Water is essential for life and has unique properties that make it suitable for life. It exists in nature as a liquid, solid, and gas and has an anomalous property of becoming less dense when frozen. Water's polarity and ability to form hydrogen bonds give it high surface tension, heat capacity, and ability to dissolve many substances, making it a universal solvent. These properties allow water to moderate climate, support biological processes in cells, and drive the water cycle which is crucial for life on Earth.

1. THE WATER AND ITS PROPERTIES
Harjinder Singh
Associate Professor
Dept. of Botany, Meerut
College, Meerut.
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2. Faculty: Science
Department: Botany
Name: Harjinder Singh
Name of degree programme: M Sc. 3rd Sem.
Course: IX: Plants -Soil - Water Relations; Growth
and Development
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3. INTRODUCTION
 Water is Transparent, colourless,odourless and tasteless biological substance.
 Water is the medium of life of Earth. Life on Earth began in water It is a major component of all living things.
 Water is one of the more abundant molecules in living cells and the most critical to life.
 Because of its unique properties it is essential for life. Cells are composed of 70 to 95 % of water.
 water is transparent, and thus aquatic plants can live within the water because sunlight can reach them.
Only strong UV light is slightly absorbed.
 World Water Day is held on 22 of March every year since 1993 to focus on importance of water and climate
change.
 Water is the most abundant compound on Earth’s surface. In nature, water exists in the liquid, solid, and gaseous
states. It is in dynamic equilibrium between the liquid and gas states at 0 °C and 1 atm of pressure.
 At room temperature (approximately 25°C ), it is a tasteless, odorless, and colorless liquid.
 Water is transparent in the visible part of the electromagnetic spectrum.
 Water can act as either an acid or a base.
 Water is a universal solvent, dissolving many substances found in nature.
 Its liquid phase is the most common phase on Earth.
 It covers 2/3 area of Earth and every living organism depends on it.
 It is made of two hydrogen atoms bond to one oxygen atom with a total atomic weight of 18 daltons.
 Most of the water found on this planet is held within the oceans ( 97.25%). The use of this sink of water by humans
is limited because of the dissolved salts it contains.
 Icecaps and glaciers contain about 2 % of the world's total water, and about 60 % of the freshwater supply. The use
of this water by humans is very restricted because of its form and location.
 Humans primarily use the freshwater found in groundwater, lakes, rivers, soil, and the atmosphere. This water
makes up less than 1 % of the Earth's supply.
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4. MOLECULAR CHARACTERISTIC OF WATER
The properties of water can best be understood by considering the structure and bonding of the water
molecule.
A Water molecule made up of two hydrogen atoms and one oxygen atom. Oxygen and hydrogen atoms
combine (H-O-H) together by single covalent bonds at 104.5 degree forming a V –shape.
 Though water molecules are electrically neutral, but the large oxygen atom holds a small negative charge
while the two small hydrogen atoms hold small positive charges.
 Oxygen attracts electrons much more strongly than does hydrogen, resulting in a partial positive charge on the
hydrogen atoms and a partial negative charge on the oxygen atom.. Because of the unequal sharing electrons (e-)
of polar covalent bonds creates two electric dipoles ( An object with such a charge difference is called a
dipole meaning “two poles”)in water molecules . It makes water dipolar and asymmetrical molecule with
an ability to make strong electrostatic interaction with itself , other molecules and ions.
Most of the physical and chemical properties of water is due to its polarity.
Water can b described as an amphoteric molecule.
 Water molecules are attracted to each other (Cohesiveness), forming hydrogen bonds ( intermolecular
interaction) that makes water a sticky molecule.
 Asymmetrical shape of water molecules gives it a variety of bonding formation.
 When a substance readily forms hydrogen bonds with water, it can dissolve in water and is referred to as
hydrophilic (e.g. NaCL)
 When a substance nonpolar and does not forms hydrogen bond with water ,is called hydrophobic ( e.g. oils, fats)
 Hydrogen bond is a weak bond but are stronger in greater numbers
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5. PROPERTIES OF WATER
 Polarity and Hydrogen bonds
 High Specific heat
 Surface tension
 Cohesion
 Adhesion
 Capillary Action
 Universal Solvent
 Evaporation, Condensation and Precipitation
 Density
 Polyphasic nature
 Osmosis and Diffusion
 Electrical Conductivity
 Neutral pH
 Amphoteric Nature and Redox Reactions
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6. KEY PHYSICAL PROPERTIES OF WATER
Property Value
Molar mass 18.015
Molar Volume 55.5 moles/lire
Boiling Point (BP) 100°C at 1 atm
Freezing point (FP) 0°C at 1 atm
Triple point 273.16 K at 4.6 torr
Surface Tension 73 dynes at 20°C
Vapour pressure 0.0212 atm at 20°C
∆H of vaporization 40.63 kJ/mol
Dielectric Constant (ε) 78.54 at 25°C
Viscosity 1.002 centipoise at 20°C
Density 1 g/cc
Density maxima 4°C
Heat Capacity (cp) 4.22 kJ/kg.k
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7. POLARITY and HYDROGEN BONDS
 Polarity: Water is a polar molecule( A molecule that has
one side is more negative than the other). Due to polarity
water molecules held together by weak electrical
interaction called hydrogen bonds.
