Hydrogen Bonding and Water: Key Points

Polarity and Partial Charges

Covalent bonds can be polar when electrons are shared unequally, creating partial charges on atoms. Water is the classic example: the molecule \mathrm{H_2O} has oxygen more electronegative than hydrogen, giving oxygen a partial negative charge and each hydrogen a partial positive charge.

Hydrogen Bonding

Hydrogen bonds are a relatively weak type of chemical interaction, stronger than van der Waals forces but weaker than covalent or ionic bonds. In a hydrogen bond, a hydrogen atom covalently bonded to an electronegative atom (the donor) interacts with another electronegative atom (the acceptor). The hydrogen carries a partial positive charge; the acceptor carries a partial negative charge.

Water: Structure and Consequences

Hydrogen bonding in water underpins its unique properties: high surface tension, high heat capacity, and solvent ability. Water is a versatile solvent for hydrophilic molecules. Water molecules are cohesive, contributing to high surface tension. Liquid water is denser than ice: \rho{\text{water}} > \rho{\text{ice}}.

Properties and Functions of Water

Water has a high heat capacity: when heat energy is added (e.g., from the sun), many hydrogen bonds break and reform, so the temperature rises slowly. The heat of vaporization is 540\ \text{cal/g}. About 60\% of body mass is water. Water also acts as a solvent, a lubricant, and supports cohesion and surface tension.

Biological Significance of Hydrogen Bonding

In DNA, hydrogen bonds form between complementary base pairs, specifically A\text{-}T and G\text{-}C, helping hold the double helix together. In proteins, hydrogen bonds help determine three-dimensional structure and stability. Hydrogen bonding also influences solvation, acid–base reactions, and the behavior of many organic and inorganic compounds.

Temperature Regulation and Body Water

Water’s high heat capacity helps regulate body temperature; heat energy from the environment is used to disrupt and reform hydrogen bonds, so the temperature changes are minimized. Approximately 60\% of body mass is water. Evaporation of water via sweat glands provides effective cooling.