Polar Bonds of Water
The molecule H2O (water) has a funny shape, the two hydrogen atoms form a "V"
shape, rather than a straight line.
The electrons spend more time orbiting the oxygen nucleus than orbiting the two
hydrogen nuclei, leaving the oxygen more negatively charged and the hydrogen
more positively charge. Therefore, the entire molecule is "Polar", that is with one
end slightly positive and one end slightly negative.
Water and its Properties
Water is one of the most remarkable compounds in the universe. It has several
unique physical and chemical properties, all based on its "bent", "polar",
structure.
Water has an unusually high freezing point (OoC) and a high boiling point (100oC)
in comparison to other small molecules, such as CO2. Since the temperatures on
the earth's surface commonly fall between waters melting and boiling point (O C
and 100 C), water exists as a liquid most of the time.
The thermal properties of water have important biological implications. Two
particularly significant characteristics are a high "heat of vaporization", the
amount of heat needed to turn a given amount of liquid water into water vapor
(gas). Water also has a high "specific heat", the amount of heat needed to raise
the temperature of 1 gm of water by 1 C. (Waters heat of vaporization is 580
calories per gram, and its specific heat is 1 calorie per gram per degree
centigrade).
A high heat vaporization means that in order for water to reach the gaseous
state, it must absorb a great deal of heat from the surroundings. For many plants
and animals, this property is the basis of a natural cooling system. Water
evaporating from leaves, skin or lungs uses up heat from the organisms in the
process of changing from liquid to gas. That is the reason mammals have
evolved sweat glands, when the body is overheated, the glands pour watery
"sweat" onto the skin, as the water evaporated, large amounts of body heat are
used up and the body is cooled.
Due to it high specific heat, water is slow to undergo changes in temperature.
Much heat must be added or removed before the temperature of water changes
much. The temperature of the water within living organisms tends to change
more slowly than does that of the surrounding air or soil, so that the living cells
are buffered somewhat against temperature fluctuations. This kind of protection
is important because many biochemical reactions will take place only within a
narrow range of temperatures.
Water has several other properties that make it important to living things. Water
molecules exhibit strong "cohesion" ie. the tendency of "like" molecules to stick
together (water to water). This is the result
of the polar nature of water molecules and
the resulting hydrogen bonding.
Cohesion
→
Water shows strong "adhesion" ie.
tendency of "unlike" molecules to cling
together (such as water and the molecules if
silicon dioxide on the walls of a drinking
glass).