Introduction:

The self-ionization of water was first proposed in 1884 by Svante

Arrhenius as part of the theory of ionic dissociation which he
proposed to explain the conductivity of electrolytes including water.
Arrhenius wrote the self-ionization as At that time, nothing was yet
known of atomic structure or subatomic particles, so he had no
reason to consider the formation of an ion from a hydrogen atom on
electrolysis as any less likely than, say, the formation of a ion from a
sodium atom.
In 1923 Johannes Nicolaus Brønsted and Martin Lowry proposed that
the self-ionization of water actually involves two water molecules:
By this time the electron and the nucleus had been discovered
and Rutherford had shown that a nucleus is very much smaller than
an atom. This would include a bare ion which would correspond to a
proton with zero electrons. Brønsted and Lowry proposed that this
ion does not exist free in solution, but always attaches itself to a
water (or other solvent) molecule to form the hydronium ion (or
other protonated solvent).
Later spectroscopic evidence has shown that many protons are
actually hydrated by more than one water molecule. The most
descriptive notation for the hydrated ion is , where aq (for aqueous)
indicates an indefinite or variable number of water molecules.
However, the notations and are still also used extensively because
of their historical importance. This article mostly represents the
hydrated proton as , corresponding to hydration by a single water
molecule.
Water:

Water Droplets

Water is a substance composed of the chemical elements i.e.,

hydrogen and oxygen and existing in gaseous, liquid, and solid states.
It is one of the most essential of compounds. A tasteless and
odourless liquid at room temperature, it has the important ability to
dissolve many other substances. A water molecule has an oxygen
atom with two hydrogen atoms connected to it by covalent bonds.
The V shape that the molecule makes is caused by the way that the
oxygen atom's valence electrons repel one another.

Structure of a water molecule

Water is an amphoteric substance, which means water can accept a

proton acting as a base, and it can also donate A proton acting as an
acid. Water acts as both an acid and a base. The ability of a species to
act as either an acid or a base is known as amphoterism.
Water as a solvent:
Water is considered a very good solvent in the biochemical reactions.
The following figure illustrates how water dissolves salts.

Interaction of NaCl with water molecules

Table salt (NaCl) consists of a positively charged sodium ion and a

negatively charged chloride ion. The oxygen of water is attracted to
the positive Na ion. The hydrogens of water are attracted to the
negative Cl ion.
Hardness of water:
Although water is typically crystal clear, it contains minerals and
chemicals. The concentration of certain minerals that creates the
“hardness” of water.

Which One Is Healthier? Hard Water

or Soft Water?
The hardness of water is determined primarily by the amount
of calcium and magnesium it contains. Higher levels of these and
other minerals make water hard.

Water softening systems work by reducing the concentrations

of minerals from the water. Instead of having higher levels of calcium
and magnesium, soft water tends to have higher concentrations
of sodium, or salt.

There is no clear consent on whether hard water or soft water

is better for drinking, but soft water is gentler on the skin and hair.
Testing water quality can help determine if it is hard or soft.

Water softeners or filters can be used to alter water hardness

based on personal preference or specific needs.
How can you tell if water is hard or soft?
Signs of hard water includes:

 Feeling a film on your hands after washing them. This is caused

by the soap reacting with calcium to form soap scum. You may
need to rinse your hands longer if the water is hard.
 Spots can appear on glasses and silverware coming out of the
dishwasher. These are usually deposits of calcium carbonate.
 Mineral stains show up on clothes when they come out of the
washing machine. Clothes can wear out faster because of the
harshness of hard water.
  Less water pressure in your home. Mineral deposits can form in
the pipes, essentially shrinking the interior diameter of the
pipes and reducing water flow.

Signs of soft water include:

 A healthy lather when washing clothes, dishes, and even your

hands and body.
 Clothes that are cleaner, with no mineral stains and less wear-
and-tear damage.
 Healthy water pressure in your home.
 A slight sodium taste in drinking water, though in many cases a
difference in taste is imperceptible.

Hard water:

Hard water is formed when through deposits of limestone and.

