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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.

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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

Physical properties of water

Where did it all come from?

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" — polar or charged, so attracted to 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. Which observation is explained by cohesion between water molecules?

  1. Oil forms a separate layer on top of water.
  2. A lake changes temperature little between day and night.
  3. Sodium chloride dissolves when it is added to water.
  4. Water is pulled up a xylem vessel as an unbroken column.
Show the answer

Answer: D. Hydrogen bonds hold water molecules together, so the water in a xylem vessel behaves as a continuous column that can be pulled upwards under tension.

Question 2. Why is a water molecule polar?

  1. Hydrogen bonds form between its two hydrogen atoms.
  2. The oxygen atom attracts the shared electrons more strongly.
  3. The oxygen atom gains an electron from each hydrogen atom.
  4. Its covalent bonds share the electrons equally.
Show the answer

Answer: B. Oxygen is more electronegative than hydrogen, so the shared electrons are pulled towards it: oxygen becomes δ− and each hydrogen δ+.

Question 3. The function of which molecule depends on it being hydrophobic?

  1. Glucose carried in blood plasma
  2. Sodium ions moving in nerve cells
  3. Amino acids dissolved in cytoplasm
  4. The lipid tails in cell membranes
Show the answer

Answer: D. Some molecules work because they do not dissolve: hydrophobic lipid tails form a barrier inside the membrane that water-soluble substances cannot easily cross.

Linking questions

Questions that connect A1.1 to other parts of the course, the kind that come up in Paper 2.

Practise A1.1

30 quiz questions5 data questions6 exam questionsmarkschemes included

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