Chemical Formula of Water: Definition, Structure, Properties of Water

Chemical Formula of Water: A major part of all living organisms is made up of water. The human body has about \(65\% {\rm{ }},\) and some plants have as much as \(95\% \) water. It is a crucial compound for the survival of all life forms. It is a solvent of great Importance. The distribution of water over the earth’s surface is not uniform. To learn more about the chemical formula of water, read the below article.

Define Water

Water is a substance that exists in gaseous, liquid, and solid states and is made up of the chemical elements hydrogen and oxygen, which are held together by covalent bonds. The chemical formula of water is \({{\text{H}}_2}{\text{O}}.\) It is one of the most abundant and necessary compounds. Next to oxygen, water is the most important for human life. One can survive for a few days without food, but not without water. It is also used as a solvent for many substances and participates in a wide range of chemical reactions. Water is also a source of heavy water \(\left({{{\text{D}}_2}{\text{O}}} \right),\) which is critical for nuclear reactors to control neutron speed.

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Structure of Water

In the \({{\text{H}}_2}{\text{O}}\) molecule, oxygen is \({\text{s}}{{\text{p}}^3}\)-hybridized and therefore has four \({\text{s}}{{\text{p}}^3}\)-hybridised orbitals. Two of these \({\text{s}}{{\text{p}}^3}\) orbitals are half-filled and therefore overlap with \(1{\text{s}}\) orbitals of hydrogen atoms to form two \({\text{O-H}}\) sigma bonds by \({\text{s}}{{\text{p}}^3} -{\text{s}}\) overlapping, while the other two contain lone pairs. Since the oxygen atom is \({\text{s}}{{\text{p}}^3}\) hybridized, the geometry of the \({{\text{H}}_2}{\text{O}}\) molecule must be tetrahedral, and the \(\angle {\text{HOH}}\) bond angle must be \({109.5^ \circ }.\) Experimentally, however, the actual bond angle \(\angle {\text{HOH}}\) was determined to be \({104.5^ \circ }.\) This may be explained as follows:

In \({{\text{H}}_2}{\text{O}},\) the oxygen atom is surrounded by two shared pairs and two lone pairs of electrons. Still, according to VSEPR (Valence Shell Electron Pair Repulsion) theory, Ione pair-lone pair repulsions are stronger than bond pair-bond pair repulsions. As a result, the \(\angle {\text{HOH}}\) bond angle in water is slightly smaller than the regular tetrahedral angle of \({109.5^ \circ }.\) The actual bond angle is \({104.5^ \circ },\) and each \({\text{OH}}\) bond has a bond length of \(95.7\,{\text{pm}}.\)

In the gaseous state, water exists as discrete molecules. It is a bent-shaped molecule with a bond angle of \({104.5^ \circ }\) and a bond length of \(95.7\,{\text{pm}}.\)

Structure of Ice

Ice has a highly ordered three-dimensional hydrogen-bonded structure. \({\text{X}}\)-ray analysis of the ice crystals reveals that four other oxygen atoms surround each oxygen atom in a tetrahedral arrangement at a distance of \(276\,{\text{pm}}.\) Hydrogen bonds give ice a fairly open structure with wide holes. These holes may interstitially contain some other molecules of appropriate size.

Hydrogen Bonding in Water and Ice

The \({{\text{H}}_2}{\text{O}}\) molecule is highly polar. As a result of polarity, the \({{\text{H}}_2}{\text{O}}\) molecules show intermolecular hydrogen bonding in ice and liquid water. The presence of hydrogen bonding explains some unique properties as discussed below:

A. Ice floats on water

In ice (solid form of water), \({{\text{H}}_2}{\text{O}}\) molecules are arranged tetrahedrally in space. The oxygen atom in each \({{\text{H}}_2}{\text{O}}\) molecule is bonded to the two hydrogen atoms by covalent bonds and, at the same time, forms hydrogen bonds with the hydrogen atoms of the neighbouring \({{\text{H}}_2}{\text{O}}\) molecules. This results in a cage-like structure.

