Water: One of the most significant oxides of hydrogen is water. The speciality of water is that most life forms on Earth cannot survive without it. That is the reason why the life forms stock up the water in their body, with the human body storing up water to make up for about \(65\,\% \) in total. There are several plant species which is made up of about \(95\,\% \) of water, making it direly necessary for their survival. However, water is not distributed evenly on Earth’s surface, even if it forms \(71\,\% \) of the Earth’s surface.

Water is distributed in oceans, polar ice, and glaciers, like groundwater, freshwater lakes, as soil moisture, rivers, saline lakes, and inland seas and in the atmosphere as water vapour. Water is present in nature in three phases- solid, liquid and gas. Water is not just essential to all life forms, but it forms a very significant part of chemical reactions too. Hence, it is important to learn all about the structure of water, its properties and why it is called the universal solvent.

Water Structure and Bonding

The structure of the water molecule is as shown below:

The oxygen in a water molecule is \({\text{s}}{{\text{p}}^3}\) hybridised, and hence, consists of four such orbitals. The two \({\text{s}}{{\text{p}}^3}\) orbitals, which are half-filled, overlap with the \({\text{1s}}\) orbitals of the two hydrogen atoms. The overlapping results in two \({\text{s}}{{\text{p}}^3} – {\text{s}}\) bonds or the sigma bonds between oxygen and hydrogen. The other two \({\text{s}}{{\text{p}}^3}\) hybridised orbitals contain the lone pair of electrons each. Theoretically, with an \({\text{s}}{{\text{p}}^3}\) hybridised central atom, the geometry of the water molecule should be tetrahedral with the \({\text{HOH}}\) bond angle \({109.5^ \circ }.\) However, the water molecule shows a bond angle of \({104.5^ \circ }.\)

The reason behind this is that the oxygen atom has two shared pairs of electrons and two lone pairs surrounding it. According to the VSPER theory, when compared to bond pair-bond pair repulsion, the lone pair- lone pair repulsions are stronger. Hence, the \({\text{HOH}}\) bond angle in water is smaller than the normal tetrahedral bond angle of \({109.5^ \circ }.\) The \({\text{HOH}}\) bond angle of water is \({104.5^ \circ },\) and the bond length of \({\text{O-H}}\) is \(95.7\,{\text{pm}}.\) Water molecule exists as discrete molecules and has a bent shape with a bond angle of \({104.5^ \circ }.\)

Partial Negative Charge and Dipole Moment

The electronegativity difference between oxygen and hydrogen is higher (Oxygen \( = 3.5,\) Hydrogen \( = 2.1\)). Due to this fact, the shared pair of electrons between hydrogen and oxygen lies slightly towards the oxygen atom. Hence, while the two hydrogen atoms have partial positive charges, oxygen has a partial negative charge. This creates a dipole, which is inclined at an angle of \({104.5^ \circ },\) making water a polar molecule. The dipole moment of water is \(1.84{\text{D}}.\)

Hydrogen Bonding in Water

In its liquid and solid form (as water and ICE), water exhibits intermolecular hydrogen bonding. The two lone pairs of electrons on the oxygen atom can form two intermolecular hydrogen bonds with the neighbouring water molecule (hydrogen in the water molecule). Hence, each water molecule is bonded through hydrogen bonding with four other water molecules. In the liquid state, therefore, water exists as associated liquids.

In the solid form, water takes up a crystalline form, as ICE and, there are nine crystalline forms of ice, depending upon how thezing of water is carried out. When frozen at atmospheric pressure, ice crystallises into normal hexagonal form, and at very low temperatures, ice crystallises in cubic form. When in hexagonal form, each oxygen atom is surrounded by four hydrogen atoms, making it a tetrahedral structure, as shown below:

Physical Properties of Water

One of the most significant and unique physical properties of water is its ability to form hydrogen bonding between themselves in liquid and solid states. Some of the constants of both liquid water and heavy water or \({{\text{D}}_2}{\text{O}}\) are as follows:

Physical Constants	water \(\left({{{\text{H}}_2}{\text{O}}} \right)\)	Heavy water \(\left({{{\text{D}}_2}{\text{O}}} \right)\)
Molecular Mass \(\left({{\text{g}}/{\text{mol}}} \right)\)	\(18.0151\)	\(20.0276\)
Boiling point \(\left({\text{K}} \right)\)	\(373.0\)	\(374.4\)
Melting point \(\left({\text{K}} \right)\)	\(273.0\)	\(276.8\)
Density (at \(298\,{\text{k}}\) & \({\text{g/c}}{{\text{m}}^3}\))	\(1.0000\)	\(1.1059\)
Dielectric	\(78.39\)	\(78.06\)
Viscosity (at \(293\,{\text{k}}\,\) & in Centipoise)	\(0.8903\)	\(1.107\)

Other physical properties include:

1. Due to the presence of intermolecular hydrogen bonding in water, the physical constants such as melting point, boiling point, viscosity, etc., are higher than the other hydrides of the same group (group \(16\)).
2. Since water has high thermal conductivity, specific heat and surface tension, it plays a very vital role in moderating the climate and the body temperature of living beings on Earth.

