WATER – physical & chemical properties
V. MAGENDIRA MANI
ASSISTANT PROFESSOR
ISLAMIAH COLLEGE (AUTONOMOUS)
VANIYAMBADI
[email protected]; [email protected]
Magendira mani vinayagam Academia.edu
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Water is a chemical substance with the chemical formula H2O. Its molecule contains
one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient
conditions, but it often co-exists on Earth three common states (solid state, ice, and
gaseous state, water vapor or steam) of matter and may take many different forms on Earth:
water vapor and clouds in the sky; seawater and icebergs in the polar oceans; glaciers and
rivers in the mountains; and the liquid in aquifers in the ground. Life is believed to have
originated in the aqueous solutions of the world’s oceans, and living organisms depend on
aqueous solutions, such as blood and digestive juices, for biological processes.
Water exhibits very complex chemical and physical properties. For example, its melting point,
0 °C (32 °F), and boiling point, 100 °C (212 °F),
The water molecule is not linear but bent in a special way. The two hydrogen atoms are bound to
the oxygen atom at an angle of 104.5°.
The O−H distance (bond length) is 95.7 picometres (9.57 × 10−11
metres, or 3.77 × 10−9
inches).
Because an oxygen atom has a greater electronegativity than a hydrogen atom, the O−H bonds
in the water molecule are polar, with the oxygen bearing a partial negative charge (δ−) and the
hydrogens having a partial positive charge (δ+).
Water is a special chemical substance consisting of two atoms of hydrogen and one atom of
oxygen. O--H bond length = 95.7 picometers, H--O---H angle = 104.5°, O-H bond energy =
450 kJ/mol, Dipole moment = 1.83 debyes
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The water on the surface of the Earth is found mainly in its oceans (97.25 percent) and polar ice
caps and glaciers (2.05 percent), with the balance in freshwater lakes, rivers, and groundwater.
Water covers 70.9% of the Earth's surface, and is vital for all known forms of life. On Earth, it is
found mostly in oceans and other large water bodies, with 1.6% of water below ground
in aquifers and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles
suspended in air), and precipitation. A very small amount of the Earth's water is contained
within biological bodies and manufactured products.
As the Earth’s population grows and the demand for fresh water increases, water
purification and recycling become increasingly important. Interestingly, the purity requirements
of water for industrial use often exceed those for human consumption. For example, the water
used in high-pressure boilers must be at least 99.999998 percent pure.
Because seawater contains large quantities of dissolved salts, it must be desalinated for most
uses, including human consumption.
Water on Earth moves continually through
a cycle of evaporation or transpiration (evapotranspiration), precipitation, and runoff, usually
reaching the sea. Over land, evaporation and transpiration contribute to the precipitation over
land.
Water occurs as a liquid on the surface of the Earth under normal conditions, which makes it
invaluable for transportation, for recreation, and as a habitat for a myriad of plants and animals.
The fact that water is readily changed to a vapor (gas) allows it to be transported through
the atmosphere from the oceans to inland areas where it condenses and, as rain, nourishes plant
and animal life.
Water plays an important role in the world economy, as it functions as a solvent for a wide
variety of chemical substances and facilitates industrial cooling and transportation.
Approximately 70% of freshwater is consumed by agriculture.
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The major physical and chemical properties of water are:
Selected physical properties of water
molar mass 18.0151 grams per mole
melting point 0.00 °C
boiling point 100.00 °C
maximum density (at 3.98 °C) 1.0000 grams per cubic centimetre
density (25 °C) 0.99701 grams per cubic
centimetre
vapour pressure (25 °C) 23.75 torr
heat of fusion (0 °C) 6.010 kilojoules per mole
heat of vaporization (100 °C) 40.65 kilojoules per mole
heat of formation (25 °C) –285.85 kilojoules per mole
entropy of vaporization (25 °C) 118.8 joules per °C mole
viscosity 0.8903 centipoise
surface tension (25 °C) 71.97 dynes per centimeter
Water is a liquid at standard temperature and pressure. It is tasteless and odorless. The
intrinsic color of water and ice is a very slight blue hue, although both appear colorless in
small quantities. Water vapor is essentially invisible as a gas.
