A1.1 Water Concepts and Notes

Build an A1.1 Water explanation from the molecules upwards: identify partial charges, distinguish hydrogen from covalent bonds, and connect each physical property to a biological consequence. Use the diagrams to test your explanation before reading on, then try the Water questions for your selected level.

Syllabus
First assessment 2025
Topic
A1.1
Level
—

Water: life’s medium

Early life

The first cells originated in water. Ocean water shielded early life from harmful ultraviolet (UV) radiation from the Sun, allowing life to evolve.

Cells today

  • Water remains the medium—the environment—in which most life processes occur.
  • Cells are water-rich: about 80% of a human cell consists of water.

From polar bonds to hydrogen bonds

Each O–H covalent bond involves a shared pair of electrons. Oxygen attracts these electrons more strongly than hydrogen, so sharing is unequal: oxygen becomes slightly negative and each hydrogen slightly positive. Together with water’s bent shape, this creates an uneven distribution of charge. Water is polar, although the molecule remains electrically neutral overall.

Two bent water molecules, each with an oxygen labelled δ− and two hydrogens labelled δ+. Solid lines join each oxygen to its hydrogens. A dashed line connects a hydrogen of the left molecule to the oxygen of the right molecule.
1

Draw complete molecules

Draw at least two separate, bent water molecules. Each must contain one O and two H atoms. Join each O to its two H atoms with solid lines: these are covalent bonds within each molecule.

2

Mark partial charges

Label every oxygen δ− and every hydrogen δ+, as in the figure. The δ symbol indicates a partial charge, not a full ionic charge.

3

Connect different molecules

Opposite partial charges attract. Draw a dashed line from a δ+ hydrogen on one molecule to the δ− oxygen on another: this represents a hydrogen bond between molecules. Hydrogen bonds are weaker than covalent bonds, but help hold water molecules together.

Align Water Molecules

Practice only · progress is not saved

Identify bonding in water

[ 1 ]

Where do hydrogen bonds form?

Keeping a xylem water column connected

Many weak bonds, strong cohesion

Cohesion is attraction between molecules of the same substance. Hydrogen bonds between neighbouring water molecules make them stick together. Individual hydrogen bonds are weak and continually break and reform, but many acting together give water strong cohesion.

Where the column travels

Find the large hollow vessel among the smaller cells. Its lumen is the space through which a water column travels. The micrograph shows vessel structure, not cohesion or water under tension.

Green-tinted micrograph showing a large, cut-open xylem vessel surrounded by smaller plant cells. Two unlabeled black arrows point toward the vessel’s interior and edge.

Withstanding tension

Evaporative water loss from leaves creates a pull on xylem water. The column is under tension—a pulling force. Cohesion keeps neighbouring water molecules connected, allowing the column to transmit the pull and move upwards without pulling apart. Cohesion maintains the column; it does not create the pull.

A habitat on the water surface

Water molecules attract one another through hydrogen bonding: cohesion. Each hydrogen bond is weak, but many together produce a large cohesive force.

Feature Within the water At the surface
Water neighbours Surround the molecule Beside and below; none above
Cohesive forces Cancel out: no net force Do not cancel: net inward force

The net inward attraction makes the surface contract and resist stretching or breaking. This is surface tension. It also pulls water droplets into nearly spherical shapes.

A pond skater

  • A pond skater’s waxy cuticle prevents its body from becoming wet.
  • Its mass is small enough that its weight does not break the water surface.

Surface tension supports the insect, allowing it to move across the surface and use it as a habitat.

How water rises in a narrow channel

Capillary action is the movement of water upward against gravity in a very narrow channel.

  • Black dashed arrows between blue particles: cohesion—attraction between water molecules, produced collectively by many hydrogen bonds.
  • White arrows beside the grey walls: adhesion—attraction between water and polar or charged materials.
  • Large grey arrows: upward capillary force opposes the downward weight of the water column.
Water between two grey capillary walls curves upward at each wall. Black dashed arrows connect blue water particles, and white arrows connect water with the walls. A large arrow labelled “capillary force” points upward; another labelled “weight of the water column” points downward.

Notice that the water surface rises beside both walls. Adhesion draws water upward at the walls; cohesion keeps neighbouring water molecules connected as water is drawn up. Together, these attractions support capillary action.

In xylem transport, cohesion is more important than adhesion: it allows the water column to resist tension without breaking.

Capillary pathways in soil and cell walls

Capillary pathways need not be separate vessels: tiny soil channels and spaces between cellulose fibres also provide narrow routes for water.

