Water: Structure, Polarity, and Hydrogen Bonding

Structure of Water

Water (H2O) is a small, bent molecule composed of two hydrogen atoms covalently bonded to one oxygen atom. The oxygen atom is more electronegative, causing an unequal sharing of electrons and resulting in a polar molecule.

• Covalent bonds involve the sharing of electrons between atoms.

• Polar covalent bonds share electrons unequally (e.g., O-H), while nonpolar covalent bonds share electrons equally (e.g., C-H).

• The bent shape and difference in electronegativity make water asymmetrical and polar.

Polarity and Molecular Comparison

Polarity arises from the unequal sharing of electrons and the molecular geometry. Water is polar, while methane (CH4) is nonpolar and symmetrical.

• Hydrophilic molecules are water-loving and dissolve in water (polar).

• Hydrophobic molecules are water-fearing and do not dissolve in water (nonpolar).

Electronegativity and Bond Types

Electronegativity is the tendency of an atom to attract electrons. Oxygen is highly electronegative compared to hydrogen, leading to polar covalent bonds in water.

• Electronegativity values: H = 2.1, O = 3.44, C = 2.55, Cl = 3.16, Na = 0.93.

• Bond polarity increases with greater differences in electronegativity.

Hydrogen Bonding

Hydrogen bonds are weak attractions between the partially positive hydrogen of one molecule and a partially negative atom (O or N) of another. While individually weak, collectively they are crucial for water's properties and biological molecules like DNA and proteins.

• Hydrogen bonds are noncovalent; electrons are not shared.

• Represented by dotted or dashed lines in diagrams.

Emergent Properties of Water

Cohesion and Adhesion

Water molecules stick to each other (cohesion) and to other polar substances (adhesion), both due to hydrogen bonding.

• Cohesion leads to surface tension, allowing insects to walk on water.

• Adhesion enables capillary action, helping water move up plant stems.

Temperature Moderation

Water has a high specific heat and heat of vaporization, meaning it resists temperature changes and absorbs heat when evaporating. This moderates Earth's climate and helps organisms maintain stable internal temperatures.

• Specific heat: Amount of energy required to raise 1g of water by 1°C.

• Evaporative cooling: As water evaporates, it removes heat from surfaces (e.g., sweating).

Ice Floats

At low temperatures, water forms a crystalline structure with stable hydrogen bonds, making ice less dense than liquid water. This allows ice to float, insulating aquatic life in winter.

• Most substances are denser as solids, but water is an exception due to hydrogen bonding.

Water as a Solvent

Water is an excellent solvent for polar and ionic substances due to its polarity. It dissolves salts, sugars, and many biomolecules, facilitating chemical reactions in cells.

• Hydrophilic (polar) substances dissolve easily in water.

• Hydrophobic (nonpolar) substances do not dissolve in water.

Water Ionization and pH

Ionization of Water

Water can ionize into hydrogen ions (H+) and hydroxide ions (OH–). The concentration of H+ determines the solution's acidity or basicity.

• Neutral water: [H+] = [OH–] = 1 × 10–7 M, pH = 7.

The pH Scale

The pH scale measures the concentration of hydrogen ions in a solution. It is logarithmic: each unit change represents a tenfold change in [H+].

• pH = –log10[H+]

• Acidic solutions: pH < 7 (higher [H+])

• Basic solutions: pH > 7 (lower [H+])

Acids, Bases, and Buffers

Acids donate H+ ions, lowering pH. Bases accept H+ ions, raising pH. Buffers are substances that minimize changes in pH by absorbing or releasing H+ as needed, maintaining homeostasis in biological systems.

• Example: Bicarbonate buffer system in blood.

Biological Importance of Water

Role in Life and Biomolecules

Water is essential for life, making up 70–95% of cells. Its unique properties support life by enabling chemical reactions, temperature regulation, and structural stability of biomolecules like proteins and DNA.

• Hydrogen bonds stabilize DNA double helix and protein structures.

• Water drives membrane formation via hydrophobic interactions.

Summary Table: Bond Types and Molecular Properties

Key Terms

• Polar molecule: A molecule with an uneven distribution of charge, resulting in partial positive and negative regions.

• Hydrogen bond: A weak attraction between a partially positive hydrogen and a partially negative atom (O or N).

• Hydrophilic: Water-loving; dissolves in water.

• Hydrophobic: Water-fearing; does not dissolve in water.

• Buffer: A substance that resists changes in pH.

Additional info: The emergent properties of water, such as cohesion, adhesion, high specific heat, and solvent abilities, are critical for the maintenance of life and the structure/function of biomolecules.