IB Biology SLTopic 1 — Water & Its PropertiesPaper 1 & 2Core idea~13 min read
Water’s Physical & Chemical Properties
Water climbs a hundred metres up a tree, dissolves almost anything a cell needs, keeps a lake liveable through winter, and floats when it freezes. Every one of those is the same trick repeated — hydrogen bonds — and the exam wants you to trace each property back to them.
📘 What you need to know
Cohesion = water sticking to itself. Adhesion = water sticking to other things.
Together they pull continuous columns of water up the xylem of plants.
Cohesion at the surface creates surface tension, strong enough for insects to stand on.
Water dissolves polar and charged substances (hydrophilic) but not non-polar ones (hydrophobic).
Water has a high specific heat capacity, so its temperature changes slowly — stable habitats, stable cells.
Water has high thermal conductivity and high viscosity compared with air, which shapes how animals stay warm and move.
Ice floats, because it is less dense than liquid water. That is unusual, and it matters enormously.
Cohesion and adhesion
Two words that look similar and mean different things. The spelling helps: COhesion is water molecules co-operating with each other; ADhesion is water sticking to something else, the way an advert sticks to a wall.
Both come straight from hydrogen bonding. Water molecules hydrogen-bond to each other, which is cohesion. Water also hydrogen-bonds to any other polar or charged surface — like the cellulose lining a xylem vessel — and that is adhesion.
The pull comes from the top, not a push from the roots. Because cohesion holds the water together as one continuous thread, tugging the top drags the whole column upwards.
Surface tension
At the surface of a pond there is no water above to bond to, so the top layer of molecules hydrogen-bond more tightly sideways to each other. That creates a kind of elastic skin — surface tension — strong enough for pond skaters to walk on without breaking through.
If you get muddled in an exam, use the spelling. COhesion — water co-operating with itself. ADhesion — water sticking to additional materials. It takes two seconds and saves a mark.
Water as a solvent
Most of the chemistry of life happens dissolved in water, so what water will and will not dissolve shapes everything a cell can do.
The rule is simple: polar dissolves polar. If a substance is polar or carries a charge, water molecules can hydrogen-bond to it, surround it, and pull it into solution. Such substances are hydrophilic, meaning water-loving.
Non-polar substances have no charged regions for water to grip, so water molecules bond to each other instead and leave them out. These are hydrophobic, water-hating, and they end up clustered together — which is exactly what happens to the fatty tails inside a cell membrane.
Notice what happens on the right: the oil is not being pushed away by a force. Water molecules simply prefer bonding to each other, and the oil gets squeezed out of the way.
Substance
Hydrophilic or hydrophobic?
Consequence for the organism
Sodium chloride, glucose, amino acids
Hydrophilic — dissolve easily
Transported freely dissolved in blood plasma or sap
Oxygen
Only slightly soluble
Needs haemoglobin to carry enough of it around the body
Fats and oils
Hydrophobic — insoluble
Carried in lipoproteins; stored as compact energy reserves
Phospholipid tails
Hydrophobic
Cluster inwards to form the core of every cell membrane
Phospholipid heads
Hydrophilic
Face the watery cytoplasm and outside of the cell
Oxygen is worth remembering. It is only sparingly soluble — and less soluble at 37 °C than at 20 °C. Enough dissolves in rivers and oceans for fish to breathe, but not nearly enough for blood plasma alone, which is exactly why haemoglobin exists.
Thermal properties
High specific heat capacity
Specific heat capacity is the energy needed to raise 1 kg of something by 1 °C. For water it is about 4200 J; for air it is about 1000 J. So warming water takes roughly four times as much energy as warming the same mass of air.
The reason is hydrogen bonds. Before the molecules can move faster — which is what a higher temperature means — energy has to go into breaking hydrogen bonds first. That energy is not raising the temperature, so the temperature climbs slowly.
The payoff is stability. Oceans and lakes change temperature far more slowly than the air above them, giving aquatic organisms a habitat that stays within a liveable range. Inside your body, cytoplasm resists sudden temperature swings, keeping enzymes near their optimum.
Almost every other substance is denser as a solid and would sink. If ice sank, ponds and polar seas would freeze solid from the bottom up and stay that way.
Thermal conductivity
Thermal conductivity is how well a substance carries heat. Water conducts heat roughly thirty times better than air — which is bad news if you are a warm-blooded animal in cold water, because your body heat is drawn away quickly.
Animals solve it in two different ways, and the IB likes comparing them:
The black-throated loon (Gavia arctica), a diving bird, traps a layer of air in its feathers. Air is a poor conductor, so the trapped layer insulates it.