Feature Soil Plant cell walls
Narrow pathway Thin, vertical soil channels Spaces between cellulose fibres
Surface water adheres to Channel walls Partly polar, hydrophilic cellulose
Water movement Up from the water table toward roots From nearby xylem into cell walls, like a wick
Biological consequence Helps supply roots when the ground becomes dry Keeps air-exposed mesophyll cell walls moist

Exam practice: Recognize an adhesion consequence

[ 1 ]

Which of the following is a consequence of the adhesive properties of water?

How water surrounds ions

Water dissolves ionic substances such as sodium chloride. Each dissolved ion becomes surrounded by a layer of oriented water molecules: hydration.

Ion Water end facing the ion
Na⁺: positive Oxygen: δ−
Cl⁻: negative Hydrogens: δ+

Opposite charges attract, so water’s partial charges explain the contrasting orientations. Once dissolved, the ions are free to move through the water.

The figure shows already-separated, hydrated ions—not a salt crystal dissolving.

Diagram of water surrounding Na⁺ and Cl⁻ ions. Each bent water molecule has a red oxygen marked δ− and two grey hydrogens marked δ+. Oxygen points toward Na⁺; hydrogens point toward Cl⁻.

What dissolves in water?

A wide variety of hydrophilic molecules dissolve in water through interactions involving charged or polar groups. Water is the solvent; the dissolved substance is the solute.

Example Interaction with water Outcome
Amino acids Ionized groups attract water’s partial charges Dissolve
Sugars Polar hydroxyl (–OH) groups form hydrogen bonds with water Dissolve
Oil Non-polar; hydrophobic Remains separate rather than dissolving

Hydrophobic substances do not mix readily with water, but low solubility need not mean zero solubility. O₂ and N₂ are only slightly soluble: they are non-polar, do not form hydrogen bonds with water, and do not undergo dissociation or ionization in it.

Water as a working medium

An aqueous solution has water as its solvent. A wide variety of hydrophilic substances, including ions and sugars, dissolve in it.

Transport

In both plants and animals, water provides a medium for transport: dissolved hydrophilic substances can be carried with moving water.

Metabolism

Dissolved molecules and ions are free to move by diffusion. This mobility makes them more chemically reactive than in an undissolved solid, supporting metabolic reactions.

  • Most enzymes catalyse reactions in aqueous solution. Water helps maintain their shape and stability so they can function effectively.
  • This does not mean enzymes must themselves dissolve. Some are fixed within cell membranes, localizing reactions to particular sites.

When insolubility is useful

Cell membranes contain phospholipids with hydrophobic tails. These non-polar tails do not dissolve in water; they form the membrane’s internal structure.

Here, remaining insoluble is essential to the function: the tails contribute to a cellular structure rather than becoming dispersed as dissolved solutes. Being water-soluble is not useful for every molecular role.

Buoyancy: support that divers must overcome

Left: the black-throated loon (Gavia arctica) floats partly submerged. Right: the ringed seal (Pusa hispida) rests on ice beside water. These are the two animal contexts for our comparison.

Left: a black-throated loon floats with its body partly submerged and its head and neck above water. Right: a ringed seal lies on an ice edge, with flippers visible beside the water.

Buoyancy is the upward force a fluid exerts on an object. Water is much denser than air, so it provides greater buoyant support for the same displaced volume. This supports floating animals but opposes downward movement during diving.

Density is mass per unit volume. An object floats if its average density is lower than water’s and sinks if it is higher.

Feature Black-throated loon Ringed seal
Relevant adaptation Solid bones, unlike the light, hollow bones of many birds; compression of air in lungs and feathers Blubber: a thick layer of fat beneath the skin
Buoyancy consequence Solid bones add weight; air compression reduces buoyancy, helping the loon dive for prey Blubber acts as a buoyancy aid, helping support the seal in water

Movement: reduce drag, generate thrust

Viscosity is resistance to flow. Water is more viscous than air, making resistance to movement a greater challenge in water.

A swimming animal needs two complementary features: a body that reduces drag and surfaces that push against water to propel it.

Movement task Black-throated loon Ringed seal
Reduce drag Hydrodynamic body shape; feet positioned laterally near the rear reduce turbulent eddies Hydrodynamic body shape allows easier movement through water
Generate propulsion Webbed feet provide a large surface area to push against water Flippers push against water to propel the body

In less viscous air, the loon’s light feathers move with less friction. Its plumage holds and deflects air to produce lift during flight. In water, its webbed feet push against the fluid instead: resistance at a propelling surface is useful, even while drag on the body is reduced.