Capacity of Water to Dissolve Substances

The high dielectric constant of water \(\left({80} \right)\) gives it the ability to dissolve most of the inorganic or ionic compounds. That is why water is called the universal solvent. While the ionic compounds dissolve in water through ion-dipole interactions, the organic solvents such as alcohols, urea, amines, sugar, etc., dissolve in water by forming a hydrogen bond with it.

Chemical Properties of Water

Water reacts in different ways with different substances and act as an acid, base, reductant, and oxidant and also as a ligand to take part in various reactions. Some of the examples are as given below:

1. Amphoteric Nature of Water:

Since water is a weak electrolyte, it undergoes ionisation to a small extent to form \({{\text{H}}_3}{{\text{O}}^ + }\) and \({\text{O}}{{\text{H}}^ – }.\) Hence, pure water has very less electrical conductivity.
\({{\text{H}}_2}{\text{O}}\left({\text{l}} \right) + {{\text{H}}_2}{\text{O}}\left({\text{l}} \right) \to {{\text{H}}_3}{{\text{O}}^ + }\left({{\text{aq}}} \right) + {\text{O}}{{\text{H}}^ – }\left({{\text{aq}}} \right);{{\text{K}}_{\text{w}}} = 1.0\,{\text{X}}\,{\text{1}}{{\text{0}}^{ – 14}}\,{\text{mo}}{{\text{l}}^2}{{\text{L}}^{ – 2}}\) at \(298\,{\text{K}}\)

Water, therefore, can act as an acid and a base, and hence, has an amphoteric character. It can act as a base towards acids that are stronger than water and as an acid with stronger bases.

Water as an acid:

\({{\text{H}}_2}{\text{O}}\left({\text{l}} \right) + {\text{N}}{{\text{H}}_3}\left({{\text{aq}}} \right) \to {\text{NH}}_4^ + \left({{\text{aq}}} \right) + {\text{O}}{{\text{H}}^ – }\left({{\text{aq}}} \right)\)

Water as a base

\({{\text{H}}_2}{\text{O}}\left({\text{l}} \right) + {{\text{H}}_2}{\text{S}}\left({{\text{aq}}}\right) \to {{\text{H}}_3}{{\text{O}}^ + }\left({{\text{aq}}} \right) + {\text{H}}{{\text{S}}^ – }\left({{\text{aq}}} \right)\)

2. Oxidation – Reduction Reaction:

Water acts both as an oxidising agent and as a reducing agent.

As an oxidising agent:

Water reacts with many active metals such as sodium, calcium, and so on, which have an electrode potential of less than \( – 0.83\,{\text{V}}\) to liberate dihydrogen gas.
\({\text{2Na}}\left({\text{s}} \right){\text{ + }}{{\text{H}}_2}{\text{O}}\left({\text{l}} \right) \to {\text{2NaOH}}\left({{\text{aq}}} \right){\text{ + }}{{\text{H}}_2}\left({\text{g}} \right)\)

Also, with carbon, water acts as an oxidant to form syngas when steam is passed over red hot coke.

\({\text{C}}\left({\text{s}} \right){\text{ + }}{{\text{H}}_2}{\text{O}}\left({\text{g}} \right)1273\,{\text{K}} \to \left[{\left\{{{\text{CO}}\left({\text{g}} \right) + {{\text{H}}_2}\left({\text{g}} \right)} \right\}} \right]{\text{syngas}}\)
The reaction takes place in the presence of \({\text{Ni}}\) catalyst.

As a reducing agent:

Water acts as a reducing agent when it reacts with electronegative elements such as fluorine which have an electrode potential of higher than \( + 1.23\,{\text{V}},\) to liberate dioxygen and trioxygen.
\(2{{\text{F}}_2}\left({\text{g}} \right) + 2{{\text{H}}_2}{\text{O}}\left({\text{l}} \right) \to {{\text{O}}_2}\left({\text{g}} \right) + 4{{\text{H}}^ + }\left({{\text{aq}}} \right) + 4{{\text{F}}^ – }\left({{\text{aq}}} \right)\)
\({\text{3}}{{\text{F}}_2}\left({\text{g}} \right) + 3{{\text{H}}_2}{\text{O}}\left({\text{l}} \right) \to {{\text{O}}_3}\left({\text{g}} \right) + 6{{\text{F}}^ – }\left({{\text{aq}}} \right) + 6{{\text{H}}^ + }\left({{\text{aq}}}\right)\)

Water also hydrolyses many oxides, both metallic and non-metallic, hydrides, nitrides, carbides, and other such salts to form corresponding acids or bases. Water also forms coordination compounds by associating with central metal ions.

