Water exists in three states and has unique properties due to its molecular structure and ability to form hydrogen bonds. As a polar molecule, water can form up to four hydrogen bonds with neighboring molecules, giving liquid water high cohesion and surface tension. This also accounts for water's unusually high melting and boiling points compared to similar compounds. The hydrogen bonding in ice locks molecules into a rigid crystalline structure that floats, allowing aquatic life to survive under frozen ponds and lakes.

1. Water: Structure and Properties
BY EKHLAQUE BIOTECH
H
O
H

2. What Is Life?
Defined as an organized genetic unit capable of
Metabolism, Reproduction, and
Evolution.

3. Exist in three states: solid (ice), liquid, or gas (vapor).
Medium in which life originated on Earth, and it is in water that life
evolved for its first billion years.
The water molecule, H2O, has unique chemical features.
As water is a polar molecule that can form hydrogen bonds.
The shape of water is a tetrahedron.
The four pairs of electrons in the outer shell of oxygen repel one
another, producing a tetrahedral shape.
Water

4. Weak Interactions in Aqueous
Systems
Hydrogen bonds between water molecules provide the cohesive forces that make
water a liquid at room temperature and favor the extreme ordering of molecules
that is typical of crystalline water (ice).
Polar biomolecules dissolve readily in water because they can replace water-water
interactions with more energetically favorable water-solute interactions.
In contrast, nonpolar biomolecules interfere with water-water interactions but are
unable to form water-solute interactions consequently, nonpolar molecules are
poorly soluble in water.
In aqueous solutions, nonpolar molecules tend to cluster together.
Hydrogen bonds and ionic, hydrophobic (Greek, “water-fearing”), and van der
Waals interactions are individually weak, but collectively they have a very significant
influence on the three-dimensional structures of proteins, nucleic acids,
polysaccharides, and membrane lipids.

5. Hydrogen Bonding Gives
Water Its Unusual Properties
Water has a higher melting point, boiling point, and heat of vaporization than most
other common solvents (Table 2–1).
These unusual properties are a consequence of attractions between adjacent
water molecules that give liquid water great internal cohesion.

6.  A look at the electron structure of the H2O molecule reveals the cause of these
intermolecular attractions.
 Each hydrogen atom of a water molecule shares an electron pair with the central oxygen
atom. The geometry of the molecule is dictated by the shapes of the outer electron orbitals
of the oxygen atom, which are similar to the sp3 bonding orbitals of carbon (see Fig. 1–14).
Cont..

7. These orbitals describe a rough tetrahedron,
with a hydrogen atom at each of two corners
and unshared electron pairs at the other two
corners (Fig. 2–1a).
The H—O—H bond angle is 104.5, slightly less
than the 109.5 of a perfect tetrahedron
because of crowding by the nonbonding
orbitals of the oxygen atom.
The oxygen nucleus attracts electrons more
strongly than does the hydrogen nucleus (a
proton); that is, oxygen is more electronegative.
Cont..

8. This means that the shared electrons are more often in the vicinity of the oxygen
atom than of the hydrogen.
The result of this unequal electron sharing is two electric dipoles in the water
molecule, one along each of the H—O bonds; each hydrogen bears a partial
positive charge, and the oxygen atom bears a partial negative charge equal in
magnitude to the sum of the two partial positives (2).
As a result, there is an electrostatic attraction between the oxygen atom of one
water molecule and the hydrogen of another (Fig. 2–1b), called a hydrogen bond.
Cont..

9. Hydrogen bonds are relatively weak.
Those in liquid water have a bond dissociation energy (the energy
required to break a bond) of about 23 kJ/mol, compared with 470 kJ/mol
for the covalent O—H bond in water or 348 kJ/mol for a covalent C—C
bond.
The hydrogen bond is about 10% covalent, due to overlaps in the bonding
orbitals, and about 90% electrostatic.
When water is heated, the increase in temperature reflects the faster
motion of individual water molecules.
At any given time, most of the molecules in liquid water are hydrogen
bonded, but the lifetime of each hydrogen bond is just 1 to 20 picoseconds
(1 ps);
When one hydrogen bond breaks, another hydrogen bond forms, with the
same partner or a new one, within 0.1 ps.
Cont..

