properties of water

1. Physical Properties of Water
Water is a liquid at physiological temperatures
•High boiling and melting point compared to other similar-sized
molecules e.g.NH4, CO2, H2S (gases at room temperature).
•The property is important for life process occurring between approx.
0 and 1000c.
Temperatures below 00c are too cold to permit significant
metabolism, while temperatures above 1000c. Tend to disrupt the
bonds.
• Water in liquid state is taken up by plant roots easily.
Water has a high heat of vaporization
•A lot of energy (ca.44kJmol-1) is needed to convert water from a
liquid to gas (i.e. water resists evaporation).
•This property is important in evaporative cooling systems, e.g.
respiration (in animals), and transpiration (in plant leaves).

2. Cont…..
Water has a high specific heat capacity
•A lot of energy is required to break/make hydrogen bonds & raise
the temperature of water (4.184 J g-1 C-1, or 1 calorie)
•Thus, water resists temperature changes (i.e. slow to heat up & to
cool down), for biological metabolic processes to proceed.
• This property is important in water's role as a thermal buffer
•The phenomenon helps desert (succulent) plants to resist
temperature fluctuations.
Water has a high heat of fusion
•Water resists freezing, thus requires a lot of energy to convert water
from a solid to a liquid.
•Energy is required to break the collective hydrogen bonds holding
water in its solid configuration.

3. Cont………
•Conversely, a lot of energy (6 kJ mol-1) must be released by water to
freeze.
•This property is used by citrus growers–fruits are sprayed with water
that freezes to form ice, releasing the heat of fusion which helps
protect the crop from serious damage.
Water has a high surface tension
•Water molecules have strong cohesive forces operating among
them, thus a lot of energy is required to break through the surface
ofwater.
•Water molecules at the surface are attracted by cohesion to others
within the liquid much more than they are to air narrowing surface of
the liquid.
• Thus, water acts as though it has a skin, a phenomena known as
surface tension

4. Cont………
• This phenomenon is important at air/water interfaces & explains
why:
i. Water rises up xylem vessels or thin columns (i.e. capillary
action);
ii. Rain drops are round (i.e. the molecules at the surface attract one
another);
iii. Water striders and other bugs can "walk on water"; and
iv. A meniscus forms in water columns
The density of water decreases on crystallization
•This occurs because when ice forms, each water molecule is
hydrogen bonded to exactly four others (i.e. water molecule is
attracted to slightly more than four others).
Thus, as water cools it gets denser and denser until it reaches 40C,
then; it gets less dense
as it cools to 00c and ice floats.

5. Cont….
• Importance of this property are;
- When ice floats, there is less damage to aquatic life
-Ice insulates the water body, thus the upper surface of the water
body is cold while the down part is warm, this allows physiological
reactions/metabolism which are enzyme controlled to proceed.
Water has high tensile strength and is incompressible
• This is the ability of water to resist breaking when stretched
•Water is good for hydraulic systems because when squeezed it
doesn't compress & produces positive pressures (hydrostatic
pressures).
•This pressure provides the driving force for cell growth and other
plant movements. The pressure is measured in units of Pascal

6. Cont……..
Water is colourless (transparent to light)
•This is important for ecological purpose, since the transmission of
visible light in aquatic habitats is important for photosynthesis
•Determines the distribution of aquatic plants (i.e. depth of light
penetration
Isotopic composition of water
• Two types, natural (1H2 &
18O) & heavy water (2H2 & 16O),the later is
used in trace experiments to follow up metabolic pathways inplants

7. Functions of Water
o Water is a major component of cells
o A solvent for the uptake and transport of materials
o A good medium for biochemical reactions
o A reactant in many biochemical reactions e.g photosynthesis
o Provides structural support via turgor pressure e.g in leaves
oThe medium for the transfer of plant gametes (sperms swim to eggs
in water, some aquatic plants shed pollen underwater)
o Seed dispersal (e.g. coconut)
o Plant movements are the result of water moving into and out of
those parts(i.e. diurnal movements, stomata opening, flower
opening)
o Cell elongation and growth
o Thermal buffer

8. Plant cells-Water relation
Water is the main constituent of all living plant cells. However, water
content of plant cells varies depending on the type & physiological
status of the cells e.g
•Most plants are comprised of atleast 70% water, e.g. lettuce leaves
constitute about 95% water
•However, some dormant plant organs e.g. seeds & buds have less
than 10% water
Water is therefore essential for chemical transformation in plants and
is the limiting resource for crop productivity in most agricultural
systems.
•The water status of plant cells generally fluctuate with time of the
day and climatic conditions.
•Water of most plant cells is normally low during hot days and is high
during night or cold days

9. Cont….
•During the day evaporation is high and water content is lost and this
causes changes in cell volume. In young tissues the changes in cell
volume may range between 20-40%