The ringed seal (Pusa hispida) uses a thick layer of blubber under its skin instead — fat conducts heat poorly and does not get squashed flat at depth the way trapped air does.
Buoyancy and viscosity
Buoyancy is the tendency to float; viscosity is resistance to flowing. Water is far more buoyant and far more viscous than air, so moving through it is a completely different problem from flying.
Property
What it means
Loon — adapted for both air and water
Seal — adapted for water
Thermal conductivity
Water pulls heat away fast
Traps an insulating layer of air in its feathers
Thick blubber layer under the skin
Buoyancy
Water pushes bodies upwards
Dense solid bones, and squeezes air out before diving
Blubber adds buoyancy as well as warmth
Viscosity
Water resists movement
Streamlined body, webbed feet set far back to reduce drag
Streamlined shape with flippers to push against the water
Most bird bones are hollow to save weight for flight. The loon’s are solid, because a bird that needs to dive has the opposite problem — it has to beat buoyancy, not gravity. That trade-off is a lovely exam point.
WORKED EXAMPLE
Explain how water reaches the leaves of a tall tree
Explain the roles of cohesion and adhesion in the movement of water up the xylem. (4 marks)
Point 1: what starts the movement
Water evaporates from the leaves during transpiration, creating a pull at the top.
Point 2: cohesionHydrogen bonds hold water molecules to each other, so the water forms one continuous column.
Pulling the top therefore drags the molecules below it upwards.
Point 3: adhesionWater also hydrogen-bonds to the cellulose walls of the xylem.
This helps hold the column up and stops it sliding back down.
Point 4: the result
An unbroken stream of water moves from root to leaf without any pumping.
Transpiration pulls • cohesion holds the column • adhesion grips the wallssay “pulled”, never “pushed” — roots do not push water up
WORKED EXAMPLE
Why does a high specific heat capacity help living things?
Explain, in terms of hydrogen bonding, why water’s high specific heat capacity is important for organisms. (4 marks)
Point 1: define itSpecific heat capacity is the energy needed to raise 1 kg by 1 °C — about 4200 J for water.Point 2: why it is high
Energy must first break hydrogen bonds between molecules before they can move faster.
So a lot of energy produces only a small temperature rise.Point 3: effect on habitats
Lakes and oceans warm and cool slowly, giving aquatic organisms a stable environment.
Point 4: effect inside the body
Cytoplasm is mostly water, so cell temperature stays steady.
Enzymes stay near their optimum and are not denatured by sudden changes.Hydrogen bonds absorb the energy, so temperature changes slowlyalways finish on enzymes — that is what makes it biologically important
WORKED EXAMPLE
Comparing the loon and the seal
Both the black-throated loon and the ringed seal are adapted to cold water. Compare how each deals with the thermal conductivity and buoyancy of water. (4 marks)
Heat loss: the shared problem
Water conducts heat about thirty times better than air, so both lose body heat quickly.
Different solutionsLoon: traps air in its feathers — air is a poor conductor.Seal: a thick blubber layer, which is not compressed at depth.Buoyancy: opposite problems
The loon must overcome floating to dive.
It has solid rather than hollow bones, and presses air out of feathers and lungs.
The seal’s blubber adds buoyancy while also insulating it.
Same two challenges, two different evolutionary answersa “compare” question needs both similarity and difference — do not just describe each animal
💡 Exam tip
Trace every property back to hydrogen bonding. That link is usually where the marks sit.
Use COhesion = water to water, ADhesion = water to other materials. State which is which explicitly.
Water is pulled up xylem by transpiration, never pushed up by roots.
For thermal questions, end on the biological consequence: stable habitat or enzymes not denatured.
Learn the loon and the seal as a pair. Examiners like comparisons, and either common or scientific names are accepted.
“Universal solvent” is a useful phrase but slightly overstated — say water dissolves polar and charged substances.
⚠ Common mix-up
Swapping cohesion and adhesion. The most common error on this whole topic.
Saying water is pushed up the xylem. It is pulled from above.
Claiming water dissolves everything. Non-polar substances such as lipids do not dissolve.
Confusing specific heat capacity with thermal conductivity. One is how much energy to warm it; the other is how fast heat travels through it.
Saying ice floats because it is lighter. Say less dense, and explain the open lattice.
Forgetting that oxygen is only slightly soluble. That limitation is the whole reason haemoglobin exists.
Describing an adaptation without naming the property. Link blubber to thermal conductivity, solid bones to buoyancy.
Up next: Carbohydrates and Lipids — the first of the big biological molecules, and a direct application of everything on this page about hydrophilic and hydrophobic behaviour.
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