IB Bio A1.1 Water

Cohesion

Force by which molecules of the same type attract and stick together; in water, caused by hydrogen bonding

Cohesion: consequence for plants

Cohesion allows water molecules to be pulled up the xylem under tension without the water column breaking

Adhesion

Force by which molecules stick to surrounding surfaces/materials, not to each other

Adhesion: example

Water adheres to cellulose in plant cell walls/xylem via hydrogen bonds

Universal solvent

Water's ability to dissolve a wide range of polar and ionic substances

Universal solvent: how it works

Water molecules orient around a dissolved solute — the negative (oxygen) pole faces cations, the positive (hydrogen) pole faces anions, surrounding and separating the ions

Universal solvent: biological importance

Enables transport of nutrients and waste (e.g. in blood, in plant sap) and provides the medium in which most enzymes catalyze reactions

High specific heat capacity

The large amount of energy needed to raise water's temperature

High specific heat capacity: why

Much of the added energy goes into breaking hydrogen bonds restricting molecule movement, rather than raising kinetic energy/temperature

High specific heat capacity: consequence

Aquatic habitats and organisms' internal environments stay temperature-stable rather than fluctuating rapidly

Heat of vaporization

The large amount of energy required to convert liquid water into vapor

Heat of vaporization: why

Hydrogen bonds between water molecules must be broken for molecules to escape as vapor, which requires a lot of energy

Heat of vaporization: consequence

Evaporation removes a large amount of heat, making it an effective cooling mechanism (e.g. sweating)

Water is less dense as a solid

Ice is less dense than liquid water, so it floats — unlike most substances, which are denser as solids

Why ice is less dense than liquid water

In ice, hydrogen bonds become fixed/stable, holding molecules in a more open lattice structure than in liquid water, where bonds constantly break and re-form

Consequence of ice floating

Ice insulates the water below it, allowing aquatic organisms to survive freezing conditions

Surface tension

Property of water's surface that resists an external force, due to cohesion between water molecules

Why water has high surface tension

Surface molecules have no water molecules above them, so they form stronger hydrogen bonds with the molecules beside/below them, creating a net inward force

Surface tension: consequence for organisms

Water surfaces can be used as a habitat by small organisms (e.g. insects walking on water)

Viscosity

A measure of a fluid's resistance to flow

Viscosity of water vs. other liquids

Water has relatively low viscosity, letting it flow through narrow tubes, gaps, and pores; oil is more viscous than water

Effect of temperature on viscosity

As temperature increases, viscosity decreases (e.g. water at 25°C has about half the viscosity of water at 4°C)

Diffusion and viscosity relationship

Diffusion of molecules through a solvent is inversely proportional to the solvent's viscosity

Buoyancy

The ability of a fluid to exert a vertical upward force on an object placed in or on it

Archimedes' principle

A body immersed in a fluid is buoyed up by a force equal to the weight of the fluid it displaces

When does an object float vs. sink?

An object floats if its average density is less than the fluid's density, and sinks if its density is greater

Thermal conductivity

A measure of how easily heat flows through a material

Thermal conductivity of water vs. air

Water conducts heat about 28 times better than air

Hydrophilic

"Water-loving" — attracted to water (e.g. polar or charged molecules)

Hydrophobic

"Water-fearing" — repelled by water and insoluble in it (e.g. non-polar molecules)

Hydrophilic/hydrophobic example: cell membrane

The phospholipid bilayer has hydrophilic phosphate heads (face water) and hydrophobic fatty acid tails (face each other, away from water)

Capillary tubes

Channels with a very small internal diameter (e.g. in soil or plant cell walls)

Capillary action

The tendency of a liquid to move upward against gravity when confined within a narrow tube

Capillary action: mechanism

Adhesion between water and the tube's walls draws water upward, forming a curved meniscus

Solute

A dissolved molecule or ion in a solution

Solvent

A liquid in which another substance can be dissolved

Gas solubility in water: CO2

Carbon dioxide is soluble in water because it reacts to form carbonic acid, which dissociates into soluble hydrogen and hydrogencarbonate ions

Gas solubility in water: O2 and N2

Oxygen and nitrogen are only slightly soluble in water because they are non-polar and don't dissociate

Evolutionary consequence of low O2 solubility

Respiratory pigments like hemoglobin evolved to increase the oxygen-carrying capacity of blood

Enzymes and water

Most enzymes can only fold into their active shape and function while in an aqueous (water-containing) environment

Covalent bond (in water)

A bond within a water molecule where a pair of electrons is shared unequally between oxygen and each hydrogen atom

Why water is V-shaped

Water is not linear because of the geometry of oxygen's shared and unshared electron pairs, giving it a bent/V shape

Why water is a polar molecule

Oxygen is more electronegative than hydrogen, so shared electrons sit closer to oxygen, making oxygen slightly negative (δ-) and the hydrogens slightly positive (δ+), even though the molecule as a whole is neutral

Hydrogen bond (definition)

A weak intermolecular (between molecule) attraction between a slightly positive hydrogen on one water molecule and a slightly negative oxygen on a neighboring molecule

How hydrogen bonds form, step 1

Electrons in the covalent O-H bonds are shared unequally due to oxygen's higher electronegativity

How hydrogen bonds form, step 2

Electrons sit closer to oxygen, making oxygen δ- and the hydrogens δ+

How hydrogen bonds form, step 3

The δ+ hydrogen of one water molecule is electrostatically attracted to the δ- oxygen of a neighboring water molecule, forming a hydrogen bond between them

Common mistake: intra vs. inter bonds

Hydrogen bonds occur BETWEEN (inter) water molecules, never WITHIN (intra) a single molecule — covalent bonds are within a molecule, hydrogen bonds are between molecules

Common mistake: are individual hydrogen bonds strong?

