Structure of Water and Hydrogen Bonding

Water is an inorganic liquid chemical that is colorless, odorless, tasteless that makes up most of the Earth’s hydrosphere and the fluids in the body of all living beings.

• Water is an extremely important component for the existence of life as it is vital for all biological processes. It doesn’t, however, have any calorific value or nutritional value.
• Water is in a liquid state at standard atmospheric temperature and pressure.
• It occupies 71% of the total land on Earth and about 70% of total body weight in humans.
• The amount of water on Earth is maintained by a continuous movement of water from the ground to the atmosphere and back, called the water cycle.
• Water is also important for various chemical processes as it is a universal solvent.

Structure of Water

• The chemical formula for water is H₂O which indicates that a single molecule of water is made up of two hydrogen atoms and one oxygen atom.
• The atoms in a water molecule are connected to each other by polar covalent bonds. The molecule in itself is electrically neutral but polar with negative and positive charges localized in different areas.

In the gaseous phase

• In a water molecule, the s and p orbitals of the valence shell are sp³ hybridized to form four sp³ hybrid orbitals oriented tetrahedrally around the oxygen atom.
• The two of the hybrid orbitals are singly occupied, while the lone pairs of the electrons occupy the other two.
• Each single occupied sp³ orbital overlaps with the half-filled orbital of the H atom.
• As a result, an oxygen atom is bonded to the two hydrogen atoms by two O-H covalent bonds, and there are two lone pairs of electrons on the oxygen atom.
• The most stable arrangement of the atoms is the one where they are the farthest. The angle between the O-H bonds is around 104° rather than a perfect tetrahedron (109°) due to the repulsion between the lone pairs.
• Thus, the structure of the water molecule is an angular of bent structure.
• The molecule of water is polar because oxygen is more electronegative than hydrogen. The oxygen atom thus attracts the shared electrons towards itself.
• As a result, a partial negative charge is developed on the oxygen atom while the hydrogen atom develops a partial positive charge.

In the liquid phase

• In the liquid phase, water molecules are held together by intermolecular hydrogen bonds.
• A single water molecule is capable of forming four hydrogen bonds as it can form two bonds with the lone pair on oxygen and donate two electrons on hydrogen.
• In water, the formation of four hydrogen bonds results in an intermolecular tetrahedral structure forming an open structure and three-dimensional bonding network.

In solid phase

• The solid form of water is ice, which can exist in different crystalline forms depending on the conditions for the freezing of water.
• In the regular hexagonal ice, each oxygen atom is tetrahedrally surrounded by four other oxygen atoms, whereas one hydrogen atom lies in between each pair of oxygen.
• Thus, each hydrogen atom is covalently bonded to one oxygen atom and linked to another oxygen atom by a hydrogen bond.
• This arrangement causes the packing of atoms with large open spaces which results in the decrease in density of ice as compared to liquid water.
• When ice melts, some of the hydrogen bonds are broken, and the water molecules become more tightly packed.

Hydrogen bonding in water

• Due to polarity in water molecules, they are capable of attracting each other. These interactions are weak attractions called hydrogen bonds.
• Hydrogen bond in water is a weak interaction between the partially positive hydrogen atom and a partially negative oxygen atom.
• Hydrogen bonding in water is intermolecular and occurs between two atoms of two different molecules.
• A single water molecule is capable of forming four hydrogen bonds as it can form two bonds with the lone pair on oxygen and donate two electrons on hydrogen.
• In water, the formation of four hydrogen bonds results in an intermolecular tetrahedral structure forming an open structure and three-dimensional bonding network.
• The structure formed after hydrogen bonding results in the collective ground state of liquid water to have energy lower than the ground state in single gaseous molecules.
• This creates a stable structure of water molecules in liquid water.
• Hydrogen bond formed in water is a weak bond, and its strength is one-twentieth of that of the O-H covalent bond.
• The lifetime of these bonds is also very short, and they are continuously broken and formed within short periods of time. Thus, a dynamic equilibrium is maintained in liquid water.
• Similarly, all water molecules in liquid water have at least one hydrogen bond with a neighboring water molecule with effectively no free water molecules.
• Hydrogen bonding in water holds water molecules about 15% closer than if only Van der Waals interactions existed.
• However, hydrogen bonding in water is directional, which restricts the number of neighboring water molecules to just about four than the larger numbers in other liquids.