Water is transparent in the visible electromagnetic spectrum. Thus aquatic plants can
live in water because sunlight can reach them. Ultra-violet and infrared light is
strongly absorbed.
Since the water molecule is not linear and the oxygen atom has higher electro
negativity than hydrogen atoms, it carries a slight negative charge, whereas the
hydrogen atoms are slightly positive. As a result, water is a polar molecule with
an electrical dipole moment.
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Water also can form an unusually large number of intermolecular hydrogen bonds (four)
for a molecule of its size. These factors lead to strong attractive forces between molecules
of water, giving rise to water's high surface tension and capillary forces.
The capillary action refers to the tendency of water to move up a narrow tube against the
force of gravity. This property is relied upon by all vascular plants, such as trees.
Water is a good solvent and is often referred to as the universal solvent. Substances that
dissolve in water, e.g., salts, sugars, acids, alkalis, and some gases – especially
oxygen, carbon dioxide (carbonation) are known as hydrophilic (water-loving)
substances, while those that do not mix well with water (e.g., fats and oils), are known
as hydrophobic (water-fearing) substances.
Hydrophylic Molecules
Substances that dissolve readily in water are termed hydrophilic. They are composed of
ions or polar molecules that attract water molecules through electrical charge effects.
Water molecules surround each ion or polar molecule on the surface of a solid substance
and carry it into solution. Ionic substances such as sodium chloride dissolve because
water molecules are attracted to the positive (Na+) or negative (Cl_) charge of each ion.
Polar substances such as urea dissolve because their molecules form hydrogen bonds with
the surrounding water molecules.
Hydrophobic Molecules
Molecules that contain a preponderance of nonpolar bonds are usually insoluble in water
and are termed 'hydrophobic'. This is true, especially, of hydrocarbons, which contain
many C-H bonds. Water molecules are not attracted to such molecules as much as they
are to other water molecules and so have little tendency to surround them and carry
them into solution.
All the major components in cells (proteins, DNA and polysaccharides) are also
dissolved in water. Pure water has a low electrical conductivity, but this increases
significantly with the dissolution of a small amount of ionic material such as sodium
chloride.
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Boiling Point: The boiling point of water (and all other liquids) is dependent on
the barometric pressure. For example, on the top of Mt. Everest water boils at 68 °C
(154 °F), compared to 100 °C (212 °F) at sea level. Conversely, water deep in the ocean
near geothermal vents can reach temperatures of hundreds of degrees and remain liquid.
Specific heat capacity: Water has the second highest molar specific heat capacity of any
known substance, after ammonia, as well as a high heat of vaporization(40.65 kJ·mol−1
),
both of which are a result of the extensive hydrogen bonding between its molecules.
These two unusual properties allow water to moderate Earth's climate by buffering large
fluctuations in temperature.
Density: The maximum density of water occurs at 3.98 °C (39.16 °F). It has the
anomalous property of becoming less dense, not more, when it is cooled down to its solid
form, ice. It expands to occupy 9% greater volume in this solid state, which accounts for
the fact of ice floating on liquid water.
Water is miscible with many liquids, such as ethanol, in all proportions, forming a
single homogeneous liquid. On the other hand, water and most oils are immiscible
usually forming layers according to increasing density from the top. As a gas, water
vapor is completely miscible with air.
Water forms an azeotrope (mixing of two or more liquids) with many other solvents.
Water can be split by electrolysis into hydrogen and oxygen.
As an oxide of hydrogen, water is formed when hydrogen or hydrogen-containing
compounds burn or react with oxygen or oxygen-containing compounds. Water is not
a fuel, it is an end-product of the combustion of hydrogen. The energy required splitting
water into hydrogen and oxygen by electrolysis or any other means is greater than the
energy that can be collected when the hydrogen and oxygen recombine.
Elements which are more electropositive than hydrogen such
as lithium, sodium, calcium, potassium and caesium displace hydrogen from water,
forming hydroxides. Being a flammable gas, the hydrogen given off is dangerous and the
reaction of water with the more electropositive of these elements may be violently
explosive.
