Importance of Water

Water, with the chemical formula of H₂O, is not the most normal of molecules. Now, each of those water molecules has two hydrogen atoms and one oxygen atom. The oxygen is more electronegative, so it pulls the electrons slightly closer to itself in its neighborhood. Because of that, the water molecule is polarized with a partial negative charge close to the oxygen and a partial positive charge close to the hydrogens. This polarity enables water molecules to form hydrogen bonds among themselves, thus resulting in their high boiling and melting points compared with other similar-sized molecules.

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This Story also Contains

1. Structure of Water
2. Structure of Ice
3. Heavy Water
4. Heavy Water, D2O
5. Chemical Properties of Water
6. Summary

In a solid-state like ice, water has molecules in a hexagonal lattice. This structure is less dense compared to the water in its liquid state, hence ice floating. The hydrogen bonds formed are stable and maintain a regular arrangement, while in a liquid state, they are always breaking and reforming. This is a unique characteristic of water, and its solid phase being less dense than its liquid plays a significant function in ensuring aquatic life does not freeze in the coldest environment.

Structure of Water

The oxygen atom in a water molecule is sp³ hybridized, and four hybrid orbitals directed toward the corners of a tetrahedral are formed. Two of the hybrid orbitals having one electron each overlap with two hydrogen atoms and form two sigma bonds. Thus, the molecule has a bent structure with a bond angle of 104.5^oC. The bond angle is less than the expected angle in tetrahedron due to the presence of two lone pairs of electrons on two uncombined hybrid orbitals which repel each other and the bonded pairs cause them to come closer thereby reducing the bond angle from 109^o28^' to 104.5^o.

Because of high electronegativity of oxygen, the O-H bonds are polar, i.e., oxygen is partially negatively charged and each hydrogen is partially positively charged The molecule is polar and possesses dipole moment. Molecules are linked with each other by hydrogen bonding.

Structure of Ice

Ice has a highly ordered three-dimensional hydrogen-bonded structure as shown in the figure given below. Examination of ice crystals with X-rays shows that each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm. Hydrogen bonding gives ice a rather open type structure with wide holes. These holes can hold some other molecules of appropriate size interstitially.

Heavy Water

Heavy water or deuterium oxide (D₂O) is a type of water in which the hydrogen atoms are replaced by deuterium, an isotope of hydrogen. The deuterium has one proton with one neutron making it twice heavier than a regular hydrogen. Heavy water has the same kind of chemical characteristics compared to those of normal water while its boiling and density are more. It is used in nuclear reactors as a neutron moderator, to slow down the speed of neutrons so they can sustain the nuclear reaction. Even though it is not toxic with few amounts, more heavy water can interfere with biological processes because of the difference in atomic mass.

Heavy Water, D₂O

It is extensively used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms. It can be prepared by exhaustive electrolysis of water or as a by-product in some fertilizer industries. It is used for the preparation of other deuterium compounds, for example

$\begin{array}{r}{\mathrm{CaC}}_2+2{\mathrm{D}}_2\mathrm{O}\to {\mathrm{C}}_2{\mathrm{D}}_2+\mathrm{Ca}(\mathrm{OD}{)}_2\\ {\mathrm{SO}}_3+{\mathrm{D}}_2\mathrm{O}\to {\mathrm{D}}_2{\mathrm{SO}}_4\\ {\mathrm{Al}}_4{\mathrm{C}}_3+12{\mathrm{D}}_2\mathrm{O}\to 3{\mathrm{CD}}_4+4\mathrm{Al}(\mathrm{OD}{)}_2\end{array}$

Chemical Properties of Water

Water is known for having solvent capabilities, and colloquially it is often called the "universal solvent" because it dissolves more substances than any other liquid. This property is very important for a large number of physical, biological, and chemical processes. This makes water moderate climate and body temperature. Because there is a surface tension present in the water, caused by hydrogen bonding of water molecules, processes such as capillary action occur, which is one of the driving forces of water flow in plants. In addition, water's polarity enables hydrolysis reactions—those in which water breaks complex molecules apart—to take place in digestion and other metabolic processes.

