DR FAIZAN WATOO ASSOCIATE PROFESSOR PMAS ARID AGRICULTURE UNIVERSITY RAWALPINDI

1. Water
Water (H2O, HOH) is the most abundant molecule on
earth's surface, constituting about 70% of the planet's
surface.
In nature it exists in liquid, solid, and gaseous states.
It is in dynamic equilibrium between the liquid and gas
states at standard temperature and pressure.
At room temperature, it is a nearly colorless with a hint of
blue, tasteless, and odorless liquid.
Many substances dissolve in water and it is commonly
referred to as the universal solvent.

2. Properties and Behavior
• Water is the essential solvent of life and the main
chemical constituent of all cells.
• In many ways, water is a miracle liquid.
• It is essential for all living things (on this planet at
least), and it is often referred to as a universal
solvent because many substances dissolve in it.
• These unique properties of water result from the
ways in which individual H2O molecules interact
with each other.

3. Unique properties
• Despite its low molecular weight, it is liquid
at most environmental temperature. Other
molecules having similar molecular
weights (e.g., ammonia, methane, and
H2S) are gasses.

4. In comparison with other solvents, water has high
values for
• Melting point:- The melting point of a solid is the
temperature range at which it changes state from solid
to liquid. At the melting point the solid and liquid
phase exists in equilibrium. The melting point of ice
is 0 °C (32 °F, 273 K) at standard pressure,
• Heat of fusion:- The heat of fusion is the amount
of heat energy needed to convert one gram of a solid
into its liquid form. The heat of fusion for ice/water is
333 J/g
• Boiling point- The boiling point of an element or a
substance is the temperature at which the vapor
pressure of the liquid equals the environmental
pressure surrounding the liquid. The boiling point of
water is 100 °C (212 °F) at standard pressure

5. In comparison with other solvents, water has
high values for
• Heat of vaporization - It is the energy
required to transform a given quantity of a
substance into a gas, heat of vaporization of
water (40.65 kJ·mol−1
• Specific heat- It is the measure of the heat
energy required to increase the temperature of a
unit quantity of a substance by unit degree
• Surface tension- Surface tension is a property
of the surface of a liquid. It is what causes the
surface portion of liquid to be attracted to another
surface, such as that of another portion of liquid

6. • Each of these properties serves to constrain
major temperature fluctuation and to keep water
in a liquid state.
• Water absorbs more heat energy per gram for
every degree rise in temperature than other
common solvents and therefore acts to moderate
temperature changes.
• Likewise , to convert one gram of water from
liquid to the vapor (gas) state at its boiling point,
an additional 2259J(540cal) must be absorbed.
• This high heat of vaporization , together with
high surface tension of liquid water, tend to keep
water in liquid state.

7. • At other temperature extreme, large amount of energy
(335J or 80cal/ gm) must be lost for water to be converted
from liquid to the solid state.
• Calories is define as amount of energy required to raise
the temperature of one gram of water from 14.5 to 15.5°C
• Water has the second highest specific heat capacity
(Specific heat capacity, often shortened to specific heat
)of all known substances, after ammonia, as well as a
high heat of vaporization , both of which are a result of the
extensive hydrogen bonding between its molecules.
• These two unusual properties allow water to moderate
Earth's climate by buffering large fluctuations in
temperature.

8. Temperature-dependency of the heats of
vaporization for water, methanol, benzene, and
acetone

9. • The specific enthalpy of fusion of water is
333.55 kJ·kg−1 at 0 °C. Of common
substances, only that of ammonia is higher.
• This property confers resistance to melting
upon the ice of glaciers and drift ice.
• Drift ice is ice that floats on the surface of
the water in cold regions, as opposed to
fast ice, which is attached ("fastened") to a
shore.
• Usually drift ice is carried along by winds
and sea currents, hence its name, "drift
ice".

10. Enthalpy of fusion
• The standard enthalpy of fusion (symbol:
ΔHfus), also known as the heat of fusion or
specific melting heat
• It is the amount of thermal energy which must be
absorbed or evolved for 1 mole of a substance
to change states from a solid to a liquid or vice
versa.
• It is also called the latent heat of fusion or the
enthalpy change of fusion, and the
temperature at which it occurs is called the
melting point.