10. The nearly tetrahedral arrangement of the orbitals about the oxygen
atom (Fig. 2–1a) allows each water molecule to form hydrogen bonds
with as many as four neighboring water molecules.
In liquid water at room temperature and atmospheric pressure,
however, water molecules are disorganized and in continuous motion,
so that each molecule forms hydrogen bonds with an average of only
3.4 other molecules.
In ice, on the other hand, each water molecule is fixed in space and
forms hydrogen bonds with a full complement of four other water
molecules to yield a regular lattice structure (Fig. 2–2).
Breaking a sufficient proportion of hydrogen bonds to destabilize the
crystal lattice of ice requires much thermal energy, which accounts for
the relatively high melting point of water (Table 2–1).
Cont..

12. Properties of water
These chemical features explain some of the interesting properties of
water
The ability of ice to float
The melting and freezing temperatures of water
The ability of water to store heat
The ability of water droplets to form

13. ICE FLOATS
Individual water molecules are held in place by hydrogen bonds,
creating a rigid, crystalline structure in which each water molecule is
hydrogen-bonded to four other water molecules.
Although the molecules are held firmly in place, they are not as tightly
packed as they are in liquid water.
In other words, solid water is less dense than liquid water, which is why
ice floats in water.

14. If ice were to sink in water, as almost all other solids do in their
corresponding liquids, ponds and lakes would freeze from the bottom up,
becoming solid blocks of ice in winter and killing most of the organisms
living in them.
Once the whole pond had frozen, its temperature could drop well below
the freezing point of water.
But, because ice floats, it forms a protective insulating layer on the top
of the pond, reducing heat flow to the cold air above.
Thus fish, plants and other organisms in the pond are not subjected to
temperatures lower than 0°C, the freezing point of pure water.
Cont..

16. MELTING AND FREEZING
Compared with other nonmetallic substances of the same size, molecular
ice requires a great deal of heat energy to melt.
Melting 1 mole (6.02 × 1023 molecules) of water requires the addition
of 5.9 kJ of energy.
This value is high because hydrogen bonds must be broken in order for
water to change from solid to liquid.
In the freezing a great deal of energy is lost when water is transformed
from liquid to solid.

17. EVAPORATION AND COOLING
Water has a high heat of vaporization, which means that a lot of heat
is required to change water from its liquid to its gaseous state.
Once again, much of the heat energy is used to break hydrogen bonds.
This heat must be absorbed from the environment in contact with the
water.
Evaporation thus has a cooling effect on the environment whether a
leaf, a forest, or an entire land mass.
This effect explains why sweating cools the human body: as sweat
evaporates off the skin, it uses up some of the adjacent body heat

18. COHESION AND
SURFACE TENSION
In liquid water, individual water molecules are free to move about.
The hydrogen bonds between the molecules continually form and break. In other
words, liquid water has a dynamic structure.
On average, every water molecule forms 3.4 hydrogen bonds with other water
molecules. This number represents fewer bonds than exist in ice, but it is still a
high number. These hydrogen bonds explain the cohesive strength of liquid
water.
This cohesive strength permits narrow columns of water to stretch from the roots
to the leaves of trees more than 100 meters high.
When water evaporates from the leaves, the entire column moves upward in
response to the pull of the molecules at the top.

19.  Water also has a high surface tension,
which means that the surface of liquid
water exposed to the air is difficult to
puncture.
 The water molecules in this surface
layer are hydrogen-bonded to other
water molecules below.
 The surface tension of water permits a
container to be filled slightly above its
rim without overflowing, and it permits
small animals to walk on the surface of
water
Cont..