The presence of soluble calcium and magnesium salts in water cause
so called water hardness. Calcium and magnesium salts enter the
water percolates the soils and rocks containing limestone and chalk-
containing minerals such as calcium and magnesium. During this
process the sparingly soluble in water salts converts into a well
water-soluble calcium and magnesium bicarbonates. This reaction is
called chemical weathering of limestone and occurs due to the
following reactions:

CaCO3 + CO2 + H2O = Ca(HCO3)2

MgCO3 + CO2 + H2O = Mg(HCO3)2

Calcium and magnesium bicarbonates (Ca(HCO3)2, Mg(HCO3)2)

cause carbonate hardness, also called temporary hardness T T.
Sulphates and chlorides of calcium and magnesium (CaSO 4, MgSO4,
CaCl2, MgCl2) cause permanent hardness of water TP. The sum of the
temporary and permanent hardness of water is a general hardness
TG. It Contains high levels of dissolved salts and minerals like calcium
and magnesium. Water with 17–60 parts per million (PPM) of
calcium and magnesium is slightly hard, and 60–120 PPM is
moderately hard. Hard water contains high concentrations of
magnesium and calcium. So, drinking hard water may help you get
essential minerals.

Soft water

Soft water can form naturally when surface water drains

through calcium-poor, impermeable rocks, or when it comes from
igneous or peat rock sources, like granite or sandstone.

Soft water has low concentrations of ions, especially calcium

and magnesium ions, and contains very small amounts of dissolved
salts. It Contains low levels of calcium and magnesium, and less than
17 PPM overall. Soft water is treated water that contains only
sodium.

Soft water is preferred for cleaning, as it doesn’t tend to cause

soap scum or mineral stains. Since it’s a more efficient and effective
cleaning agent, you may save money on your water bill by not having
to re-wash clothes or dishes, or taking longer showers to feel fully
cleaned and rinsed.

Soft water can also be created by a water softening process

that removes calcium, magnesium, and other metal cations from
hard water. This process is usually done using ion-exchange resins or
lime softening, but can also be done using nanofiltration or reverse
osmosis membranes.

In the ion exchange process, tiny resin beads charged with

sodium or potassium ions collect the hardness ions, calcium and
magnesium. Softened water can have high levels of bicarbonate and
sodium ions
In general, water with less than 60 ppm can be considered soft,
water with 60-120 ppm moderately hard, and water with greater
than 120 ppm hard.

Dissociation of water:
Dissociation of water is essential for many chemical and biological
processes. it is extremely important because in the lab we can
dissolve a ton of different stuff in water. It is an equilibrium reaction
in which

Water molecules dissociate into hydroxide ions (OH-) and hydrogen

ions (H+) through a process called self-ionization or autoionization.

2H2O(l) H3O+(aq) + OH-(aq)

Autoionization

This occurs when two water molecules transfer a proton from one
molecule to another, resulting in the formation of a hydroxide ion
and a hydronium ion.
Ionic compound dissociation: When ionic chemicals dissolve in
water, they dissociate to some extent. Ionic compounds are made up
of ions (charged atoms) with opposite charges. The ionic bond is
destroyed when an ionic substance dissociates in water.
Covalent compound dissociation: When covalent chemicals are
dissolved in water, they usually do not separate. When dissolved in
water, however, some covalent substances dissociate. Glucose is a
covalently bound molecule. It does not dissociate when dissolved in
water. In water, the molecules split and they move apart, but no
bonds break. In water, each glucose molecule remains whole.
Water dissociates, or autoionizes, due to a number of factors,
including:
1. Electric field fluctuations Between neighbouring molecules
2. Dipole liberations
3. Thermal effects
4. Localized hydrogen bonding
5. Nuclear quantum effects
 Water dissociation constant (Kw):
The dissociation of water is an equilibrium reaction. The
concentrations of H3O+ and OH- produced by the dissociation of
water are equal.
It means the rate of the forward reaction is equal to the rate of
the reverse reaction and the concentration of the reactants and
products do not change at equilibrium.
Kw describes the dissociation of water at equilibrium. The
water dissociation constant, or Kw, is the product of the molar
concentrations of hydroxide ions (OH-) and hydronium ions (HO) in
water.
It is also known as the autoionization constant. The constant is
the same whether the aqueous solution is acidic, basic, or neutral. At
25°C, the value of K is 1.0 x 10-14.
Degree of dissociation (α):
The fraction of a mole of the reactant that dissociates is known as
the degree of dissociation. It is represented by the symbol (α). The
degree of dissociation refers to the extent to which the dissociation
occurs.
The degree of dissociation is calculated by dividing the amount of
dissociated material by the total amount of the substance, which can
be expressed in molecules or moles.

