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
- SL
Water Provides a Medium for Life
Water is the medium in which most life processes occur. The first cells originated in water, and modern cells still carry out most reactions in aqueous cytoplasm.
Water keeps many ions and polar molecules dissolved, so reactants can diffuse, collide and be acted on by enzymes. It also transports dissolved substances within organisms.
When explaining water as a medium, link three ideas: dissolved substances, movement or mixing, and a biological process such as metabolism or transport.
In cytoplasm, dissolved glucose and enzymes can move and interact, allowing enzyme-catalysed reactions to occur.
A medium is the environment in which a process occurs. Water does not have to be a reactant in every reaction that takes place in it.
Water as the medium for life
Assessment in practice
8 marksHow it is assessed
This objective is assessed through structured response, commonly using Explain.
Command terms
Explain
What earns marks
In exam answers, connect each biological role to the relevant molecular property rather than listing uses of water without explaining why they are possible.
Watch for
Describing water only as a habitat without explaining its role as a cellular reaction medium.
Representative question
Question 1
[Maximum number: 8]
Explain the relationship between the properties of water and its uses in living organisms as a coolant, a medium for metabolic reactions and a transport medium.
water is a polar molecule;
oxygen has a partial negative charge / hydrogen has a partial positive charge;
hydrogen bonds form between adjacent water molecules;
water remains liquid over wide range of temperatures/ 0 to 100∘C;
moderates temperature fluctuation / stable environment;
accurate reference to specific heat;
sweating/evaporation cools organisms;
accurate reference to high heat of vaporization;
polarity makes water a good/universal solvent for polar/ionic substances;
(all) metabolic reactions of cells take place in (aqueous) solutions;
blood/xylem/phloem transport solutes in water;
cohesive properties allow capillary action/transpiration stream/water column in xylem;
Polarity Leads to Hydrogen Bonds Between Water Molecules
Each O-H bond in water is polar covalent because oxygen attracts the shared electrons more strongly than hydrogen. Oxygen is partially negative (delta-) and each hydrogen is partially positive (delta+).
Because a water molecule is V-shaped, its bond dipoles do not cancel. The delta+ hydrogen of one molecule is attracted to the delta- oxygen of another, forming an intermolecular hydrogen bond.
To represent two water molecules: draw solid O-H covalent bonds within each V-shaped molecule, label O as delta- and H as delta+, then draw a dotted line from H(delta+) on one molecule to O(delta-) on the other and label it hydrogen bond.
Many individually weak hydrogen bonds act together. This collective attraction helps explain cohesion and the relatively high energy needed to separate water molecules.
Do not draw a hydrogen bond between O and H within the same water molecule: that connection is a polar covalent bond.
Hydrogen bonds in water
Assessment in practice
1–2 marksHow it is assessed
This objective is assessed through structured response, multiple choice, commonly using Identify / Describe / Explain.
Command terms
Identify / Describe / Explain / Outline / Draw
What earns marks
Build the answer around this relationship: Unequal electron sharing makes each O–H bond polar.
Watch for
Drawing a hydrogen bond between atoms within the same water molecule.
Representative question
Question 1
[Maximum number: 3]
Outline how hydrogen bonds form in water.
a water (molecules) are polar/dipolar/have partially positive and negative poles/have δ+ and δ-;
b attraction/bonding between positive and negative (poles);
c hydrogen bond formed between hydrogen and oxygen;
Reject if H and O in same molecule
d bond/attraction between different water molecules/intermolecular;
Marks can be awarded in an annotated diagram Reject answers stating or implying that there are whole positive or negative charges for mpa
3 max
Turn a water molecule until attraction appears
Loading interaction assets…
Cohesion Keeps Water Molecules Together
Cohesion is attraction between molecules of the same substance; in water it supports continuous columns and surface tension.
Hydrogen bonding resists separation. This allows water to transmit tension through xylem and lets the surface support small organisms.