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We live on a planet that is dominated by water. More than 70% of the Earth's surface is covered with
this simple molecule. Scientists estimate that the hydrosphere contains about 1.36 billion cubic
kilometers of this substance mostly in the form of a liquid (water) that occupies topographic
depressions on the Earth. The second most common form of the water molecule on our planet is ice.
If our entire planet's ice melted, sea-level would rise by about 70 meters.
Water has a very simple atomic structure. This structure consists of two hydrogen atoms bonded to
one oxygen atom. The nature of the atomic structure of water causes its molecules to have unique
electrochemical properties. The hydrogen side of the water molecule has a slight positive charge. On
the other side of the molecule a negative charge exists. This molecular polarity causes water to be a
powerful solvent and is responsible for its strong surface tension.
When the water molecule makes a physical phase change its molecules arrange themselves in
distinctly different patterns. The molecular arrangement taken by ice (the solid form of the water
molecule) leads to an increase in volume and a decrease in density. Expansion of the water molecule
at freezing allows ice to float on top of liquid water.
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The three diagrams above illustrate the distinct arrangement patterns of water molecules as they
change their physical state from ice to water to gas. Frozen water molecules arrange themselves
in a particular highly organized rigid geometric pattern that causes the mass of water to expand
and to decrease in density. The diagram above shows a slice through a mass of ice that is one
molecule wide. In the liquid phase, water molecules arrange themselves into small groups of
joined particles. The fact that these arrangements are small allows liquid water to move and flow.
Water molecules in the form of a gas are highly charged with energy. This high energy state
causes the molecules to be always moving reducing the likelihood of bonds between individual
molecules from forming.
Water has several other unique physical properties. These properties are:
Water has a high specific heat. Specific heat is the amount of energy required to change the
temperature of a substance. Because water has a high specific heat, it can absorb large amounts of
heat energy before it begins to get hot. It also means that water releases heat energy slowly when
situations cause it to cool. Water's high specific heat allows for the moderation of the Earth's climate
and helps organisms regulate their body temperature more effectively.
Water in a pure state has a neutral pH. As a result, pure water is neither acidic nor basic. 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.
Water conducts heat more easily than any liquid except mercury. This fact causes large bodies of
liquid water like lakes and oceans to have essentially a uniform vertical temperature profile.
Water molecules exist in liquid form over an important range of temperature from 0 - 100° Celsius.
This range allows water molecules to exist as a liquid in most places on our planet.
Water is a universal solvent. It is able to dissolve a large number of different chemical compounds.
This feature also enables water to carry solvent nutrients in runoff, infiltration, groundwater flow,
and living organisms.
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Water has a high surface tension. In other words, water is adhesive and elastic, and tends to
aggregate in drops rather than spread out over a surface as a thin film. This phenomenon also causes
water to stick to the sides of vertical structures despite gravity's downward pull. Water's high surface
tension allows for the formation of water droplets and waves, allows plants to move water (and
dissolved nutrients) from their roots to their leaves, and the movement of blood through tiny vessels
in the bodies of some animals.
The following illustration shows how water molecules are attracted to each other to create
high surface tension. This property can cause water to exist as an extensive thin film over
solid surfaces. In the example above, the film is two layers of water molecules thick.
The adhesive bonding property of water molecules allows for the formation of water droplets
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Water molecules are the only substance on Earth that exists in all three physical states of
matter: solid, liquid, and gas. Incorporated in the changes of state are massive amounts of heat
exchange. This feature plays an important role in the redistribution of heat energy in the Earth's
atmosphere. In terms of heat being transferred into the atmosphere, approximately 3/4's of this
process is accomplished by the evaporation and condensation of water.
The freezing of water molecules causes their mass to occupy a larger volume. When water freezes it
expands rapidly adding about 9% by volume. Fresh water has a maximum density at around 4°
Celsius. Water is the only substance on this planet where the maximum density of its mass does not
occur when it becomes solidified.