Why aquatic temperatures change slowly

Specific heat capacity is the heat energy needed to raise the temperature of a unit mass by 1°C. Unlike the heat capacity of a whole body, it does not depend on the amount of material.

Property or response Water Air
Specific heat capacity Higher Lower
Temperature rise for the same heat input per unit mass Smaller Larger

Hydrogen bonds restrict movement between water molecules. Disrupting many of these bonds requires substantial energy, helping explain water’s high specific heat capacity.

  • Aquatic habitats change temperature slowly as surrounding air temperatures change. Relatively stable sea temperatures allow ringed seals to live and feed throughout the year.
  • Cells and organisms contain much water. Absorbing or losing heat without a large temperature change cushions them against temperature fluctuations.

Stable water can still carry heat away

Specific heat capacity concerns temperature change; thermal conductivity concerns how readily heat is transferred by conduction. A habitat can change temperature slowly yet conduct heat away from an animal readily.

Water has higher thermal conductivity than air. When the surroundings are colder than an animal’s body, water conducts heat away more readily than air, creating a greater heat-loss challenge.

Insulation feature Black-throated loon Ringed seal
Insulating layer Air trapped in feathers Thick blubber beneath the skin
Effect on heat transfer Air’s low thermal conductivity reduces conduction Blubber reduces heat transfer from the body
Additional protection Trapped air moves little, restricting convection currents Blubber provides insulation in cold water

Thermal conductivity and seals

[ 1 ]

Which characteristic of the ringed seal (Pusa hispida) is a consequence of the thermal conductivity of water?

Water’s biological roles and aquatic habitats: checkpoint

Name the property for each consequence before checking the right-hand column.

Attraction and dissolved substances

Biological consequence Property to retrieve
Xylem water column resists tension Cohesion: water–water attraction
Pond skater uses the water surface as a habitat Surface tension, a consequence of cohesion
Water rises through soil channels and wets cellulose cell walls Adhesion to polar or charged surfaces supports capillary action
Hydrophilic solutes are transported in plants and animals; most enzymes catalyse reactions in aqueous solution Solvent properties

Support, movement and temperature

Aquatic consequence Property to retrieve
Water provides upward support; the black-throated loon must overcome it to dive Buoyancy
Streamlined loons and ringed seals reduce resistance during movement Viscosity: water is more viscous than air
Aquatic temperatures change relatively slowly compared with air temperatures Specific heat capacity: higher in water than air
Ringed seal blubber limits heat loss in water Thermal conductivity: water conducts heat better than air
  • Thermal distinction: changing temperature → specific heat capacity; transferring heat → thermal conductivity.
  • Solubility boundary: hydrophobic, insoluble phospholipid tails form the membrane interior—their function depends on remaining insoluble.

Water’s arrival and retention (HL)

HL only

Delivery hypothesis

Water-bearing asteroids are hypothesised to have delivered water to Earth. Heating during impacts could release their water as vapour. This is the assessed origin hypothesis—not proof that all Earth’s water came from space.

Reasons for retention

Delivery brings water to Earth; retention keeps it there through two conditions:

  • Low enough temperatures: water vapour could condense into liquid water as Earth cooled.
  • Gravity: Earth’s gravitational attraction retained water rather than allowing it to disperse into space.

Why persistence mattered

The abundance of water over billions of years of Earth’s history allowed life to evolve.

HL: Identify a water-retention condition

HL only

[ 1 ]

It has been hypothesized that asteroids were the primary source of Earth's water. What has caused the retention of water on Earth?

Goldilocks zones and the search for life — HL

HL only

Life as we know it depends on water. Searching for water therefore helps scientists identify planets that might support life.

The Goldilocks zone, or habitable zone, is the range of distances from a star where surface liquid water could exist under suitable conditions.

  • Mercury: “too hot” — the near-star cue illustrates conditions where water can boil and evaporate.
  • Earth: “just right” — Earth lies within the shaded habitable band, between the hot and cold cues.
  • Jupiter: “too cold” — the farther-from-star cue illustrates conditions where water can freeze.
Solar System diagram with a shaded band labelled habitable zone. Mercury is marked “too hot”, Earth “just right”, and Jupiter “too cold”. Earth lies within the band. The diagram is labelled “Not to scale”.

Being in the zone does not demonstrate that liquid water or life is present: atmospheric conditions also matter. The figure is not to scale, so use its labelled contrasts, not measured distances.