IB Biology HLWater & Its PropertiesPaper 1 & 2~12 min read
Water’s Physical & Chemical Properties
Water sticks to things, dissolves almost everything, warms up slowly and holds objects afloat. These sound like four unrelated facts. They are really one fact — hydrogen bonding — showing up in four different ways.
📘 What you need to know
Cohesion — water molecules stick to each other. This gives surface tension and lets columns of water be pulled up the xylem.
Adhesion — water molecules stick to other polar or charged materials, such as the cellulose in cell walls. This gives capillary action.
Water is the universal solvent: polar and charged substances are hydrophilic and dissolve; non-polar substances are hydrophobic and do not.
Specific heat capacity is high (4200 J/kg/°C vs about 1000 for air), so water temperature changes slowly.
Thermal conductivity of water is about 30 times higher than air, so air is a much better insulator.
Buoyancy and viscosity are both far higher in water than in air.
You must be able to compare these four properties in water and air using the black-throated loon (Gavia arctica) and the ringed seal (Pusa hispida).
Cohesion
Hydrogen bonds between water molecules pull them towards each other. That attraction between molecules of the same substance is called cohesion.
Cohesion
Water molecules are attracted to other water molecules by hydrogen bonds
Two consequences matter for the exam:
Mass transport in plants. Because the molecules hold on to each other, a whole column of water can be pulled up under tension through the xylem without snapping apart. This is how water reaches the leaves at the top of very tall trees.
Surface tension. Where a body of water meets the air, the top layer of molecules can only hydrogen bond sideways and downwards, not upwards. This creates a kind of film on the surface — strong enough for insects such as pond skaters to walk on it.
Adhesion
Water is also attracted to other substances, as long as those substances are polar or charged. That is adhesion. Cellulose, which makes up plant cell walls, is polar — so water sticks to it well.
Adhesion
Water molecules are attracted to other polar or charged materials
Adhesion helps water climb the xylem during transpiration by sticking to the vessel walls.
Water is drawn up narrow channels in soil, called capillary tubes, by capillary action.
Spaces between the cellulose fibres in cell walls draw water out of the xylem, letting water flow through plant tissue.
Hydrogen bonding produces both forces, which is why the column of water does not break as it is pulled up a tall tree.
🧠
Memory trick: CO and AD
COhesion = water sticking to each other (CO- means together, like co-worker). ADhesion = water sticking to additional materials.
Water as a solvent
Because water is polar, it can surround and pull apart any particle that also carries charge. Biological molecules fall into two groups depending on whether water can do this:
Hydrophilic — “water loving”. Polar molecules and charged ions can form hydrogen bonds with water, so they dissolve.
Hydrophobic — “water hating”. Non-polar molecules carry no charge, cannot form hydrogen bonds with water, so they do not dissolve.
Hydrophobic molecules end up clustered together in groups. This is not because they attract each other — it is because the water molecules around them hydrogen bond to each other instead, pushing the non-polar molecules out of the way. This is called a hydrophobic interaction.
Since most biological molecules are hydrophilic and can be dissolved, water is called the universal solvent.
Water pulls ionic and polar substances apart by surrounding each particle. A non-polar molecule has nothing for water to grip, so it stays undissolved.
Solubility of different biological molecules
Group
Examples
What it means for the organism
Highly soluble
Sodium chloride, urea, glucose, amino acids
Easily transported in solution around the body. Even amino acids with hydrophobic R groups are soluble enough to travel in water
Sparingly (less) soluble
Oxygen
Dissolves just enough in oceans, rivers and lakes for aquatic animals to breathe, but needs haemoglobin to carry useful amounts in blood. Oxygen is less soluble at 37 °C than at 20 °C
Insoluble
Fats, phospholipid tails
Being hydrophobic is the whole point — phospholipid hydrocarbon tails form the hydrophobic core of the cell membrane
Enzymes need water too. Most enzymes need water to hold their shape and stay stable, which lets them catalyse reactions in aqueous solution. Hydrogen bonds also help the substrate bind into the active site to form the enzyme–substrate complex.
Specific heat capacity
Definition
The energy needed to raise the temperature of 1 kg of a substance by 1 °C
Water’s specific heat capacity is 4200 J/kg/°C, compared with about 1000 J/kg/°C for air. So it takes a lot of energy to warm water up, and water releases a lot of energy as it cools down.
The reason is the hydrogen bonds. A large amount of the thermal energy going in is used to break hydrogen bonds rather than to speed the molecules up, so the temperature does not rise much.
Why this matters to living things:
Aquatic habitats are stable — sea temperature changes far more slowly than air temperature.
Organisms can maintain a fairly constant internal temperature, because their watery cytoplasm absorbs heat without large swings.
Stable temperatures keep enzymes working near their optimum.
Arctic and sub-Arctic species such as the ringed seal (Pusa hispida) can survive all year round because sea temperatures stay stable.
Ice floats. Solid water is less dense than liquid water, so ice forms on the surface rather than the bottom. That gives seals a habitat on top of the floating ice sheets and underneath them, and it stops lakes and seas freezing solid.
Thermal conductivity
Definition
The ability of a substance to conduct heat
Water conducts heat almost 30 times better than air. Flip that around and it gives you the useful conclusion: air is a very good insulator, which is exactly why animals in cold climates trap air.
The black-throated loon (Gavia arctica) traps an insulating layer of air in its feathers to regulate body temperature.
The ringed seal uses a thick layer of fat called blubber under the skin to insulate it from the outside air instead.
