Properties of Water
- Some extraordinary properties of water
- The cohesiveness of Liquid Water
- Cohesion = Phenomenon of a substance being held
together by hydrogen bonds.
- Though hydrogen bonds are transient, enough
water molecules are hydrogen bonded at any given time to
give water more structure than other liquids.
- Contributes to upward water transport in plants.
- Surface tension = Measure of how difficult it is to
stretch or break the surface of a liquid.
- Water has a greater surface tension than
- Function of the fact that at the air/water interface,
surface water molecules are hydrogen bonded to each other
and to the water molecules below.
- Causes water to bead (shape with smallest area to
volume ratio and allows maximum hydrogen bonding).
- Adhesion = Clinging of water to hydrophilic substances
- Hydrophilic = (Hydro=water; philo=loving) Property of
having a affinity for water.
- Imbibition = Process of water soaking into a porous
hydrophilic substance (e.g. a sponge).
- Imbibition by seeds, ruptures seed coat and
- Water's High Specific Heat
- Heat and Temperature
- Kinetic energy = The energy of motion.
- Heat = Total kinetic energy due to molecular motion
in a body of matter.
- Calorie (cal) = Amount of heat it takes to raise the
temperature of one gram of water by one degree
- Kilocalorie (kcal or Cal) = Amount of heat required
to raise the temperature of one kilogram of water by one
degree Celsius (1000 cal).
- Temperature = Measure of heat intensity due to the
average kinetic energy of molecules in the body of
- The Celsius Thermometer: (Figure 3.6) The
Celsius scale at sea level:
- 100 degrees C (212 degrees F) = water
- 37 degrees C (98.6 degrees F) = human body
- 23 degrees C (72 degrees F) = room
- 0 degrees C (32 degrees F) = water freezes
- To convert temperature scales:
- degrees C = 5 (degrees F - 32) / 9
- degrees F = 9 degrees C / 5 + 32
- degrees K = degrees C + 273
- Specific heat = Amount of heat that must be absorbed
or lost for one gram of a substance to change its
temperature by one degree Celsius.
- Specific heat of water = One calorie per gram per
degree Celsius (1 cal/g/degree C).
- How Water Stabilizes Temperature
- Water has a high specific heat, which means
that it resists temperature changes when it absorbs or
- As a result of hydrogen bonding among
water molecules, it takes a relatively large heat loss
or gain for each 1 degree C change in
- Hydrogen bonds must absorb heat to break, and they
release heat when they form.
- Much absorbed heat energy is used to disrupt
hydrogen bonds before water molecules can move faster
- A large body of water can act as a heat sink --
absorbing heat from sunlight during the day and summer and
releasing heat during the night and winter as the water
gradually cools. As a result:
- Water, which covers three-fourths of the
planet, keeps temperature fluctuations within a range
suitable for life.
- Coastal areas have milder climates than inland.
- The marine environment has a relatively stable
- Water's High Heat of Vaporization
- Vaporization (evaporation) = Transformation
from liquid to gas.
- Molecules with enough kinetic energy to
overcome the mutual attraction of molecules in a liquid,
can escape into the air.
- Heat of vaporization = Quantity of heat a liquid must
absorb for 1 g to be converted to the gaseous state.
- For water molecules to evaporate, hydrogen
bonds must be broken which requires heat energy.
- Water has a relatively high heat of vaporization (540
cal/g). (See Campbell, Table 3.2)
- Evaporative cooling = Cooling of a liquid's surface when
a liquid evaporates.
- The surface molecules with the highest
kinetic energy are most likely to escape into gaseous
form; the average kinetic energy of the remaining surface
molecules is thus lower.
- Water's high heat of vaporization:
- Moderates the earth's climate.
- Solar heat absorbed by tropical seas
dissipates when surface water evaporates (evaporative
- As most tropical air moves poleward, water vapor
releases heat as it condenses into rain.
- Stabilizes temperature in aquatic ecosystems
- Helps organisms from overheating by evaporative
- Freezing and Expansion of Water
- Water is densest at 4 degrees C.
- As water cools from 4 degrees C to freezing
(0 degrees C), it expands and becomes less dense than
liquid water (ice floats).
- When water begins to freeze, the molecules do not
have enough kinetic energy to break hydrogen bonds.
- As the crystalline lattice forms, each water molecule
forms a maximum of 4 hydrogen bonds, which keeps water
molecules further apart than they would be in the liquid
- Expansion of water contributes to the fitness of the
environment for life:
- Prevents deep bodies of water from freezing
solid from the bottom up.
- Since ice is less dense, it forms on the surface
first. As water freezes it releases heat to the water
below and insulates it.
- Makes the transitions between seasons less abrupt. As
water freezes, hydrogen bonds form releasing heat. As ice
melts, hydrogen bonds break absorbing heat.
- Water as a Versatile Solvent
- Solution = A liquid that is a homogenous
mixture of two or more substances.
- Solvent = Dissolving agent of a solution.
- Solute = Substance dissolved in a solution.
- Aqueous solution = Solution in which water is the
- Water is a versatile solvent owing to the polarity of
the water molecule.
- Ionic compounds dissolve in water.
- Charged regions of polar water molecules
have an electrical attraction to charged ions.
- Water surrounds individual ions, separating and
shielding them from one another.
- Polar compounds in general, are water-soluble.
- Charged regions of polar water molecules
have an affinity for opposite charged regions of other
- Nonpolar compounds (which have symmetric distribution
in charge) are NOT water-soluble.