Forms of Water

Water, when it occurs in nature, contains dissolved salts. Hence, depending upon the amount or concentration of these salts in water, they behave differently with the soap solution or how it forms lather with the soap. Accordingly, water is classified as:

a. Hard Water: Water, which does not produce lather easily, is called hard water. Some examples of hard water include seawater, spring water, lake water and Well water.
b. Soft Water: Water that has less dissolved salts, and therefore, produces lather readily is called soft water. Examples include distilled water, demineralised water, and rainwater.

Causes of Hardness in Water and Types:

Hardness in water is due to the presence of chlorides, bicarbonates, and sulphates of calcium and magnesium. The ions \({\text{C}}{{\text{a}}^{2 + }}\) and \({\text{M}}{{\text{g}}^{2 + }},\) when reacted with sodium or potassium salt of fatty acids (or soaps), form a curdy white precipitate or scum instead of lather. This prevents the lather from forming in hard water.

According to the types of salts present in the hard water, it is classified as:

Permanent Hardness	Temporary Hardness
Permanent hardness results due to the presence of chlorides and sulphates (soluble) of calcium and magnesium.	The hardness is due to the presence of bicarbonates of calcium and magnesium.
Permanent hardness cannot be removed by filtering or boiling since it contains soluble salts.	The temporary hardness can be removed by boiling or filtering water.
Permanent hardness is called non-carbonate hardness.	Temporary hardness is called carbonate hardness.

Summary

Water is an essential part of life systems on Earth and makes up a large percentage of several living organisms. Water, as a molecule, has a central oxygen atom that is bonded to two hydrogen atoms, presenting a bent shape. Although a tetrahedral geometry is expected due to the two lone pairs of electrons on the oxygen atom, the lone pair-lone pair repulsions give it a bent shape and a bond angle lesser than expected. Water molecule exhibits intermolecular hydrogen bonding in their liquid and solid-state. Due to its higher dielectric constant, water can dissolve most inorganic and many organic compounds, making it a universal solvent. It exhibits amphoteric nature too.

FAQs

Q.1. What is the structure and bond angle of water?
Ans: Water has a bent structure and exhibits a bond angle of \({104.5^ \circ }\) and a bond length of \(95.7{\text{pm}}.\) The bond angle is less than the \({109.5^ \circ }\) expected.

Q.2. Why does a water molecule have a bent shape?
Ans: According to the VSEPR theory, the lone pair-lone pair repulsions are stronger than the bond pair-bond pair repulsions, thereby shortening the \({\text{HOH}}\) bond angle (oxygen has two lone pairs of electrons). Therefore, since the bond angle is reduced from \({109.5^ \circ }\) to \({104.5^ \circ },\) water has a bent shape.

Q.3. What are the four main properties of water?
Ans: The properties of water are as follows:
a. Water has a high dielectric constant, and therefore, can dissolve several compounds, both organic and inorganic.
b. Due to the presence of intermolecular hydrogen bonding, the rest of the physical parameters,zing point, boiling point, specific heat, etc., are abnormally higher.
c. Water as an acid:
\({{\text{H}}_2}{\text{O}}\left({\text{l}} \right) +{\text{N}}{{\text{H}}_3}\left({{\text{aq}}} \right) \to {\text{NH}}_4^ + \left({{\text{aq}}} \right) +{\text{O}}{{\text{H}}^ – }\left({{\text{aq}}}\right)\)
d. Water as a base:
\({{\text{H}}_2}{\text{O}}\left({\text{l}} \right) + {{\text{H}}_2}{\text{S}}\left({{\text{aq}}} \right) \to {{\text{H}}_3}{{\text{O}}^ + }\left({{\text{aq}}} \right) + {\text{H}}{{\text{S}}^ – }\left({{\text{aq}}} \right)\)

Q.4. How does the structure of water affect its properties?
Ans: The presence of two lone pairs of electrons in the oxygen atom gives water a bent shape. This makes water molecules polar, and therefore, it increases the power of attraction; therefore, when it comes near other molecules, such as \({\text{NaCl,}}\) it readily reacts with them.

Q.5. What type of hydrogen bonding is present in water?
Ans: The two lone pairs of electrons on the central oxygen atom of a water molecule forms intermolecular hydrogen bonding with the other two hydrogen atoms of neighbouring water molecules.

Study About Conservation Of Water Here

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