Role of water
Water is the basis of life
- Life on earth probably evolved in water
- Living cells are 70-95% H O
- Water covers about ¾ of the earth
- In nature, water exists in all three physical states of matter- solid, liquid and gas.
- It can be quite correctly argued that life exists on earth because of the abundant liquid water
- Other plants have water, but it is either gas (Venus) or water (Mars)
- Recent studies of Mars reveal the presence sometime in the past of running fluid, possibly
Polarity of water
- Water is a polar molecule. Its polar bonds and asymmetrical shape give water molecules
opposite charges on opposite sides
- 4 valance orbitals of O point to corners of a tetrahedron
- Two corners are orbitals with unshared pairs of electrons and weak negative charge
- Two corners are occupied by H atoms which are in polar covalent bonds with O.
- Oxygen is also electronegative, that shared electrons spend more time around O causing a weak
positive charge near H’s.
- Hydrogen bonding orders water into a higher level of structural organization.
- The polar molecules of water are held together by hydrogen bonds.
- Positively charged H of one molecule is attracted to the negatively charged O of another
- Each water molecule can form a maximum of 4 hydrogen bonds with neighboring water
- Water has extraordinary properties that emerge as a consequence of its polarity and hydrogen-
bonding. Come of these properties are that water:
o Has cohesive behavior
o Resists changes in temperature
o Has a high heat of vaporization and cools surfaces as it evaporates
o Expands when it freezes
o Is a versatile solvent
- Water is a polar molecule (having both a positive and negative side)
- Consequently, water has a great connectivity of individual molecules, which is caused by the
individually weak hydrogen bonds that can be quite strong when taken by the billons Cohesion of water molecule
- Collectively, the hydrogen bonds hold the substance together- a phenomena called cohesion.
- Cohesion contributes to the transport of water against gravity in plants. Adhesion of water to
the walls of the vessels helps counter the downward pull of gravity.
- Water has a great surface tension. At interface between water and air, hydrogen is bounded to
one another and to the water below.
Water moderates temperatures on Earth
- Water absorbs heat from the warmer air and releases the stored hear to the cooler air.
- Everything that moves has kinetic energy. The faster a molecule moves, the greater E . k
- Heat is a measure of the total quantity of E due to molecular motion in a body of matter.
- Temperature measures the intensity of heat due to average E of the mklecules.
- When the 2 objects are brought together, molecules in the cooler object speed up at the
expense of the E ok the warmer one.
- A calorie is the amount of heat energy required for raising the temperature of 1g of water by 1
- Conversely, one calorie is the amount of heat given by 1 g of water releases when it cools down
by one degree Celsius.
Water’s high specific heat
- Water has a high specific heat, which means it resists temperature changes when it absorbs or
- The specific heat of a substance is defined as the amount go heat that must be absorbed or lost
for 1 g of that substance to change its temperature by 1 degree Celsius.
- The specific heat of water is 1 calorie per gram per degree C, abbreviated as 1 cal/g/C
- It is unusually high when compared to other substances, (0.6 for alcohol).
- As a result of hydrogen bonding water molecules, it takes a relatively large heat loss or gain for
each 1 degree change in temperature
- Hydrogen bonds must absorb heat to break, and they release heat when they form
- Much of the absorbed heat energy is used to disrupt hydrogen bond s before water molecules
can move faster (increase temperature)
- A large body of water can act as a heat sink, absorbing heat from sunlight during the day and the
summer (while warming only a few degrees) and releasing heat when the night and winter as
the water gradually cools.
- As a result
o Water, which covers ¾ of the planet keeps temperature fluctuations within a range
suitable for life
o Coastal areas have milder climates then inland
o The marine environment has a relatively stable temperature Evaporative cooling
- The transformation from the liquid to gas is called vaporization
o Molecules with enough E tokovercome the mutual attraction of molecules in a liquid;
can escape into the air
- Heat of vaporization is the quantity of heat a liquid must absorb for 1g of it to be converted to
the gaseous state
o For water molecules to evaporate, hydrogen bonds must be broke which requires
- Water has a high heat of vaporization- for each gram of water 580 cal are needed.
- With liquid evaporation the surface of the