These are the various chemical properties of water:

• Water is neutral in nature. pH of the pure water is 7. It is a weak electrolyte and ionizes into H⁺ and OH^- ions.${\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}^{+}+{\mathrm{OH}}^{-}$
• It reacts with active metals and evolves hydrogen The reaction is exothermic in the case of alkali and alkaline earth metals.$2\mathrm{Na}+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{NaOH}+{\mathrm{H}}^2$
• Reaction with non-metals: Water reacts with non-metals like fluorine and chlorine as follows:$2{\mathrm{F}}_2+2{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{H}}_2{\mathrm{F}}_2+{\mathrm{O}}_2$
• Action on non-metallic oxides: Acidic oxides combine with water to form acids viz:${\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}_2{\mathrm{CO}}_3{\mathrm{SO}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{SO}}_3$
• Action on metallic oxides: Basic oxides combine with water to form alkalies.${\mathrm{Na}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{NaOH}$
• Action on hydrides, carbides, nitrides, and phosphides: Water decomposes these compounds with the evolution of hydrogen, acetylene, ammonia, and phosphine respectively.
• $\begin{array}{r}{\mathrm{CaH}}_2+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ca}(\mathrm{OH}{)}_2+2{\mathrm{H}}_2\\ {\mathrm{CaC}}_2+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}}_2{\mathrm{H}}_2+\mathrm{Ca}(\mathrm{OH}{)}_2\\ {\mathrm{Ca}}_3{\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{NH}}_3+3phosphine\mathrm{Ca}(\mathrm{OH}{)}_2\\ {\mathrm{Ca}}_3{\mathrm{P}}_2+6{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{PH}}_3+3\mathrm{Ca}(\mathrm{OH}{)}_2\end{array}$
• Hydrolysis: Many salts undergo hydrolysis with water.${\mathrm{CH}}_3\mathrm{COONa}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{CH}}_3\mathrm{COOH}+\mathrm{NaOH}$
• Decomposition: Water containing either alkali or acid when electrolysed gets decomposed into H₂ and O₂.$2{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{H}}_2+{\mathrm{O}}_2$
• The water of crystallization: It combines with many salts during crystallization to form hydrates. For example, CuSO₄.5H₂O, FeSO₄.7H₂O, etc.
• Water as a catalyst: Water acts as a catalyst in many reactions. Perfectly dry gases generally do not react but the presence of moisture brings the chemical change. Ammonia and hydrochloric acid gas combine only in the presence of moisture.

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Some Solved Examples

Example 1

Question:

Which one of the following statements about water is FALSE?

1) Water is oxidized to oxygen during photosynthesis.

2) Water can act both as an acid and as a base.

3) There is extensive intramolecular hydrogen bonding in the condensed phase.

4) Ice formed by heavy water sinks in normal water.

Solution:

The false statement about water is:

1) Water is oxidized to oxygen during photosynthesis.

Explanation: During photosynthesis, water is reduced to oxygen (O₂), not oxidized. The correct reaction is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Therefore, option (1) is false.

Example 2

Question:

When two ice cubes are passed over each other, they unite to form one cube. Which of the following forces is responsible for holding them together?

1) Hydrogen bond formation

2) Vander Waals forces

3) Covalent attraction

4) Ionic interaction

Solution:

When two ice cubes are pressed over each other, hydrogen bond formation takes place between them, causing them to unite into one cube. Therefore, the answer is option (1), hydrogen bond formation.

Example 3

Question:

The boiling point of water is exceptionally high because:

1) There is a covalent bond between H and O.

2) The water molecule is linear.

3) Water molecules associate due to hydrogen bonding.

4) Water molecules are not linear.

Solution:

The boiling point of water is exceptionally high because water molecules are associated with hydrogen bonding. This association leads to stronger intermolecular forces and a higher boiling point. Hence, option (3) is

correct.

Summary

Water is an extraordinarily important molecule in organisms and the environment because of its unique structure and properties. From its molecular structure, which leads to it being polar and capable of making hydrogen bonds to the unusual properties of heavy water, H₂O plays a role from natural to industrial processes. Universal solvent and a solvent with significant heat capacity are its chemical properties, which suggest its importance to the sustenance of life and environment stability. Knowledge about its properties has made apparent the many roles it plays in the ecosystem.