 Hydrogen bonding in liquid water allows water to self-
associate, which significantly changes its behavior.
 Due to strong hydrogen bonds between water molecules it
has high melting and boiling point, high latent heat of
condensation and crystallization, and low vapor pressure
 Being a polar molecule and hydrogen bonds of water are
the main reason of its’ nature of a life supporting
substance.
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8. CHEMICAL STRUCTURE OF A WATER MOLECULE
The delta ( δ ) symbol indicates
slightly positive/negative on the diagram above
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9. CHEMICAL STRUCTURES OF WATER MOLECULES
A. Hydrogen bonds between water
molecules. Water attracts other Polar
molecules(e.g. sugars, NaCL)forming
hydrogen bonds
B. Tetrahedral Lattice structure of ice where
each water molecule makes H- bonds with fur
other water molecules. it makes less dense
than the freely flowing molecules of
liquid water. Ice’s lower density enables
it to float on water 9H SINGH

10. HIGH SPECIFIC HEAT
 Specific Heat is the amount of heat that must be absorbed or lost for one gram of a substance to
change its temperature by 1°C.
 Water has a high specific heat.
 Because so much heat loss or heat input is required to lower or raise the temperature of water,
the oceans and other large bodies of water have relatively constant temperatures In winter and
summer.
 The high water content of plants and animals living on land helps them to maintain a relatively
constant internal temperature and prevent organism from overheating.
 Water has high heat (540 to 580 ca/gm.) of vaporization. This leaves a cooling effect on the
water evaporating surfaces of soils ,plants and other bodies.
 The energy require to convert liquid in to gas (vapour) at constant temp. is called latent heat
of vaporization.
 High heat of vaporization of water enables the plant to cool by transpiration.
 When hydrogen bonds are formed(Freezing) between water molecules energy(heat) is
released.
 To break hydrogen bonds energy (heat) is required.
 Water absorbs heat from warmer air and release stored heat in cooler air, thus helps in moderating the
Earth’s temperature.
Because of high boiling point, it exists predominantly in its liquid form in the range of
environments where life flourishes, ice and vapour also play an essential role in shaping the
environment on Earth.
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11. SURFACE TENSION
 Surface tension is the measure of the force required to
break the surface of any liquid.
 Surface tension of water results due to the attraction
between the water molecules at open boundary surface
of water.
 Water has greater surface tension than many other liquid
due to hydrogen bonds between water molecules.
 Surface of water can behave as an elastic sheet due to
the cohesion between water molecules.
 Surface tension is responsible for transpiration pull ,
which helps in ascent of sap in plants
 The high surface tension of water is relevant, First,
below a length scale of about 1 mm surface tension
forces dominate gravitational and viscous forces, and
the air–water interface becomes an effectively
impenetrable barrier. It helps life style of small
insects, bacteria and other microorganisms in the
environment.
 Secondly, at the molecular (0.1–100 nm) scale the
surface tension plays a key role in water’s solvent
properties. The high dielectric constant of water is
important in its action as a solvent
Rain drop are spherical because of surface tension
Needle floats
On water surface
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12. COHESION and ADHESION
 Cohesion is the ability of water molecules to cohere or stick to each other by
hydrogen bond.
 Cohesion holds water column together in capillary sized xylem elements
 Cohesion is vital for transpiration in plants. Due to cohesion water moves up as
a continuous column in the xylem . It has great advantage as it is easier to
draw up a column rather than individual molecules without energy input.
 Cohesion also helps in maintaining surface tension.
 Cohesion also related to the other property of water : Adhesion.
 Adhesion is the attraction of Water molecules to the molecules of other
substance due to its polar nature.
 Water molecules holds firmly , rich in O and N e.g. cellulose, protiens, soil
colloids, and glass etc.
