IB Biology · Theme A: Unity and diversity · SL and HL
A1.1 Water
A one-page summary of A1.1 Water, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What physical and chemical properties of water make it essential for life?
What are the challenges and opportunities of water as a habitat?
What A1.1 covers
Chemical properties of water
- A1.1.1Life began in water
- A1.1.2Hydrogen bonds
- A1.1.3Cohesion in the xylem
- A1.1.3Cohesion at the surface
- A1.1.4Adhesion
- A1.1.5The universal solvent
- A1.1.5Except what it excludes
Physical properties of water
Where did it all come from?
- A1.1.7 · HLWhere did Earth's water come from?
- A1.1.8 · HLLooking for life, looking for water
A1.1 Water: summary
Structure
- Polar bond: O pulls electrons → δ− O, δ+ H
- Hydrogen bond: δ+ of one molecule to δ− of another
Cohesion & adhesion
- Cohesion: water-to-water → surface tension, xylem columns
- Adhesion: water-to-surface → capillary action
Solvent
- Hydrophilic dissolves; hydrophobic excluded
- Enzymes react in aqueous solution
Physical properties
- Buoyancy: water ≈800× denser than air
- Viscosity: water ≈50× more viscous
Thermal
- Conductivity: water 0.58 vs air 0.024 W/m·K
- Specific heat: water 4.18 vs air ≈1.0 J/g·°C
HL · Origins
- Water delivered by icy asteroids; retained by gravity + temperature
- Goldilocks zone: where liquid water can persist
Key terms
- Polar
- an uneven spread of charge across a molecule.
- Hydrogen bond
- the weak attraction between one molecule's δ+ and another's δ−.
- Cohesion
- water molecules attracted to each other.
- Adhesion
- water attracted to other polar or charged surfaces.
- Hydrophilic
- "water-loving" — dissolves readily in water.
- Hydrophobic
- "water-fearing" — does not dissolve in water.
- Buoyancy
- the upward force a fluid exerts on a body within it.
- Viscosity
- a fluid's internal resistance to flowing.
- Thermal conductivity
- how quickly a material transfers heat.
- Specific heat capacity
- energy needed to raise 1 g of a substance by 1°C.
- Retention HL
- a planet keeping water rather than losing it to space.
- Goldilocks zone HL
- the orbital distance where liquid water can persist.
Sample exam questions
Three of the 30 multiple-choice questions for A1.1. Try each one before opening the answer.
Question 1. A hydrogen bond between two water molecules forms between:
- two oxygen atoms of neighbouring molecules sharing a pair of electrons
- the covalent O–H bonds within one single water molecule
- the δ+ hydrogen of one molecule and the δ− oxygen of another
- two hydrogen atoms of adjacent molecules attracting each other
Show the answer
Answer: C. A hydrogen bond is an intermolecular attraction: the δ+ hydrogen of one water molecule is attracted to the δ− oxygen of another. It is much weaker than a covalent bond but, in large numbers, collectively strong.
Question 2. Water is described as a polar molecule. What is the underlying reason for this polarity?
- The two hydrogen atoms repel each other, which bends the molecule
- Oxygen attracts the shared electrons more strongly than hydrogen
- Hydrogen bonds form between the H and O atoms inside one molecule
- Oxygen and hydrogen have equal electronegativity, so sharing is even
Show the answer
Answer: B. Oxygen is more electronegative than hydrogen, so the bonding electrons sit closer to the oxygen. Oxygen becomes slightly negative (δ−) and the hydrogens slightly positive (δ+). That uneven distribution of charge is what 'polar' means.
Question 3. Which pair of water's physical properties most directly explains why a fast-swimming fish is streamlined and a seal has a thick layer of blubber?
- Specific heat capacity and its solvent properties
- Adhesion to surfaces and cohesion between molecules
- Surface tension at the surface and buoyancy
- Viscosity, causing drag, and thermal conductivity
Show the answer
Answer: D. Water's viscosity produces drag, so a streamlined shape reduces the energy needed to swim; water's high thermal conductivity draws heat from the body, so blubber insulates against heat loss.
Linking questions
Questions that connect A1.1 to other parts of the course, the kind that come up in Paper 2.
- How do the solvent properties of water underpin transport across cell membranes? (see B2.1)
- How does water potential (osmosis) depend on water acting as a solvent and medium? (see D2.3)
- How is the cohesion of water essential to transport in plants? (see B3.2)
- Why is liquid water central to discussions of the origin of the first cells? (see A2.1)
Practise A1.1
This whole lesson is free: notes, quiz, data questions and exam-style papers, with markschemes. No payment needed.