A strong acid and a strong base will have a degree of dissociation

very near to 1. In weaker acids and bases, the degree of dissociation
is lower. The degree of dissociation also depends on several factors
like dilution, temperature, the nature of the solvent and the
composition of the electrolyte.
Factors affecting degree of
dissociation:
1. Dilution: Under typical dilution, the value of (α) is nearly 1 for
strong electrolytes and less than 1 for weak electrolytes.
2. Temperature: As the temperature of a solution rises, the
degree of dissociation of an electrolyte also increases.
 3. The Electrolyte's Composition: The degree of dissociation is
inversely proportional to the solution's concentration and
weight, but directly proportional to the solution's dilution and
the amount of solvent contained in the solution.
4. The Nature of the Solvent: The degree of dissociation is also
affected by the nature of the solvent, whether it is polar or
nonpolar.

Water samples and their

dissociation:
Water molecules dissociate into equal amounts of H 3O+ and
OH−, so their concentrations are almost exactly 1.00×10−7 mol dm−3 at
25 °C and 0.1 MPa. A solution in which the H 3O+ and
OH− concentrations equal each other is considered a neutral solution.
Pure water is neutral, but most water samples contain
impurities. If an impurity is an acid or base, this will affect the
concentrations of hydronium ion and hydroxide ion. Water samples
that are exposed to air will absorb some carbon dioxide to form
carbonic acid (H2CO3) and the concentration of H3O+ will increase due
to the reaction H2CO3 + H2O = HCO3− + H3O+. The concentration of
OH− will decrease in such a way that the product [H 3O+] [OH−]
remains constant for fixed temperature and pressure. Thus, these
water samples will be slightly acidic.
Dissociation of acids in water:

In this instance, water acts as a base. The equation for

CH3CO2H + H2O ⇄ CH3CO2− + H3O+.

the dissociation of acetic acid, for example, is

Dissociation of bases in water:

 In this case, the water molecule acts as an acid and adds

H2O + NH3 ⇄ OH− + NH4+.

a proton to the base. An example, using ammonia as the base, is

Older formulations would have written the left-hand side of the

equation as ammonium hydroxide, NH4OH, but it is not now believed
that this species exists, except as a weak, hydrogen-bonded complex.

SAMPLE 1-Sea water (Hard water)

Seawater is slightly basic, with a typical pH range of 7.5 to 8.4 and an
average pH of around 8.1. This is because the minerals dissolved in
groundwater increase the pH of the ocean. Seawater also contains
carbonic acid, which is formed when carbon dioxide bubbles from
volcanic vents on the seafloor dissolve. Carbonic acid is relatively
weak, but if enough of it forms, it can make seawater corrosive.
However, as the ocean absorbs more carbon dioxide from the
atmosphere, its pH decreases and it becomes more acidic.

hydrolyses in water: CO2 + H2O ⇌ H2CO3

Carbonic acid is formed when carbon dioxide (CO 2) dissolves and

The double arrow in the equation shows that the reaction is in

equilibrium, meaning that carbonic acid can dissociate back into
carbon dioxide and water.
Carbonic acid (H2CO3) is a weak, unstable acid that dissociates quickly
in water into hydrogen ions (H+) and bicarbonate ions (HCO3-).
H2CO3 ⇌ H+ + HCO3-
 Formation of carbonic acid in the ocean
SAMPLE 2- Tap water (Hard or soft depends on source of water)
In much of the developed world, chlorine often is added as a
disinfectant to tap water. If the water contains organic matter, this
may produce other by-products in the water such as halo acetic
acids, which has shown to increase the risk of cancer.
Tap water is neutral, with a pH of 7, which is in the middle of
the pH scale. However, tap water can contain halo acetic acids
(HAAs), which are chemical compounds that are formed when
chlorine-based disinfectants react with organic matter in the water.
Monochloroacetic acid (MCA), Dichloroacetic acid (DCA),
Trichloroacetic acid (TCA), Monobromoacetic acid (MBA), and
Dibromoacetic acid (DBA). Trichloroacetic acid is a by-product of
disinfection that occurs when chemicals added to make tap water
safe react with naturally occurring acids in the water. Dibromoacetic
acid has been shown to cause tumors in mice and rats when
administered in drinking water.
 Haloacetic acids (HAAs) are a group of disinfection by-products
(DBPs) that form when water disinfectants, like chlorine or ozone,
react with other chemicals in the water. The structure of HAAs is
XCH2COOH, where X can be F, Cl, Br, or I.
The dissociation constant for chloroacetic acid is 2.97. The
chemical formula for chloroacetic acid is CH 2ClCOOH. Chloroacetic
acid, also known as monochloroacetic acid, is a weak acid with a pKa
of 2.86–2.87. When a weak acid dissolves in water, a proton is
transferred from the acid to water, forming a hydronium ion. This
reaction better represents the process that occurs in an aqueous
solution and establishes acid equilibrium. The balanced chemical
equation for chloroacetic acid ionizing in water is:
ClCH2COOH + H2O = ClCH2COO- + H3O+