Characteristics/ Properties of water

Physical properties of water

• Pure water is a transparent, colorless, odorless liquid that readily picks up the flavor of any substance dissolved in it.
• The freezing point, boiling point, enthalpy of fusion, and enthalpy of vaporization of water is higher compared to the hydrides of other members of the same group due to the intermolecular hydrogen bonds between the molecules.
• Water has a high dipole moment which makes it an ideal medium for the dissolution of a wide variety of compounds.
• The high specific heat capacity of water enables it to absorb the heat of various biochemical and physiological reactions, going on inside the body, with the minimum rise of temperature.
• Water is a poor conductor of heat and electricity, but the addition of a small quantity of an acid or an alkali makes it electrically conducting.

The solvent action of water

• Water is also called a universal solvent because of its ability to dissolve a larger variety of substances.
• This ability is due to the intensive hydrogen bonding and polarity of water molecules.
• The polarity of water causes water to behave differently with polar and non-polar compounds.
• Polar molecules of water can form weak electrostatic interactions with other polar molecules and ions.
• Thus, the polar molecules and ions interact with the partially positive and partially negative ends of water, with positive charges attracting negative charges.
• When the number of water molecules in a solution is more than the solute molecules, the interactions lead to the formation of a three-dimensional sphere of water, called the hydration shell around the solute molecules.
• The formation of the hydration shell enables the equal dispersion of solute molecules throughout the solution.
• Non-polar molecules, however, don’t interact with water or form hydration shells due to the lack of charged interactions.

High specific heat capacity

• By virtue of intensive hydrogen bonding, water has a very high specific heat capacity and high heat of vaporization.
• These properties allow water to moderate the climate and temperature of Earth by buffering large fluctuations in temperatures.
• A similar process occurs within the body where water prevents the rapid rise in body temperature as a result of various biochemical reactions.
• The latent heat of fusion and vaporization of water is also high, which prevents the melting of ice glaciers and drift ice.

Anomalous expansion of water

• Anomalous expansion of water is an abnormal property of water where water expands instead of contracting when the temperature goes from 4°C to 0°C.
• Thus, the density of water is maximum at 4°C and decreases as the temperature goes down.
• This property of water results because the water molecules in freezing state are held together by H-O attraction rather than O-O attraction.
• But because the H-O interaction is not as tight as the O-O interaction, a little expansion of water is observed during freezing.

Biological functions of Water

Some biological functions of water are listed below:

1. Water is a vital body fluid that is essential for regulating the processes like digestion, transport of nutrients, and excretion. Water dissolves ionic and polar organic compounds and allows the transportation of the products of digestion to the place of requirement in the body.
2. Water regulates body temperature through the process of sweating and evaporation.
3. Water is the medium for all metabolic reactions in the body as all metabolic reactions in the body take place in the solution phase.
4. Water also provides habitat for various animals in the form of ponds, rivers, seas, etc.
5. Water is essential for the germination of seed and the process of photosynthesis by which plants prepare their food.
6. Water is the medium for the transport of minerals from the soil to different parts of plants.
7. Water helps in the maintenance of the plant structures by providing the appropriate pressure to the plant tissues.
8. Water also contributes to the formation of various biological membranes by interaction with various organic compounds.
9. Water also impacts the fundamental components of all cells like DNA and proteins where hydrogen bonding regulates the folding of proteins and nucleic acids.
10. The double helix structure of DNA is also supported by water molecules that surround the DNA in an ordered fashion.

References and Sources

1. SK (2006). Conceptual Chemistry. S. Chand and Company Ltd. New Delhi.
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8. 1% – https://www.entrancei.com/question-answer/all-metabolic-reactions-in-the-body-take-place-in-the-presence-of-27075
9. 1% – https://quizlet.com/21532508/chap-3-water-and-life-flash-cards/
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