Biological importance of water | Water unique properties | Biology and chemistry lecture

Life on Earth began in water and evolved there for 3 billion years before spreading onto land.
Studied on its own, the water molecule is deceptively
simple. It is shaped like a wide V, with its two hydrogen
atoms joined to the oxygen atom by single covalent bonds.
Oxygen is more electronegative than hydrogen, so the electrons
of the covalent bonds spend more time closer to oxygen
than to hydrogen; these are polar covalent bonds. This unequal sharing of electrons and water’s
V-like shape make it a polar molecule, meaning that its
overall charge is unevenly distributed. In water, the oxygen
region of the molecule has a partial negative charge (δ-),
and each hydrogen has a partial positive charge (δ+).
The properties of water arise from attractions between
oppositely charged atoms of different water molecules: The
slightly positive hydrogen of one molecule is attracted to the
slightly negative oxygen of a nearby molecule. The two molecules
are thus held together by a hydrogen bond.
Cohesion of Water Molecules
Water molecules stay close to each other as a result of hydrogen
bonding. Although the arrangement of molecules
in a sample of liquid water is constantly changing, at any
given moment many of the molecules are linked by multiple
hydrogen bonds. These linkages make water more structured
than most other liquids. Collectively, the hydrogen
bonds hold the substance together, a phenomenon called
cohesion.
Cohesion due to hydrogen bonding contributes to the
transport of water and dissolved nutrients against gravity
in plants. Water from the roots reaches the leaves through
a network of water-conducting cells.
Water’s High Specific Heat
The ability of water to stabilize temperature stems from its
relatively high specific heat. The specific heat of a substance
is defined as the amount of heat that must be absorbed or lost
for 1 g of that substance to change its temperature by 1°C. Because of the high specific heat of water relative to other
materials, water will change its temperature less than other
liquids when it absorbs or loses a given amount of heat.
Heat of vaporization is the quantity of heat a liquid must
absorb for 1 g of it to be converted from the liquid to the
gaseous state. For the same reason that water has a high specific
heat, it also has a high heat of vaporization relative to
most other liquids. Water’s high heat
of vaporization is another emergent property resulting from
the strength of its hydrogen bonds, which must be broken
before the molecules can exit from the liquid in the form of
water vapor. As a liquid evaporates, the surface of the liquid that remains
behind cools down (its temperature decreases).
Water is one of the few substances that are less dense as a
solid than as a liquid. In other words, ice floats on liquid
water. when a deep body of water cools, the floating
ice insulates the liquid water below, preventing it from
freezing and allowing life to exist under the frozen surface,
Water is a very versatile solvent, a quality we can trace to
the polarity of the water molecule. Suppose, for example, that
a spoonful of table salt, the ionic compound sodium chloride
(NaCl), is placed in water.

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