SAMPLE 3- Ground water (Hard water)

The most common acid in groundwater is carbonic acid, as it is
formed in water just by being exposed to the carbon dioxide in the
atmosphere. The quantity of carbonic acid in the water can vary
depending on what life is present in the water and how much carbon
dioxide is being produced. Groundwater flows from areas with a
higher water table surface to areas with a lower water table. This
mixture of carbonic acid in water makes most natural surface waters
slightly acidic. In groundwater systems, carbon dioxide is produced
when heterotrophic microorganisms break down organic matter as
water filters through soil. This results in higher partial pressures of
carbon dioxide in the subsurface than in the atmosphere, which
increases the formation of carbonic acid. Carbonic acid is formed in
groundwater when carbon dioxide reacts with water, by a process
called a hydrolysis reaction. The equation for this reaction is:
H2O + CO2 ⇌ H2CO3.
 The dissociation of carbonic acid produces bicarbonate and
hydrogen ions (H+). Bicarbonate can also dissociate into carbonate
(CO32-) and more hydrogen ions. Bicarbonate (HCO 3-) dissociates from
carbonic acid (H2CO3) in groundwater when the groundwater has a
certain buffering capacity.

Formation of carbonic acid in the ground water

SAMPLE 4- Rain water (Soft water)
Rainwater is naturally soft water; it’s considered soft because it
hasn't had the chance to pass through rocks and absorb minerals.
However, rainwater can become slightly acidic due to dissolved
carbon dioxide from the atmosphere. But this is different from acid
rain, which is caused by other gases. Rainwater is slightly acidic, with
a pH of around 5.6, because carbon dioxide (CO 2) dissolves into it to
form carbonic acid. Carbonic acid is formed when carbon dioxide
(CO2) in the air reacts with water in clouds. Carbonic acid (H 2CO3)
dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3–) in
water. This dissociation is due to carbonic acid being a weak and
unstable acid. The chemical equilibrium for this reaction is
CO2 + H2O ⇌ H2CO3.
However, only a small amount of CO2 is converted into carbonic acid
in this equilibrium.
However, when rain combines with sulphur dioxide and nitrogen
oxides from power plants and automobiles, it becomes more acidic,
with a pH of around 4.0. A pH of 4.0 is 10 times more acidic than a
pH of 5.0 which leads to acid rain. Acid rain has a pH of less than 5.3.

Acid rain formation and its effects

Application of Water Ionization:

Following are some applications of Ionization of Water
 Chemical Analysis: pH measurements, titrations, and
spectroscopy.
 Water Treatment: pH adjustment for water softening,
disinfection.
 Biological systems: Influence on cellular processes, enzyme
activity.
 Industrial processes: Electroplating, metal cleaning, synthesis.
 Environmental monitoring: Assessing water body health.
 Medical applications: Pharmaceutical formulations, diagnostics.
 Food and beverage industry: Processing, fermentation,
preservation.
 Corrosion control: pH regulation for metal equipment
longevity.
  Energy production: Fuel cells, battery technologies.
Dissociation of water molecules leads to acidic and basic conditions
that affect living organisms. Although the dissociation of water is
reversible and statistically rare, it is exceedingly important in the
chemistry of life. Hydrogen and hydroxide ions are very reactive.
Conclusion:
To conclude, the ionization of water is a fundamental concept in
chemistry with widespread implications across various scientific
disciplines. It forms the basis for understanding pH, acid-base
reactions, and numerous biochemical and environmental processes.
The ionization of water plays a vital role in biochemical reactions,
such as enzymatic processes and protein folding, where pH levels
must be carefully regulated for optimal function. Understanding the
ionization of water is also crucial in fields like environmental science,
where pH levels impact aquatic ecosystems’ health and stability.
Bibliography:
www.byjus.com
www.vedantu.com
www.wikipedia.com
www.geolsoc.org.uk
www.healthline.com
healy.create.stedwards.edu