Ice has a much lower thermal conductivity than liquid water, so a sheet of ice acts as an insulating lid. Thermal energy is trapped underneath, which keeps the water below warmer.
Two properties working together: low density of ice puts the lid on, and low thermal conductivity of ice keeps the heat in.
Buoyancy
Definition
The ability of an object to float in water
Water is dense, so it pushes objects upwards strongly. Air barely does this at all. Depending on the animal, that upward push is either a problem to solve or an advantage to use.
A problem for the loon. It dives to catch prey, so floating is a nuisance. It has solid bones rather than the hollow bones most birds have. This increases its weight, and diving also compresses air out of its lungs and feathers, so it sinks more easily.
An advantage for the seal. The blubber under its skin improves buoyancy as well as providing insulation against the cold.
Viscosity
Definition
The resistance of a fluid to flow
Water is much more viscous than air, so moving through water takes far more effort than moving through air.
The loon can fly through air with very little friction because air has such low viscosity.
Both animals have a streamlined body shape to cut through the more viscous water.
The seal has flippers to propel itself.
The loon uses webbed feet to push against the water, and the feet are set far back on the body, which reduces drag as it swims.
Water vs air — the comparison the IB asks for
Property
Water
Air
Adaptation it explains
Specific heat capacity
High (4200 J/kg/°C)
Lower (about 1000 J/kg/°C)
Stable sea temperatures let the ringed seal survive all year
Thermal conductivity
About 30× higher
Low — a good insulator
Loon traps air in its feathers; seal uses blubber
Buoyancy
High
Very low
Loon has solid bones to dive; seal’s blubber aids floating
Viscosity
Much higher
Low
Streamlined bodies, flippers and webbed feet for water; easy flight through air
You may use either the common name or the scientific name for these two organisms in an exam. What you cannot do is answer a “compare water and air” question without mentioning an actual adaptation — the marks are usually for the link, not for the property.
Worked examples
WE 1
Explain how water moves up the xylem of a tall tree
Explain how the properties of water allow it to move from the roots to the leaves of a tall tree. (3 marks)
Point 1: name the force between water molecules
Hydrogen bonds between water molecules cause cohesion, so the molecules stick to each other.
Point 2: explain the continuous column
This holds the water together as an unbroken column that can be pulled upwards under tension without breaking.
Point 3: bring in the vessel wallAdhesion between water and the polar cellulose of the xylem wall helps hold the column in place and supports capillary action.
Cohesion holds the column together; adhesion holds it to the wallUse both words. Answers that only say “cohesion” usually cap at 2 marks.
WE 2
Explain why glucose dissolves in water but a fat does not
Explain, in terms of molecular structure, why glucose is soluble in water but a lipid is not. (3 marks)
Point 1: glucose is polar
Glucose has hydroxyl (–OH) groups, which are polar, so it can form hydrogen bonds with water.
Point 2: water surrounds it
Water molecules surround each glucose molecule and separate them, so glucose is hydrophilic and dissolves.
Point 3: fats are non-polar
A lipid’s hydrocarbon chains carry no charge, so no hydrogen bonds form with water. It is hydrophobic and stays undissolved.
Hydrogen bonding with water is the deciding factor, and that needs charge“It’s polar so it dissolves” is only half an answer — say why polarity matters: hydrogen bonds.
WE 3
Compare the thermal properties of water and air
Using named examples, compare the thermal conductivity of water and air and explain how animals are adapted to this difference. (4 marks)
Point 1: state the comparison
Water conducts heat about 30 times better than air, so animals lose body heat far faster in water than in air.
Point 2: air is therefore an insulator
The black-throated loon (Gavia arctica) traps a layer of air in its feathers to reduce heat loss.
Point 3: the alternative solution
The ringed seal (Pusa hispida) uses a thick layer of blubber instead of trapped air.
Point 4: add the ice point
Ice conducts heat poorly, so a surface ice layer traps thermal energy and keeps the water underneath warmer.
High conductivity in water → animals need insulation, from air or from fatName at least one organism. “Compare” questions expect a clear difference plus a consequence.
💡 Exam tips
COhesion = water to water. ADhesion = water to other materials. Mixing these up is the single most common error in this topic.
Every property here should be traced back to hydrogen bonding in your answer, not just stated.
Learn the two numbers: specific heat capacity 4200 J/kg/°C for water vs about 1000 for air, and thermal conductivity about 30× higher in water.
Use the correct terms — hydrophilic, hydrophobic, solvent, solute, solution.
Remember that oxygen is sparingly soluble, not insoluble. That is why haemoglobin exists.
For “compare water and air”, always link the property to a named adaptation in the loon or the ringed seal.
⚠ Common mistakes
Swapping cohesion and adhesion. Read the question carefully — the wrong word loses the whole mark.
Saying oxygen is insoluble in water. It is sparingly soluble, which is exactly why aquatic animals can breathe.
Saying hydrophobic molecules “attract each other”. They are pushed together because the water molecules bond to each other instead.
Confusing specific heat capacity with thermal conductivity. Capacity is about how much energy is needed to heat it; conductivity is about how fast heat travels through it.
Saying water is a good insulator. It is the opposite — air is the good insulator.
Forgetting that ice floats because it is less dense. Don’t say it floats because it is lighter or colder.
Describing water as the universal solvent without a reason. Add “because it is polar and forms hydrogen bonds with solutes”.
Up next: Origin of Water on Earth — how water arrived on a planet that was once far too hot for it, and how astronomers hunt for it on planets orbiting other stars.
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