 In biological cells and organelles , water is in contact with membrane and
protein surfaces that are hydrophilic, so that have a strong attraction to water.
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13. Significance of cohesion in plants
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14. Adhesion of water with Xylem
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15. CAPILLARY ACTION
 Capillarity is defined as the rise or fall of a
liquid in a narrow tube.
 This action is very important. Water from the
ground gets distributed to all part of the plant
due to capillary action.
 Capillary action is primarily as a result of
surface tension.
 Capillary action occurs when the adhesion to
the walls is stronger than the cohesive forces
between the liquid molecules.
 Plants absorbs water using capillary
action(.Fig.1)
 In Narrow glass tube water pull upward up to
certain distance due to capillary action (Fig.2)
 The height to which capillary action will take
water in a uniform circular tube is limited by
surface tension and, gravity.
 Adhesion of water to the walls of a vessel will cause an
upward force on the liquid at the edges and result in
a meniscus ( A meniscus is a curve in the surface of
water) which turns upward
1 Capillary action in plants
2. Water moves upward in narrow tube15H SINGH

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17. UNIVERSAL SOLVENT
 Water is an excellent solvent due to its high dielectric constant and
considered as universal solvent.
 Substances that mix well and dissolve in water are known
as hydrophilic substances e.g., salts, sugars, acids, alkalis, and some
gases – especially oxygen, carbon dioxide (carbonation) , while those
that do not mix well with water are known as hydrophobic
substances(e.g., fats and oils).
 Water readily dissolves charged biomolecules by replacing solute –
solute hydrogen bonds with solute –hydrogen bonds and weakening
electrostatic interaction between them.
 In general, ionic and polar substances such as acids, alcohols,
and salts are relatively soluble in water, and non-polar substances such
as fats and oils are not.
 When an amphipathic compound such as proteins , pigments,
phospholipids and vitamins etc. are mixed with water, their
hydrophilic regions interact favorably with water but hydrophobic
ends tend to stay away.
 Water can oxidize, dissolve,, melt other substance than Sulphuric acid.
 It is able to dissolve a large number of different chemical compounds
by hydrolysis because of its' dielectric constant.
 All the major components in cells (proteins, DNA and
polysaccharides) are also dissolved in water.
 all the molecular processes essential to life – chemical reactions,
association and binding of molecules, diffusion-driven
encounters, ion conduction – will only take place at significant
rates in solution, hence the importance of water’s solvent
properties
 This feature enables water to carry solvent nutrients
in runoff, infiltration, ground water flow, and living organisms. 17H SINGH
NaCL dissociate in water

18. EVAPORATION, CONDENSATION AND PRCIPITATION
 Evaporation, condensation and precipitation
are the significant processes of water cycle,
which is essential for the very existence of life
on Earth
 Evaporation is transformation of water into
vapour at increased temp.
 Condensation is the opposite to evaporation
where, de - energized vapour molecules stick
to other substance/surface or condense into
liquid form. Water vapour in the atmosphere
form clouds after condensation.
 During day at increased temperature water
evaporate and number of vapour molecules
increased in the air, while during night cooling
temp. de- energized vapour molecules start
condensing into dew drops.
 precipitation is the process of fall of any
product of the condensation of atmospheric
water vapor in the form of drizzle, rain, sleet,
snow, ice pellets, and hail falls under gravity
from cloud.
 Precipitation is the primary source of
freshwater for rivers, lakes, groundwater, and
glaciers on the Earth's terrestrial surface
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19. DENSITY OF WATER
 The density of water is about 1 gram per cubic centimetre (62 lb/cu ft):
 Water's liquid state is much denser than its solid state, causing ice to float on top of water
 The maximum density of water occurs at 3.98 °C . It has the anomalous property of becoming less dense, when it is
cooled down to its solid form, ice
 When water reaches 0 °C , water becomes locked into a crystalline lattice with each molecule bonded to the
maximum of four partners. – Ice is about 10% less dense than water at 4 °C.
 It expands from 4°C to 0°C. It expands to occupy 9 percent greater volume in this solid state, which accounts for the
fact of ice floating on liquid water.
 The density varies with temperature. As the temperature increases, the density rises to a peak at 3.98 °C and then
decreases.
 above 4 °C water expands as the temperature increases.
 The density of salt water depends on the dissolved salt content as well as the temperature.
 Expansion of water upon cooling below 4 °C and upon freezing has biological significance .
 It prevents the lakes from totally freezing so aquatic life can survive.
