Revise Water for IB Biology (A1.1)

A water molecule is one oxygen atom joined to two hydrogen atoms by covalent bonds — pairs of shared electrons. But oxygen is far more electronegative than hydrogen, meaning it pulls the shared electrons more strongly towards itself. The electrons therefore spend more time near the oxygen, giving it a partial negative charge (written δ-) and leaving each hydrogen with a partial positive charge (δ+). Each O–H bond is said to be POLAR. Crucially, the molecule is not straight: the two O–H bonds meet at an angle of about 105°, so the molecule has a bent, wide-V shape. Because it is bent, the two bond dipoles do not point in opposite directions and cannot cancel — the oxygen end stays δ- and the hydrogen end stays δ+. The result is a molecule with an overall separation of charge: water is a POLAR molecule. (Contrast this with a symmetrical non-polar molecule such as methane, CH₄, where any small dipoles cancel and no overall charge separation remains.)

Because each water molecule has a δ+ end and a δ- end, neighbouring molecules attract one another electrostatically: the δ+ hydrogen of one molecule is drawn to the δ- oxygen of another. This attraction is called a HYDROGEN BOND. It is an INTERMOLECULAR force — it acts between separate molecules — and it must not be confused with the far stronger covalent bonds WITHIN a molecule. A single hydrogen bond is weak, easily broken by thermal motion, and in liquid water hydrogen bonds are continually breaking and reforming. Their power lies entirely in their number: at any instant each water molecule can hydrogen-bond to up to four neighbours, so a sample of water is held together by a vast, self-healing network of these weak attractions. That collective grip is what gives rise to water's emergent properties — properties of the bulk liquid that no single molecule possesses on its own.

Cohesion is the attraction of water molecules to ONE ANOTHER through hydrogen bonding. It has two important consequences. First, it produces SURFACE TENSION: molecules at the surface are hydrogen-bonded only to neighbours below and beside them, so the surface behaves like a thin elastic skin — strong enough for pond skaters and other small insects to stand on. Second, cohesion holds a column of water together, so that a pull applied at the top is transmitted all the way down without the column snapping. ADHESION is different: it is the attraction of water molecules to OTHER polar surfaces, such as the cellulose of plant cell walls, again by hydrogen bonding. Adhesion lets water cling to and creep along such surfaces. In plants, cohesion and adhesion act together in the xylem: water evaporating from the leaves creates tension, cohesion transmits that pull down an unbroken column, and adhesion helps hold the column against the vessel walls, drawing water up against gravity.

Two thermal properties both come from having to deal with hydrogen bonds. Water has a HIGH SPECIFIC HEAT CAPACITY: raising its temperature takes a large amount of energy, because incoming heat must first break hydrogen bonds before it can speed the molecules up. So water heats and cools slowly, buffering temperature — cell contents stay thermally stable, large bodies of water resist rapid temperature swings, and aquatic habitats remain more constant than the air above them. Water also has a HIGH LATENT HEAT OF VAPORISATION: turning liquid water into vapour requires a large amount of energy, because the many hydrogen bonds holding molecules in the liquid must be broken to release them as gas. This makes evaporation a powerful cooling mechanism — when water evaporates it carries away a great deal of heat. Mammals exploit this by SWEATING: as sweat evaporates from the skin, it removes heat and lowers body temperature. Plants do the same through TRANSPIRATION: evaporation of water from the leaves cools them, which matters on a hot, sunlit day.

Because water is polar, it interacts strongly with other charged and polar particles. Around a positive ion the δ- oxygen ends of water molecules cluster; around a negative ion the δ+ hydrogen ends cluster. These HYDRATION SHELLS pull the particles apart and keep them in solution, so water dissolves ionic compounds (such as sodium chloride) and polar molecules (such as glucose and amino acids). Substances that dissolve in or interact readily with water are called HYDROPHILIC (‘water-loving’) — they are polar or charged. Substances that do not dissolve, such as fats and oils, are HYDROPHOBIC (‘water-fearing’) — they are non-polar, and water molecules prefer to hydrogen-bond with each other rather than surround them. Water's solvent power makes it the transport medium of life: blood plasma carries dissolved glucose, ions, amino acids and wastes, and xylem and phloem carry dissolved minerals and sugars. Equally, water's inability to dissolve hydrophobic molecules is essential — it drives phospholipid tails together to form the cell membranes that compartmentalise every cell.

Two further properties shape life in water. Unusually, water is LESS dense as a solid than as a liquid: in ice the hydrogen bonds lock molecules into an open lattice that holds them further apart than in the liquid. So ice floats, forming an insulating layer that keeps the water beneath from freezing — aquatic organisms survive the winter below it, and floating ice also provides buoyant support and a surface for some animals. Liquid water is also TRANSPARENT, allowing light to penetrate to a useful depth. This lets aquatic plants, algae and other photosynthesisers capture light below the surface, supporting the food chains of ponds, lakes and the sunlit upper ocean.

A1.1 explain questions are marked ANALYTICALLY, not against a level or markband: each distinct valid biological point is worth one mark. Reasoning marks (M) credit a correct piece of mechanism (for example polarity → hydrogen bonding), answer marks (A) credit the correct LINK to a biological role, and error-carried-forward (ECF) means that if you build correctly on an earlier idea you keep the later marks even if a detail slips. Equivalent wording is accepted — you do not have to use the exact phrases below — but the engine rejects vague claims with no mechanism. Study how each mark is tied to a specific named idea.

Every idea here reappears across the course. Hydrogen bonding returns in the pairing of DNA bases and in the folding of proteins; water as a solvent underlies enzyme reactions, blood transport and membrane structure; and evaporative cooling connects to thermoregulation and to transport in plants. Hold onto the single habit this lesson teaches — trace each property back to the polar, hydrogen-bonded molecule, and forward to a biological role — and much of the chemistry of life becomes a story about one small bent molecule.