Water and Life

Core Concept

Water is the substance in which all life's chemistry takes place. Its remarkable properties flow from a single structural fact: the water molecule is polar. Because oxygen is much more electronegative than hydrogen, the shared electrons in each O–H bond spend more time near oxygen, giving it a partial negative charge (δ−) and leaving the hydrogens partially positive (δ+). The molecule's bent shape prevents these charges from canceling, so water has distinct positive and negative ends. Adjacent water molecules therefore attract each other — a positive hydrogen of one is drawn to the negative oxygen of another — forming hydrogen bonds.

These hydrogen bonds explain nearly everything water does: it sticks to itself (cohesion), sticks to other polar surfaces (adhesion), resists temperature change (high specific heat), requires a lot of energy to evaporate (high heat of vaporization), expands when it freezes, and dissolves ions and polar molecules while excluding nonpolar ones. Water's ability to ionize into H⁺ and OH⁻ also gives life a chemical yardstick — the pH scale — that cells regulate tightly.

Key Concepts

Cohesion, Adhesion, and Surface Tension

Cohesion is the attraction between water molecules themselves, a consequence of hydrogen bonding. It lets water move up tall plants as a continuous column, with water evaporating from leaves pulling more water behind it. Adhesion is water's attraction to other polar or charged surfaces, such as the walls of narrow plant vessels, which helps the column cling and climb. Surface tension — the "skin" on water that lets a water strider stand on a pond — is a direct result of cohesion: surface molecules are pulled inward and sideways by their neighbors.

High Specific Heat and Heat of Vaporization

Specific heat is the amount of heat required to raise the temperature of 1 gram of a substance by 1 °C. Water's specific heat is unusually high because much of the added heat goes into breaking hydrogen bonds rather than speeding up molecular motion. This lets large bodies of water — and the human body — resist rapid temperature swings. Heat of vaporization is the energy needed to convert liquid to gas. Because water's hydrogen bonds must be broken for a molecule to escape as vapor, evaporation carries away a lot of heat; this is evaporative cooling, the principle behind sweating and panting.

Ice Floats

Most substances contract when they freeze, but water expands. As water cools, hydrogen bonds lock molecules into a crystalline lattice that holds them slightly farther apart than in liquid water, so ice is less dense and floats. The floating ice sheet insulates the liquid water beneath, allowing aquatic life to survive winter rather than freezing from the bottom up.

Water as a Solvent

Water is an excellent solvent for ionic and polar substances (solutes). Its partial charges surround and separate solute particles — the positive ends cluster around anions, the negative ends around cations — forming a hydration shell. Substances that dissolve readily are hydrophilic ("water-loving"); those that do not, because they are nonpolar, are hydrophobic ("water-fearing"). This is why oil and water separate, and it is the reason cell membranes self-assemble from phospholipids.

pH, Acids, Bases, and Buffers

A small fraction of water molecules ionize: H₂O ⇌ H⁺ + OH⁻. The concentration of hydrogen ions determines acidity, measured on the pH scale: pH = −log₁₀[H⁺], where [H⁺] is the hydrogen-ion concentration in moles per liter (M). Because the scale is logarithmic, each one-unit change is a tenfold change in [H⁺]: a solution of pH 4 has ten times the H⁺ of pH 5. A pH below 7 is acidic (more H⁺), 7 is neutral, and above 7 is basic/alkaline (fewer H⁺). Acids donate H⁺ (or increase H⁺ in solution); bases accept H⁺ or release OH⁻. Buffers are substances that minimize pH change by absorbing or releasing H⁺; the carbonic acid–bicarbonate system keeps human blood near pH 7.4.

How It Works

The chain from structure to behavior is direct: oxygen's high electronegativity → partial charges on a bent molecule → hydrogen bonds between water molecules → cohesion, high specific heat, and high heat of vaporization. For example, when you sweat, heat from your skin is used to break the hydrogen bonds that tether surface water molecules to the liquid, so those molecules escape as vapor, carrying the energy away and cooling you. Buffering works by equilibrium: H₂CO₃ (carbonic acid) ⇌ HCO₃⁻ (bicarbonate) + H⁺. If H⁺ rises, the reaction shifts left, consuming H⁺; if H⁺ falls, it shifts right, releasing H⁺ — resisting pH change in either direction.

How it works

The chain from structure to behavior is direct: oxygen's high electronegativity → partial charges on a bent molecule → hydrogen bonds between water molecules → cohesion, high specific heat, and high heat of vaporization. For example, when you sweat, heat from your skin is used to break the hydrogen bonds that tether surface water molecules to the liquid, so those molecules escape as vapor, carrying the energy away and cooling you. Buffering works by equilibrium: H₂CO₃ (carbonic acid) ⇌ HCO₃⁻ (bicarbonate) + H⁺. If H⁺ rises, the reaction shifts left, consuming H⁺; if H⁺ falls, it shifts right, releasing H⁺ — resisting pH change in either direction.

Common confusions

• "pH 6 is slightly less acidic than pH 5." It is ten times less acidic — the scale is logarithmic, not linear.
• "Hydrophobic means 'afraid of water' and such molecules are polar." Hydrophobic molecules are nonpolar; they don't dissolve because they cannot form favorable interactions with water, not because of any "fear."
• "Cohesion and adhesion are the same thing." Cohesion is water-to-water; adhesion is water-to-other-surface.
• "Bases are only things that release OH⁻." Many bases are H⁺ acceptors (e.g., ammonia, bicarbonate) without literally releasing hydroxide.
• "Ice floats because it is colder." Ice floats because it is less dense — its hydrogen-bonded lattice spaces molecules farther apart.

Quick review

• Polarity → hydrogen bonding → cohesion, adhesion, surface tension.
• High specific heat and heat of vaporization buffer temperature and power evaporative cooling.
• Ice is less dense than water and floats.
• Water dissolves polar/ionic (hydrophilic) solutes; hydrophobic molecules do not dissolve.
• pH = −log[H⁺]; each unit is a tenfold difference in H⁺ concentration.
• Buffers maintain pH; blood is buffered near 7.4.