 It also act as hunting ground for some organisms.
 Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation.
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20. POLYPHASIC NATURE
 In nature, water exists in the liquid, solid, and gaseous states.
 Its liquid phase is the most common phase of water and most significant
for a living system
 As a result of the nature of its hexagonal packing within its crystalline
structure, water’s solid form (ice) is less dense than its liquid form. Water is
primarily a liquid under standard conditions (25°C and 1 atm of pressure).
 It is in dynamic equilibrium between the liquid and gas states at 0 °C and 1
atm. of pressure.
 Water also exists in a rare fourth state called supercritical fluid, which
occurs only in extremely uninhabitable conditions. When water achieves a
specific critical temperature and a specific critical pressure (647 K and
22.064 M Pa), the liquid and gas phases merge into one homogeneous fluid
phase that shares properties of both gas and liquid.
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21. States of Water
Solid (Ice), Hexagonal crystal
Liquid (Water)
Gaseous(Vapour)
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22. Interconversion of states of water
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23. OSMOSIS AND DIFFUSION
 Osmosis is the net movement of water across a selectively
permeable membrane driven by a difference in solute
concentrations on the two sides of the membrane.
 „The water moves from the area of higher solute
concentration to the area of lower solute concentration
until equilibrium is reached.
 The tendency of substances to move from areas of high
concentration to areas of low concentration.
 „At equilibrium, they remain as far apart as possible
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24. ELECTRICAL CONDUCTIVITY
 Conductivity is the ability of a substance to convey an electrical
current, and electricity is conducted on unbound (free) electrons that
move about in a substance.
 Pure water contains only small concentrations of hydrogen and
hydroxyl ions resulting from its weak dissociation thus it is a poor
conductor.
 Pure water containing no exogenous ions is an excellent insulator.
 Natural waters, however, contain greater concentrations of dissolved
ions than pure water and are therefore better conductors.
 Because water is such a good solvent, it almost always has
some solute dissolved in it, often a salt. If water has even a tiny amount
of such an impurity, then the ions can carry charges back and forth,
allowing the water to conduct electricity far more readily.
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25. NEUTRAL pH AND DISSOCIATION OF WATER MOLECULES
 Water has a pH around 7.
 It has great biological significant as most of the
biological activities occur at neutral pH.
 Water changes its pH when substances are
dissolved in it. Rain has a naturally acidic pH of
about 5.6 because it contains natural derived
carbon dioxide and sulfur dioxide.
 Dissolved CO2 in water acts as buffer.
 Water has a very strong hydrating tendency due
to its dielectric constant. It dissolves many ionic
compounds. Some covalent and ionic
compounds can be hydrolyzed in water.
 Dissociation of water molecules occurs.
Occasionally a hydrogen atom is shared by two
molecules shifts from one molecules to other.
 The hydrogen atom leaves behind its electron and is
transferred as a single proton or hydrogen ion.
 The water molecule with an extra proton is called
hydronium ion, while the molecule that has lost a
proton is called as hydroxide ion.
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26. AMPHOTERIC NATURE AND REDOX REACTIONS
 Amphoteric nature:
 Water can act as both acid and base, i.e. it is amphoteric in nature.
E.g.
Acidic behavior:
 Basic behavior:
 Redox reactions: water can be oxidized and reduced, which is very
useful in redox reactions.
 Electropositive elements reduce water to hydrogen molecule. Thus
water is a great source of hydrogen.
E.g.2H2O(l)+2Na(s)→2NaOH(aq)+H2(g)
 During the process of photosynthesis, water is oxidized to O2.
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27. HYDROLOGICAL CYCLE
 The hydrologic cycle is a conceptual model that
describes the storage and movement of water between
the biosphere, atmosphere, lithosphere, and
the hydrosphere.
 Water on this planet can be stored in any one of the
following
reservoirs: atmosphere, oceans, lakes, rivers, soils,
glaciers, snowfields, and groundwater .
 Water moves from one reservoir to another by way of
processes
like evaporation, condensation, precipitation, dep
osition, runoff, infiltration, sublimation, transpir
ation, melting, and groundwater flow. The oceans
supply most of the evaporated water found in the
atmosphere. Of this evaporated water, only 91 % of it
is returned to the ocean basins by way of precipitation.
 Water is continually cycled between its various
reservoirs. through the processes
of evaporation, condensation, precipitation, depo
sition, runoff, infiltration, sublimation, transpira
tion, melting, and groundwater flow.
Hydrological cycle
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