Water molecules and polarity

· Water (H₂O) is a polar molecule because oxygen is more electronegative than hydrogen.
· The oxygen atom has a partial negative charge (δ−); each hydrogen atom has a partial positive charge (δ+).
· The molecule is bent, so the charges are unevenly distributed.
· Polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules or ions.

This image shows the bent shape of a water molecule and its dipole moment. It helps explain why water is polar and why it can form hydrogen bonds. Source

Hydrogen bonding between water molecules

· A hydrogen bond forms when a δ+ hydrogen atom in one water molecule is attracted to a δ− oxygen atom in another water molecule.
· Hydrogen bonds are individually weak but collectively important because many form between water molecules.
· These bonds explain water’s key biological properties: solvent action, high specific heat capacity and high latent heat of vaporisation.
· In exam answers, always link the property back to hydrogen bonding and then to the role in living organisms.

This diagram shows hydrogen bonds forming between neighbouring water molecules. It clearly links water’s polarity to the intermolecular attractions responsible for its biological properties. Source

Solvent action

· Water is an excellent solvent for many ionic and polar substances.
· Ions dissolve because the δ− oxygen end of water is attracted to positive ions, while the δ+ hydrogen end is attracted to negative ions.
· Polar molecules dissolve because they can form hydrogen bonds with water.
· This allows substances such as glucose, amino acids, mineral ions and many metabolic wastes to be transported in solution.
· Biological role: water acts as a transport medium in blood plasma, tissue fluid, xylem and phloem.
· Biological role: water provides an aqueous medium for metabolic reactions inside cells.

This diagram shows how polar water molecules surround Na⁺ and Cl⁻ ions. It illustrates solvent action, where water separates and disperses ions in solution. Source

High specific heat capacity

· Specific heat capacity = the amount of energy needed to raise the temperature of a substance.
· Water has a high specific heat capacity because a lot of energy is needed to break hydrogen bonds before water molecules move faster.
· This means water temperature changes slowly when heat is gained or lost.
· Biological role: water helps maintain a stable internal temperature in organisms.
· Biological role: water helps provide stable aquatic environments, reducing sudden temperature changes that could affect enzyme-controlled reactions.
· Exam phrase: hydrogen bonds absorb thermal energy, reducing temperature fluctuation.

High latent heat of vaporisation

· Latent heat of vaporisation = the energy needed to change water from a liquid to a gas.
· Water has a high latent heat of vaporisation because many hydrogen bonds must be broken for water molecules to evaporate.
· Evaporation removes high-energy water molecules, so the remaining surface becomes cooler.
· Biological role: sweating cools mammals as water evaporates from the skin.
· Biological role: transpiration can cool leaves as water evaporates from cell walls and diffuses out through stomata.
· Exam phrase: evaporation of water removes heat energy from the organism or surface.

This graph shows that energy can be absorbed by water without an immediate temperature rise during state changes. It supports the idea that water has a high heat capacity and a high latent heat of vaporisation. Source

Common exam links

· Hydrogen bonding → high specific heat capacity → stable temperatures → enzymes work efficiently.
· Hydrogen bonding → high latent heat of vaporisation → evaporation removes heat → cooling by sweating/transpiration.
· Polarity → solvent action → transport of dissolved substances → metabolic reactions occur in solution.
· Do not just state “water is polar”; explain how δ+ and δ− regions cause hydrogen bonding or solvent action.
· For full marks, use the structure-function chain: property → cause → biological role.