Distinguish cohesion from adhesion before explaining a transport or surface effect.; separate property, mechanism and biological use
A continuous water column in a plant xylem vessel can be pulled upward as leaves lose water, because water molecules cohere. This gives a concrete prediction from the stated water condition.
Cohesion alone does not lift water without transpiration, adhesion and a continuous pathway. Interpret the result within the stated evidence and scale limits.
Cohesion of water molecules
Assessment in practice
1–2 marksHow it is assessed
This objective is assessed through structured response, multiple choice, commonly using Identify / Explain / Outline.
Command terms
Identify / Explain / Outline
What earns marks
Strong exam answers begin with polarity or hydrogen bonding, name cohesion, and then link it to a specific biological outcome such as xylem transport or surface tension.
Watch for
Defining cohesion as attraction between water and another surface.
Representative question
Question 1
[Maximum number: 4]
Outline how the cohesive properties of water benefit living organisms.
a. cohesion is caused by the polarity of water
OR
water molecules have a negative and positive end;
b. water molecules are attracted to each other
OR
water molecules are linked by hydrogen bonds;
c. (cohesion) enables transport of water along stems/xylem
OR
enables a continuous column of water in transpiration/transpiration pull;
d. surface tension provides a habitat for some organisms
OR
some insects can walk on water surface because surface tension/other verified example
OR
cohesion is related to buoyancy allowing organisms (like ringed seal) to float;
e. helps animals regulate body temperature (through evaporative cooling);
f. (cohesion causes high specific heat) so maintains a stable temperature in aquatic environments
OR
water is liquid at a wide range of temperatures supporting many aquatic habitats;
4
Marking guidance:
max
Adhesion of water
Assessment in practice
1 marksHow it is assessed
This objective is assessed through multiple choice, commonly using Identify.
Command terms
Identify
What earns marks
A complete answer identifies the different surface involved and links the attraction to capillary movement.
Watch for
Using adhesion and cohesion as interchangeable terms.
Representative question
Question 1
[Maximum number: 1]
Which of the following is a consequence of the adhesive properties of water?
A
Stable temperatures of aquatic environments
B
Water movement by capillary action in soil
C
Transport of dissolved nutrients in blood plasma
D
Cooling effect of sweating
B
Water Dissolves Polar and Ionic Solutes
Water dissolves many ionic and polar substances because its partial charges attract ions and polar regions of molecules.
Water molecules form hydration shells around ions and hydrogen bonds with many polar solutes. Dissolved particles remain mobile, so they can be transported in blood, xylem or phloem and can take part in enzyme-catalysed reactions in aqueous solution.
Hydrophilic substances interact with water and tend to dissolve. Hydrophobic substances are non-polar and tend to remain separate; this insolubility is important for structures such as lipid membranes.
When sodium chloride dissolves, the oxygen side of water faces Na+ and the hydrogen sides face Cl-, producing hydration shells that keep the ions dispersed.
Water is not a literally universal solvent. Many non-polar substances dissolve poorly, and solubility does not mean that a substance is harmless.
Solvent properties of water
Assessment in practice
1–3 marksHow it is assessed
This objective is assessed through structured response, multiple choice, commonly using Identify / Explain.
Command terms
Identify / Explain
What earns marks
Exam questions commonly ask students to connect solvent behaviour to polarity, identify a biological transport example, or distinguish solvent properties from cohesion and thermal properties.
Watch for
Calling water a universal solvent that dissolves all substances.