11. • Before the advent of mechanical refrigeration, ice was in common use
to retard food spoilage .
• Although water is often called the "universal solvent", not all
substances dissolve in water.
• Water does dissolve most salts and other ionic compounds, as well as
nonionic polar and nonpolar groups compounds such as sugars,
alcohols and other molecules that contain hydroxyl(OH), aldehyde
and ketone groups.
• Many substances that contain both polar and non-polar groups (such
as soap, fatty acid, and glycerophosphatides) do not dissolve in water,
but they do form micelles.
• A micellar arrangement is not a true solution but is a suspension or
dispersion.

12. • The behavior of soap molecules in water is a good and
also common example of micelle formation.
• Soap molecules form by the saponification of fatty acid ,
consist of long , non-polar hydrocarbon chain terminating
in a polar carboxyl group that is ionically bonded to a
ions such as Na+ and K+
• When disperse in water, the soap molecules aggregate
to form spherical clusters called micelles, in which polar
carboxyl groups of the soap molecules are arranged at
the surface of the sphere, where they form week bonds
with the surrounding water and the non-polar
hydrocarbon chain project inward

13. Density of water and ice
• The density of water is dependent on its
temperature, but the relation is not linear
and is not even monotonic (see following
table).
• When cooled from room temperature
liquid water becomes increasingly dense,
just like other substances. But at
approximately 4°C, water reaches its
maximum density

15. • As it is cooled further under ambient conditions, it
expands to become less dense.
• This unusual negative thermal expansion is attributed to
strong, orientation-dependent, intermolecular
interactions
• The solid form of most substances is denser than the
liquid phase; thus, a block of the solid will sink in the
liquid. But, by contrast, a block of common ice floats in
liquid water because ice is less dense than liquid water.
• Upon freezing, the density of ice decreases by about
9%.
• The reason for this is the 'cooling' of intermolecular
vibrations allowing the molecules to form steady
hydrogen bonds with their neighbors and thereby
gradually locking into positions reminiscent of the
hexagonal packing achieved upon freezing to ice

16. • While the hydrogen bonds are shorter in the
crystal than in the liquid, this locking effect
reduces the average coordination number of
molecules as the liquid approaches nucleation.
Other substances that expand on freezing are
 antimony,
 bismuth,
 gallium,
 germanium,
silicon,
acetic acid.

17. Surface Tension Effects in everyday life
• Water beading

18. • Beading of rain water on the surface of a waxed
automobile.
• Water adheres weakly to wax and strongly to
itself, so water clusters into drops.
• Surface tension gives them their near-spherical
shape, because a sphere has the smallest
possible surface area to volume ratio
• Formation of drops occurs when a mass of liquid
is stretched. If a stream of water were running
from the faucet, the stream would break up into
drops during its fall.
• Gravity stretches the stream, then surface
tension pinches it into spheres.

19. • Two nice and simple experimental demonstrations can be
done to show the role of surface tension and solid-liquid
adhesion forces.
• Flotation of objects denser than water occurs when the
object is non-wettable and its weight is small enough to be
borne by the forces arising from surface tension.
• Separation of oil and water is caused by a tension in the
surface between dissimilar liquids. This type of surface
tension is called "interface tension", but its physics are the
same.
• Tears of wine is the formation of drops and rivulets on the
side of a glass containing an alcoholic beverage. Its cause
is a complex interaction between the differing surface
tensions of water and ethanol.

20. • Emulsions are a type of solution in which
surface tension plays a role.
• Tiny fragments of oil suspended in pure water
will spontaneously assemble themselves into
much larger masses.
• But the presence of a surfactant (a substance which
tends to reduce the surface tension of a liquid in which it is
dissolved), provides a decrease in surface tension,
which permits stability of minute droplets of oil
in the bulk of water (or vice versa).
• One end of surfactants molecule is attracted to water, while the
other end is attracted to dirt and grease. So the surfactant
molecules help water to get a hold of grease, break it up, and
wash it away.

21. Electronic Distribution in H2O

22. • The dipole forms across the water molecule as a result of the
polar covalent bonding between hydrogen and oxygen .
• Because the bonding electrons are shared unequally by the
hydrogen and oxygen atoms, a partial negative charge (ð-)
forms at the oxygen end of the water molecule, and a partial
positive charge (ð+) forms at the hydrogen ends.
• Since the hydrogen and oxygen atoms in the molecule carry
opposite (though partial) charges, nearby water molecules are
attracted to each other like tiny little magnets.
• The electrostatic attraction between the ð+ hydrogen and the ð-
oxygen in adjacent molecules is called hydrogen bonding.