The physical and chemical properties of seawater vary according to latitude, depth, nearness to land,
and input of fresh water. Approximately 3.5 percent of seawater is composed of dissolved
compounds, while the other 96.5 percent is pure water. The chemical composition of seawater
reflects such processes as erosion of rock and sediments, volcanic activity, gas exchange with the
atmosphere, the metabolic and breakdown products of organisms, and rain. (For a list of the
principal constituents of seawater see ocean: Composition of seawater.) In addition to carbon, the
nutrients essential for living organisms
Density of water molecules at various temperatures.
Temperature (degrees Celsius) Density (grams per cubic centimeter)
0 (solid) 0.9150
0 (liquid) 0.9999
4 1.0000
20 - 80 0.9982 – 0.9718
100 (gas) 0.0006
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Chemical Properties of Water
Chemical Description
The chemical description of water is H2O, which means that water is formed by the combination
of two hydrogen atoms and one oxygen atom. Hydrogen atom weighs one-sixteenth of an
oxygen atom, thus in water molecule, 88.8% of weight is from oxygen, while 11.2% belongs to
hydrogen.
Polar Molecule
Though overall water has a neutral charge, it tends to be slightly positive on the hydrogen side
and slightly negative on oxygen side. The electrostatic bond formed between the slightly positive
hydrogen ion of water molecule and other negative ions or polar molecules is termed as
hydrogen bond. Read more on why is water a polar molecule.
Universal Solvent
Water has the capacity to dissolve more substances than any other liquid, thus, is called universal
solvent. The universal solvent quality enables water to take other valuable minerals, nutrients or
chemicals with them wherever they go. The polar bonds in water molecule make it a universal
solvent.
Hydrophilic Compounds
Substances dissolving readily in water are called hydrophilic compounds. They consist of ions or
polar molecules that use electrical charge effects to attract water molecules. The water molecules
surround these polar molecules and carry them into the solution, thereby dissolving them. For
example, ionic substances like sodium chloride dissolve in water, as the positive sodium ions and
negative chlorine ions of sodium chloride get attracted to the polar water molecules.
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Hydrophobic Compounds
Molecules with prevailing nonpolar bonds are the ones that are mostly insoluble in water and are
called hydrophobic compounds. Hydrocarbons containing C-H bonds are examples of
hydrophobic compounds. This is because the intensity, with which water molecules are attracted
to C-H bonds, is far lesser than the intensity towards other water molecules. Hence, water
molecules do not carry these hydrocarbons into the solution.
Ph
The pH scale runs from 1 to 14, wherein ph 1-6 refers to acidic pH, while pH 8-14 refers to basic
pH. At pH 7, a chemical is known to be neutral. The pH of a chemical substance is determined
by the amount of hydrogen atoms in it. Thus, a chemical compound with high pH has higher
number of hydrogen atoms in its chemical composition and is called acids. Whereas compounds
with lower pH contains lower number of hydrogen atoms and are called bases. The pH of water
is neutral. Read more on how to make alkaline water.
Amphoteric Nature
Water has the ability to act as both an acid (proton donor) as well as a base (proton acceptor). It
acts as a base to acids stronger than itself and acts as an acid to bases stronger than itself. This
amphoteric nature is clearly visible in the below reaction, wherein water molecule reacts with
acid as a base.
H2O (l) + HCl (aq) ⇌ H3O++ Cl
-
Water molecule also reacts with a strong base as an acid.
H2O (l) + NH3 (aq) ⇌ NH4+ + OH
-
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Another interesting water chemical property is that metals such as gold, silver, copper, tin, etc.
do not react with water. Moreover, although salt water chemical properties differ from that of
regular water, due to the different dissolved salts present in them, distilled water chemical
properties are the same as chemical properties of water. Distilled water will only differ in their
physical properties. Hope this article on water chemical properties was helpful!
Physical properties if marine water
Sea water has many interesting physical properties. These properties are attributed not only to
the fact of water itself possessing interesting characteristics, but are also due to the addition of
salt, which produces behavior different from that of fresh water. Sea water on average is about
96.5% water, and is approximately only 3.5% salt. There are other dissolved elements, which
vary in concentration according to location, even in places in the same area. These dissolved
substances include major elements in parts per thousand such as chlorine, minor substances in
parts per million such as bromine, and trace elements such as nitrogen.