The structure is also porous because of the voids. When the ice melts into liquid water, the added thermal energy tends to break some hydrogen bonds between the \({{\text{H}}_2}{\text{O}}\) molecules in ice. As a result, the tetrahedral arrangements begin to collapse, and the \({{\text{H}}_2}{\text{O}}\) molecules in the water move close together. The number of voids also begins to decrease, so the density of water is greater than that of ice, which means that ice always floats on water.

B. Water has maximum density at \(4\,^\circ {\rm{C}}\)

At \(273\,{\text{k}},\) ice and water coexist. With increasing temperature, the supplied thermal energy will break the tetrahedral arrangements further due to the decrease in the hydrogen bondings. Hence the density of the water is expected to increase. But it is expected that the increase in temperature will also increase the average kinetic energy of the \({{\text{H}}_2}{\text{O}}\) molecules, resulting in an increase in volume.

However, this effect is negligible up to \({4^ \circ }{\text{C}},\) the density of water becomes maximum at \(4\,^\circ {\rm{C}}.\) If the temperature rises above \(4\,^\circ {\rm{C}},\) the effect of increasing the kinetic energy becomes greater than the effect of the increase in density. This is probably because the volume of water increases and its density decreases.

Therefore we conclude that the density increases in water up to \(4\,^\circ {\rm{C}}\) and decreases after that; that is, water has a maximum density and a minimum volume of \(4\,^\circ {\rm{C}}.\) This property of water is extremely useful for animals that live underwater in the sea. In very cold weather, the sea surface almostzes over. But beneath the surface, there is water with a temperature of around \(4\,^\circ {\rm{C}}.\) Aquatic animals can live safely in the water at this temperature.

Importance of the Structure of Water

The liquid state of water has a very complex structure, which undoubtedly involves significant molecule association. Because of the extensive hydrogen bonding among the molecules in liquid water, the values for properties such as viscosity, surface tension, and boiling point are much higher than would be expected for a typical liquid containing small molecules.

Water, for example, would be expected to have a boiling point of \(100\,^\circ {\rm{C}},\) higher than the boiling point expected based on the size of its molecules. In contrast to the condensed states (solid and liquid) of water, which have extensive association among the water molecules, the gaseous (vapour) phase has relatively independent water molecules that are separated by large distances.

Physical Properties of Water

• 1. Water is a liquid with azing point of \(273.15\,{\text{K}}\) and a boiling point of \(373.15\,{\text{K}}{\text{.}}\)
• 2. Water has the maximum density at \(277.15{\mkern 1mu} {\rm{K}}\left( {4\,^\circ {\rm{C}}} \right),{\rm{i}}{\rm{.e}}.,{\mkern 1mu} 1{\mkern 1mu} {\rm{g}}/{\rm{ml}}.\)
• 3. In water, the molecules are hydrogen-bonded. The hydrogen bonding influences all the physical properties such as state, the heat of fusion, the heat of vaporization, melting point and boiling point, etc.
• 4. Water is a naturally polar compound. As a result, most inorganic substances and many organic substances with polar bonds in their molecules are water-soluble due to intermolecular hydrogen bonding.
• 5. Because of its polar nature, strong intermolecular forces of attraction and hydrogen bonding are present in the \({{\text{H}}_2}{\text{O}}\) molecules.

Chemical Properties of Water

The important chemical properties of water are as follows:

1. Amphoteric Nature

It can act as both an acid and a base; it behaves like an amphoteric substance. In the Brönsted sense, it acts as an acid with \({\text{N}}{{\text{H}}_3}\) and as a base with \({{\text{H}}_2}{\text{S}}.\)

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

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

2. Oxidising and Reducing Characteristics

Water can act both as an oxidising and reducing agent in chemical reactions.