23. Hydrogen Bonding between Water Molecules

24. • Hydrogen bonding makes water molecules "stick" together.
While hydrogen bonds are relatively weak compared to
other types of bonds, they are strong enough to give water
many unique properties.
• For example, hydrogen bonds sank the Titanic, and
hydrogen bonds allow the Basilisk lizard to walk on water
(as a result, the Basilisk has earned the nickname "Jesus"
lizard).
• Just how does hydrogen bonding do this? Well, let's start
with the Titanic. The Titanic sank because it hit an iceberg
- a chunk of ice floating on the surface of the ocean.
• The reason ice floats is because of hydrogen bonding. In
water's liquid form, hydrogen bonding pulls water molecules
together. As a result, liquid water has a relatively compact,
dense structure.

25. • The structure that forms in the solid ice crystal actually has
large holes in it.
• Therefore, in a given volume of ice, there are fewer water
molecules than in the same volume of liquid water.
• In other words, ice is less dense than liquid water and will float
on the surface of the liquid. Throw in one really big chunk of
ice and a cruise ship, and you begin to see the problems that can
arise.

26. Surface Tension
• Neighboring water molecules are attracted to one another.
• Molecules at the surface of liquid water have fewer neighbors
and, as a result, have a greater attraction to the few water
molecules that are nearby.
• This enhanced attraction is called surface tension. It makes the
surface of the liquid slightly more difficult to break through
than the interior.

27. • When a small object that would normally sink in water is
placed carefully on the surface, it can remain suspended on the
surface due to surface tension.
• The Basilisk lizard makes use of the high surface tension of
water to accomplish the incredible feat of walking on water's
surface.
• The Basilisk can't actually walk on water; rather, it runs on
water, moving its feet before they break through the
surface. Take a look:

28. Water as a Solvent
• The partial charge that develops across the water molecule
helps make it an excellent solvent.
• Water dissolves many substances by surrounding charged
particles and "pulling" them into solution.
• For example, common table salt, sodium chloride, is an ionic
substance that contains alternating sodium and chlorine ions.

29. • When table salt is added to water, the partial charges on the
water molecule are attracted to the Na+ and Cl- ions.
• The water molecules work their way into the crystal structure
and between the individual ions, surrounding them and slowly
dissolving the salt.
• The water molecules will actually line up differently
depending on which ions are being pulled into solution.
• The negative oxygen ends of water molecules will surround
the positive sodium ions; the positive hydrogen ends will
surround the negative chlorine ions.

30. Table Salt Dissolving in Water

31. • In a similar fashion, any substance that carries a net electrical
charge, including both ionic compounds and polar covalent
molecules (those that have a dipole), can dissolve in water.
• This idea also explains why some substances do not dissolve
in water.
• Oil, for example, is a non-polar molecule. Because there is no
net electrical charge across an oil molecule, it is not attracted
to water molecules and therefore does not dissolve in water.

32. The Dipole
• Because the valence electron in the water molecule spend more
time around the oxygen atom than the hydrogen atoms, the
oxygen end of the molecule develops a partial negative charge
(because of the negative charge on the electrons).
• For the same reason, the hydrogen end of the molecule develops
a partial positive charge.
• Ions are not formed; however, the molecule develops a partial
electrical charge across it called a dipole.
• The water dipole is represented by the arrow in which the head
of the arrow points toward the electron dense (negative) end of
the dipole and the cross resides near the electron poor (positive)
end of the molecule.

33. Water Hydrogen Bonding
• The hydrogen bond is really a special case of dipole forces.
• A hydrogen bond is the attractive force between the hydrogen
attached to an electronegative atom of one molecule and an
electronegative atom of a different molecule.
• Usually the electronegative atom is oxygen, nitrogen, or fluorine,
which has a partial negative charge.
• The hydrogen then has the partial positive charge.
• To recognize the possibility of hydrogen bonding, examine the
Lewis structure of the molecule.
• The electronegative atom must have one or more unshared
electron pairs as in the case of oxygen and nitrogen, and has a
negative partial charge.
• The hydrogen, which has a partial positive charge tries to find
another atom of oxygen or nitrogen with excess electrons to
share and is attracted to the partial negative charge.
• This forms the basis for the hydrogen bond.

34. Water Hydrogen Bonding The properties of water are
closely related to hydrogen bonding. The polar nature of
the water molecule facilitates the hydrogen bond.