interactions.
• Alkali metal ions do not undergo Metal-catalyzed
Hydrolysis.

11. Chemistry of water: Hydrolysis reactions
Halogen hydrolysis
Cl2 + H2O  HCl + HOCl

12. Chemistry of water: Hydrolysis reactions
Metal hydride hydrolysis
2LiH + 2H2O → 2LiOH + 2H2
Metal oxide hydrolysis
BaO + H2O → Ba(OH) 2
K2O + H2O → 2KOH
Non-metal oxide hydrolysis
CO2 + H2O  H2CO3
SO3 + H2O → H2SO4

13. Electrolysis Reactions
• Electrolysis of water is the decomposition of
water into oxygen and hydrogen gas due to
an electric current being passed through the water.
– The reaction has a standard potential of −1.23 V
• H2O(l) → H+(aq) + OH-(aq)
• Reduction at cathode: 2H+(aq) + 2e− → H2(g)
• Oxidation at anode: 2H2O(l) → O2(g) + 4 H+(aq) + 4e−
• Cathode (reduction):2H2O(l) + 2e−→H2(g) + 2OH−(aq)
• Anode (oxidation):4OH−(aq)→O2(g) + 2H2O(l) + 4e−
• Overall reaction: 2H2O(l) → 2H2(g) + O2(g)

14. Purification of water
Conventional Treatment
• 1.1 Screening
removal of floating and suspended material and
sedimentation (coarse sand)
• 1.2 Clarification
coagulation/sedimentation/filtration e.g. sand filters)
addition of chemicals to bring about agglomeration of tiny
particles.
• 1.3 Disinfection
destruction of pathogenic organisms that may be present
- by chlorination, O3, UV

15. Purification of water
Tertiary water treatment
• 2.1 Precipitation
removal of dissolved ionic species by adding chemicals
which bring about their precipitation
• 2.2 Ion exchange
selective or complete removal of the dissolved cations and
anions in water. Replacement with less harmful ion
• 2.3 Adsorption
removal of a variety of organic compounds e.g. those
responsible for colour, taste, and odours and pesticides
• 2.4 Chemical oxidation
oxidation of various compounds that may be found in water
e.g. those responsible for taste and colour.

16. Removal of Water Hardness
To reduce soap consumption and lower
maintenance cost of plumbing fixtures, water
hardness can be removed by:
• distillation
• de-ionisation
• lime-soda process
• ion-exchange
• Membrane filtration

17. Removal of Temporary Water
Hardness only
• Temporary water hardness can be removed by
boiling,
Ca(HCO3)2 → CaCO3 + CO2 + H2O(l).
• The calcium carbonate (CaCO3) formed is the
main component of scale that accumulates in
boilers and cooking vessels

18. Removal of both temporary and
permanent water hardness
1. Distillation or de-ionisation
2. Lime-soda process:
• Na2CO3 + CaSO4 → CaCO3 (s) + Na2SO4
(Na2SO4 does not affect soap consumption)
• Ca(HCO3)2 + Ca(OH)2 → 2CaCO3(s) + 2H2O
• CaSO4 + Na2CO3 → CaCO3(s) + Na2SO4
• Mg (HCO3)2 + 2Ca(OH)2 → 2CaCO3(s)+ Mg (OH)2(s) + H2O

19. Removal of both temporary and permanent water
hardness
3. Ion exchange
• Commercial water softeners (ion exchange resins) are used.
• These substances are usually made into beads, which are packed
into cylinders called ion exchange columns.
• These can be built into machines, such as dishwashers, or plumbed
into water systems to continuously soften the water.
• The resin beads have sodium ions attached to them. As the hard
water passes through the column, the calcium and magnesium ions
swap places with the sodium ions.
• The calcium and magnesium ions are left attached to the beads,
while the water leaving the column contains more sodium ions.
• Some ion exchange resins use potassium or hydrogen ions instead of
sodium ions.