\(2{{\text{H}}_2}{\text{O}} + 2{{\text{e}}^ – } \to 2{\text{O}}{{\text{H}}^ – } + {{\text{H}}_2}\)

\(2{{\text{H}}_2}{\text{O}} + 2{\text{Na}} \to 2{\text{NaOH}} + {{\text{H}}_2}\)

As an oxidising agent: The oxidising character is because of its electron-accepting tendency by hydrogen atoms in the water molecule.

\(2{{\rm{H}}_2}{\rm{O}} \to 4{{\rm{H}}^ + } + {\rm{ }}4{{\rm{e}}^ – } + {{\rm{O}}_2}\)

\(2{{\text{F}}_2}{\text{ + 2}}{{\text{H}}_2}{\text{O}} \to 4{\text{HF}} + {{\text{O}}_2}\)

3. Hydrolytic Reaction

Water can hydrolyse many oxides, hydrides, carbides, nitrides, phosphides, and other salts. In these reactions, \({{\text{H}}^ + }\) and \({\text{O}}{{\text{H}}^ – }\) ions of the water interact with the anions or cations of the compound, respectively, resulting in the formation of an acid or a base or both, as shown below:

\({\text{CaO}} +{{\text{H}}_2}{\text{O}} \to {\text{Ca}}{\left({{\text{OH}}} \right)_2}\)

\({\text{S}}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}} \to {{\text{H}}_2}{\text{S}}{{\text{O}}_3}\)

\({{\text{P}}_4}{{\text{O}}_{10}} + 6{{\text{H}}_2}{\text{O}} \to \underset{{{\text{Phosphoric}}\,{\text{acid}}}}{\mathop {4{{\text{H}}_3}{\text{P}}{{\text{O}}_4}}} \)

4. Water as a Catalyst

Water also catalyses some chemical reactions. For example, perfectly dry gas like \({\text{N}}{{\text{H}}_3}\left({\text{g}} \right)\) and \({\text{HCl}}\left({\text{g}} \right)\) does not react. However, in the presence of traces of moisture, they combine to form \({\text{N}}{{\text{H}}_4}{\text{Cl}}.\)

Summary

Water is a substance that exists in gaseous, liquid, and solid states and is made up of the chemical elements hydrogen and oxygen. The chemical formula of water is \({{\text{H}}_2}{\text{O}}{\text{.}}\) Water is a liquid with azing point of \(273.15\,{\text{K}},\) and a boiling point of \(373.15\,{\text{K}}{\text{.}}\) Water is a polar solvent. Water is amphoteric in nature and acts as both oxidising and reducing agent. One can survive for a few days without food, but not without water. Water is also used as a solvent for a wide range of substances and participates in a wide range of chemical reactions.

FAQs on Chemical Formula for Water

Q.1. What is the full name of \({{\text{H}}_2}{\text{O}}\)?
Ans: The full name of \({{\text{H}}_2}{\text{O}}\) is water. It is also known as Dihydrogen monoxide.

Q.2. What is the formula of water in chemistry?
Ans: The chemical formula of water in chemistry is \({{\text{H}}_2}{\text{O}}.\)

Q.3. What is \({{\text{H}}_2}{\text{O}}\) called in chemistry?
Ans: In chemistry, \({{\text{H}}_2}{\text{O}}\) is known as water.

Q.4. How is water written?
Ans: The water molecule comprises two hydrogen atoms, represented by the letter \({\text{H,}}\) and a single oxygen atom, represented by the letter \({\text{O}}{\text{.}}\) Thus, water can be written as \({{\text{H}}_2}{\text{O}}.\)

Q.5. What are the three rules for writing a chemical formula?
Ans: The three rules for writing a chemical formula are as follows:
1) First, we write the symbols of the elements that make up the compound.
2) Under the symbol of each element, we write its valency.
3) Finally, we criss-cross the valences of the atoms that are combined.

Q.6. Is \({{\text{H}}_2}{\text{O}}\) acid or base?
Ans: \({{\text{H}}_2}{\text{O}}\) can act as both acid and base in chemical reactions. Hence, water is amphoteric in nature.

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