Representative question
Question 1
[Maximum number: 3]
Water has important solvent properties. Explain these properties using an example to illustrate your answer.
| a | water molecule is polar OR water has «weak» positive and negative charges |
| b | substances that dissolve in water are hydrophilic |
| c | water forms hydrogen bonds with polar substances |
| d | positive/hydrogen side/pole of water attracted to negative ions OR negative/oxygen side/pole attracted to positive ions |
| e | glucose/other example dissolves because it is polar OR sodium chloride/other example dissolves because ions are attracted to water |
Water and Air Present Different Physical Challenges
Compared with air, water is denser and more viscous, conducts heat more rapidly, and has a higher specific heat capacity. Each difference has a distinct consequence for aquatic animals.
| Property | Water compared with air | Consequence for aquatic animals |
|---|---|---|
| Density and buoyancy | Denser; provides more buoyant support | Bodies are supported, but animals must control floating and diving |
| Viscosity | Higher | Movement creates greater drag, favouring streamlined shapes |
| Thermal conductivity | Higher | Heat is transferred from a warm body more rapidly, increasing the need for insulation |
| Specific heat capacity | Higher | Aquatic temperatures change more slowly, giving a more stable thermal environment |
A streamlined ringed seal experiences less drag in viscous water, while blubber reduces heat transfer to the water.
Thermal conductivity is the rate of heat transfer; specific heat capacity is the energy needed to change temperature. Do not use the terms interchangeably.
Match Aquatic Adaptations to Water Properties
An aquatic adaptation should be explained by naming the water property, the challenge it creates, and how the feature improves survival or movement.
| Ringed seal feature | Water-related challenge | Functional consequence |
|---|---|---|
| Streamlined body | Water has high viscosity compared with air | Reduces drag during swimming |
| Thick blubber | Water conducts heat away faster than air | Reduces heat loss and also increases buoyancy |
| Flippers | Propulsion and steering are needed in a dense medium | Produce controlled movement through water |
A complete explanation is: water has higher thermal conductivity than air, so a seal loses heat rapidly; its blubber slows this heat transfer.
Do not claim that blubber is mainly an adaptation to high specific heat capacity. The direct challenge is water's higher thermal conductivity.
Physical properties of water for aquatic
Assessment in practice
1–2 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / Describe / Distinguish.
Command terms
Identify / Describe / Distinguish
What earns marks
Credit depends on matching the adaptation to the correct physical challenge.
Watch for
Confusing thermal conductivity with specific heat capacity.
Representative question
Question 1
[Maximum number: 2]
Distinguish between the physical properties of water and air.
The ringed seal (Pusa hispida) can be found in the Arctic and sub-Arctic regions of the Pacific, Atlantic and Arctic Oceans.
a. water has greater viscosity than air;
b. thermal conductivity is higher in water than in air;
c. specific heat capacity is higher in water than in air;
d. water is denser/has more buoyancy than air;
The physical property must be named. Comparative terms are expected, making reference to both eg: accept higher in water, but not high in water.
Marking guidance:
Accept the converse of any of the marking points.
2
max
Water summary
- Water is life's medium because substances can dissolve, move and react in it.
- Unequal electron sharing and bent geometry make water polar; hydrogen bonds form between molecules.
- Many hydrogen bonds produce cohesion, maintaining xylem columns and creating surface tension.
- Adhesion is water-to-surface attraction and contributes to movement through narrow soil spaces and cellulose walls.
- Polarity lets water hydrate ions and interact with polar solutes; dissolved particles can be transported and react, while non-polar substances are hydrophobic.
- Compared with air, water provides more buoyancy and drag, transfers heat faster and changes temperature more slowly. Aquatic adaptations respond to these consequences.
Objective notes
6 learning objectivesA1.1.1Water 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 mass1% of analysed papers 1 paper · 1 questionViewA1.1.2Hydrogen 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 properties11% of analysed papers 16 papers · 18 questionsViewA1.1.3Cohesion 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 insects4% of analysed papers 6 papers · 6 questionsViewA1.1.4Adhesion 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 vessels1% of analysed papers 1 paper · 1 questionViewA1.1.5Solvent 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 water3% of analysed papers 4 papers · 4 questionsViewA1.1.6Physical 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 movement1% of analysed papers 2 papers · 2 questionsView