No — a single hydrogen bond is weak, but the sheer number of hydrogen bonds collectively produces strong cohesive forces

Common mistake: heat capacity vs. specific heat capacity

Heat capacity = energy needed to change a body's temperature by 1°C (depends on mass/volume); specific heat capacity = energy needed to raise 1 kg of a substance by 1°C (independent of mass/volume)

Water vs. air: density

Water ≈ 998.2 kg/m³ vs. air ≈ 1.204 kg/m³ — water is far denser

Water vs. air: thermal conductivity

Water ≈ 0.598 W/mK vs. air ≈ 0.02154 W/mK — water conducts heat much better

Water vs. air: specific heat capacity

Water ≈ 4184 J/kg°C vs. air ≈ 1007 J/kg°C — water needs far more energy to change temperature

Water vs. air: dynamic viscosity

Water ≈ 1.002 × 10⁻³ kg/m·s vs. air ≈ 1.825 × 10⁻⁵ kg/m·s — water is far more viscous

Black-throated loon: what it is

A diving bird (Gavia arctica) adapted for both air and water

Loon: buoyancy problem

Bird anatomy (hollow bones, air trapped in feathers) is adapted for flight, which increases buoyancy — a disadvantage when trying to dive underwater for prey

Loon: buoyancy adaptation

Has solid(er) bones to add weight, and compresses air out of its lungs and feathers before diving to reduce buoyancy

Loon: viscosity/movement adaptation

Feathers are shaped to hold and deflect air for low-friction flight; uses webbed feet to generate resistance and propel itself through low-viscosity water

Loon: thermal conductivity adaptation

Traps air within its feathers to form an insulating layer and restrict convection currents, keeping body heat in despite water's high thermal conductivity

Ringed seal: what it is

A marine mammal (Pusa hispida) that lives in Arctic waters and on ice

Seal: buoyancy adaptation

Has a thick layer of subcutaneous fat (blubber) that increases buoyancy while also insulating the body

Seal: viscosity/movement adaptation

Streamlined body and flippers let it move efficiently through low-viscosity water, generating propulsion and resistance

Seal: thermal conductivity adaptation

Relies on blubber, a strong insulator, to prevent heat loss to the surrounding cold water and ice (both of which conduct heat far better than air)

Seal and specific heat capacity of the ocean

Water's high specific heat capacity keeps ocean temperature relatively stable year-round, letting the seal live and feed consistently despite changing air temperatures

Seal and ice (density link)

Because ice is less dense than water and floats, seals can live on top of it; they use their claws to dig breathing holes through the ice to live on/under it year-round

Why aquatic habitats have stable temperatures

Water's high specific heat capacity means a large amount of energy is needed to change its temperature, so bodies of water change temperature far more slowly than air does

Sweating: control

The hypothalamus in the brain controls sweating in humans

Sweating: mechanism

Sweat (water) is released onto the skin; as it evaporates, it removes a large amount of heat from the body due to water's high heat of vaporization, cooling the body

Evaporative cooling in plants

Plants increase their transpiration rate when at risk of overheating, cooling themselves as water evaporates from leaves

Evaporative cooling in dogs

Dogs pant, evaporating water from the tongue and airway, since they don't cool themselves effectively through sweating

Independent variable (IV)

The variable that is deliberately changed/manipulated by the experimenter

Dependent variable (DV)

The variable that is measured/observed, expected to change in response to the IV

Controlled variables

Variables kept constant throughout an experiment so they don't affect the DV, ensuring any change in the DV is due to the IV alone

Hypothesis

A testable, falsifiable prediction of the relationship between the IV and DV, often written as an "if…then…" statement

Control group/setup

A baseline condition (without the IV applied, or at a standard level) used for comparison against experimental results

Repeats/replicates

Repeating trials to check reliability and calculate an average, reducing the effect of random error

Extraplanetary origin of water: the problem

Earth formed about 4.5 billion years ago in conditions too hot for liquid water to exist, so Earth's water must have come from somewhere else

Extraplanetary origin of water: the source

Water likely arrived via asteroids/meteorites (e.g. carbonaceous chondrites), some of which are up to 28% water

Extraplanetary origin of water: the evidence

Deuterium-to-protium (hydrogen isotope) ratios in certain meteorites closely match Earth's ocean water, supporting them as the source

Extraplanetary origin of water: retention

Once temperatures cooled enough for water vapor to condense, Earth's gravity retained the water rather than letting it escape into space

Goldilocks Zone

Also called the habitable zone — the region around a star that's not too hot or cold, allowing liquid water to exist on a planet's surface

Goldilocks Zone: relevance to finding life

Scientists search for exoplanets in the Goldilocks Zone with a "water signature" (detected via transit spectroscopy) as candidates for supporting life