A1.1 Water
Water’s polarity and hydrogen bonding create solvent, thermal, cohesive and adhesive properties that support metabolism, transport, cooling and aquatic life.
- Syllabus
- First assessment 2025
- Topic
- A1.1
- Level
- HL
Learning objectives
A1.1.1Water as the medium for life• First cells originated in water, where oceans shielded early life from UV radiation• Water remains the medium for most life processes and forms much of cell massA1.1.2Hydrogen bonds in water• O-H covalent bonds and water's V-shape make molecules polar• Hydrogen bonds form between water molecules, not within a molecule• Hydrogen bonding explains many unusual water propertiesA1.1.3Cohesion of water molecules• Hydrogen bonding creates cohesion and surface tension• Cohesion prevents xylem water columns breaking under tension• Surface tension supports droplets and surface-skating insectsA1.1.4Adhesion of water• Water adheres to polar or charged surfaces• Adhesion supports capillary action in soil channels and cellulose cell walls• Cohesion is the stronger force in xylem transport; adhesion helps refill vesselsA1.1.5Solvent properties of water• Water dissolves ions and polar molecules by hydration and hydrogen bonding• Aqueous solution enables diffusion, transport, enzyme activity, and metabolism• Non-polar molecules are hydrophobic; O₂ and N₂ have low solubility in waterA1.1.6Physical properties of water for aquatic animals• Specific heat capacity stabilizes aquatic temperatures and body temperature• Thermal conductivity, buoyancy, and viscosity shape aquatic adaptations• Compare black-throated loon and ringed seal adaptations for diving, insulation, and movementA1.1.7(HL)—Extraplanetary origin of water on Earth• Asteroid delivery is the syllabus hypothesis for Earth's water origin• Carbonaceous chondrites and Vesta meteorites provide isotope evidence• Gravity and cooler temperatures allowed water vapour to condense and be retainedA1.1.8(HL)—Search for extraterrestrial life and water• Liquid water guides searches for extraterrestrial life• Goldilocks zones mark where surface liquid water could exist• Transit spectroscopy can detect water signatures in exoplanet atmospheres
Molecular shape turns bond polarity into a polar molecule
Oxygen attracts the shared electrons in each O–H covalent bond more strongly than hydrogen. Oxygen therefore carries a partial negative charge, δ−, while each hydrogen carries δ+. Because the molecule is bent, the two bond polarities do not cancel.
A hydrogen bond is the attraction between a δ+ hydrogen on one water molecule and a δ− oxygen on another. It is an intermolecular attraction, not the covalent bond holding O and H together inside one molecule.
One hydrogen bond is weak compared with an O–H covalent bond. Water's unusual properties arise because enormous numbers of hydrogen bonds act together while continually breaking and reforming.
Cohesion has a bulk effect and a surface effect
Cohesion is attraction between water molecules. Many hydrogen bonds together allow a body of water to transmit force without the molecules separating easily.
In xylem, evaporation from leaves creates tension. Cohesion keeps water molecules connected, so the pull is transmitted down a continuous column and the column resists breaking.
At the surface, water molecules have no neighbours above them, so cohesive attractions have a net inward effect. The surface resists stretching or breaking; a small pond skater with water-repellent legs can spread its weight without rupturing it.
Cohesion maintains the xylem column but does not create the transpiration pull. Surface tension supports an organism at a boundary; buoyancy supports an immersed body through displaced fluid.
Adhesion depends on what water is touching
| Force | Attractive contact | Main biological consequence |
|---|---|---|
| Cohesion | water ↔ water | maintains a connected xylem column that can resist tension |
| Adhesion | water ↔ polar or charged surface | wets cellulose and pulls water along narrow spaces |
In narrow soil pores and spaces between cellulose fibres, adhesion pulls water along the hydrophilic wall. Cohesion draws neighbouring water molecules with it, producing capillary movement and keeping leaf cell walls moist.
Adhesion helps water enter and wet narrow spaces, but cohesion is the dominant force preventing a transpiring xylem column from breaking under tension.
Partial charges predict what water can carry
Around Na+, the δ− oxygen ends of water face inward; around Cl−, the δ+ hydrogen ends face inward. Water–ion attractions separate the ions and stabilize a mobile hydration shell around each one.
| Substance | Interaction with water | Biological result |
|---|---|---|
| ions and polar molecules | charges or polar groups attract water; polar groups may hydrogen-bond | dissolution enables diffusion, reactions and transport |
| non-polar molecules or regions | no equivalent favourable attraction | poor solubility helps form water-excluding boundaries such as membrane interiors |
O₂ and N₂ are non-polar and only slightly soluble. Respiratory pigments such as haemoglobin therefore raise the oxygen-carrying capacity of blood far above that of water alone.
A loon and a seal meet the same habitat in different ways
| Aquatic challenge | Black-throated loon | Ringed seal |
|---|---|---|
| reduce heat loss | trapped air in feathers limits conduction and convection | thick blubber insulates beneath the skin |
| produce thrust with low drag | streamlined body; rear webbed feet push water backward | streamlined body; flippers provide propulsion |
| manage buoyancy while diving | solid bones and compressed air reduce upthrust | blubber adds buoyancy, so swimming force is needed to descend |
An adaptation is explained by linking a structural feature to the relevant property of water and then to its consequence for heat balance, propulsion or diving—not by listing the feature alone.
Water had to arrive—and then remain on Earth
Early Earth was initially too hot for water vapour to condense. One supported hypothesis is that water-bearing asteroids formed in cooler regions and delivered water during impacts.
Evidence supporting an asteroid contribution includes:
- water held in minerals of carbonaceous chondrites
- hydrogen-isotope ratios in some meteorites, including Vesta-derived samples, that resemble Earth's water
Delivery alone would not create oceans. As Earth cooled, water vapour could condense; Earth's gravity then limited escape to space, allowing persistent surface water.
The isotope similarity supports an asteroid contribution but does not prove that every water molecule came from one source. Arrival and retention are separate causal requirements.
A habitable-zone orbit is a filter, not a discovery of life
A Goldilocks or habitable zone is the range of distances from a star where temperature could permit liquid water at a planet's surface. Planet size, gravity and atmosphere also matter, so zone membership establishes possibility only.
During a transit, some starlight passes through an exoplanet's atmosphere. Molecules absorb characteristic wavelengths, so the spectrum can reveal a water-vapour signature.
| Observation | Supported claim | Not established |
|---|---|---|
| orbit lies in the habitable zone | surface liquid water could be physically possible | water is present |
| atmospheric water signature | water vapour is present in the atmosphere | surface liquid water or life exists |