Cambridge International AS & A Level Biology 9700: Water – Revision Notes

New to this content? Start with the Water study guide, which explains everything here in full with worked examples.

These notes cover section 2.4 Water (learning outcome 1) of Topic 2, Biological molecules, in the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It is AS Level content, examined on Paper 1 and Paper 2; Paper 4 can draw on it because A Level questions require knowledge of the AS content.

Other pages for this topic: Biological molecules revision notes (all of Topic 2) · Water practice questions · Cambridge A Level Biology hub · 9700 checklist · Free 9700 AS diagnostic

What the outcome asks (2.4.1)

Two jobs, both in one outcome:

1. Explain how hydrogen bonding occurs between water molecules.
2. Relate the properties of water to its roles in living organisms, limited to:
   • solvent action
   • high specific heat capacity
   • latent heat of vaporisation

Nothing else is required for 2.4. Cohesion of water in the xylem is in Topic 7; water potential is in Topic 4.

Definitions

Term	Definition to learn
Polar molecule	Molecule with an uneven spread of charge: one part slightly negative, another slightly positive
δ− / δ+	Small (partial) negative / positive charge; in water, δ− on O, δ+ on each H
Hydrogen bond	Weak attraction between a δ+ hydrogen of one molecule and a δ− oxygen of another
Solvent	Liquid in which a solute dissolves to form a solution
Specific heat capacity	Energy needed to raise the temperature of 1 kg (or 1 g) of a substance by 1 °C
Latent heat of vaporisation	Energy needed to change a liquid into vapour without a change in temperature
Hydrophilic / hydrophobic	Attracted to water (polar or charged) / not attracted to water (non-polar)

Hydrogen bonding in steps

1. O attracts the shared electrons in each O–H covalent bond more than H does
2. So O is δ−, each H is δ+
3. The molecule is bent (about 104.5°), so it has a δ− side and a δ+ side: it is polar
4. δ+ H of one molecule is attracted to δ− O of a neighbouring molecule
5. That attraction = a hydrogen bond (weak alone; very many together)
6. Each molecule can form up to 4 hydrogen bonds; in liquid water they constantly break and re-form

Drawing check: label δ+ on H and δ− on O; dotted line from H of one molecule to O of the next; the solid lines inside a molecule are covalent bonds.

The three properties at a glance

Property	Link to hydrogen bonding	Roles in living organisms
Solvent action	Polar water molecules surround ions (δ− O towards cations, δ+ H towards anions) and form hydrogen bonds with polar molecules	Transport of dissolved glucose, amino acids, ions and urea in blood plasma; mineral ions in xylem; sucrose in phloem; medium for metabolic reactions in cytoplasm; urea excreted in solution
High specific heat capacity	Energy is used to break hydrogen bonds before molecules move faster, so much energy is needed per 1 °C rise	Aquatic habitats change temperature slowly; cells and bodies (mostly water) resist temperature change, keeping enzymes near their optimum and away from denaturing
High latent heat of vaporisation	All hydrogen bonds holding a molecule must break for it to evaporate; escaping molecules carry energy away	Evaporation of sweat cools mammals; panting; transpiration cools leaves; a lot of heat lost for little water lost

Values worth knowing

Quantity	Value
Specific heat capacity of liquid water	about 4.2 kJ kg⁻¹ °C⁻¹ (4.18 J g⁻¹ °C⁻¹ at 25 °C)
Latent heat of vaporisation of water	about 2 257 J g⁻¹ at 100 °C; about 2.4 kJ g⁻¹ at body temperature
Ratio of the two	evaporating 1 g removes about 540 times the energy of cooling 1 g by 1 °C

In a calculation, always use the value printed in the question.

Method in steps: thermal calculations

Temperature change from energy (specific heat capacity):

energy (J) = mass (g) x c (J g-1 °C-1) x temperature change (°C)
temperature change = energy / (mass x c)

Mass of water evaporated (latent heat):

energy (kJ) = mass evaporated (g) x L (kJ g-1)
mass = energy / L

Check units before you divide: g with J g⁻¹, kg with kJ kg⁻¹, J with J, kJ with kJ.

Worked reminder. How much energy warms 250 g of water by 8 °C, with c = 4.2 J g⁻¹ °C⁻¹? 250 × 4.2 × 8 = 8 400 J (8.4 kJ).

Method in steps: “relate a property to a role”

A full answer has three links. Miss one and you usually lose a mark.

1. Property: name it (for example, high latent heat of vaporisation).
2. Reason: hydrogen bonds (for example, many hydrogen bonds must break for water to evaporate).
3. Role: name an organism or process and the effect (for example, sweat evaporating from skin removes heat and cools a mammal).

Solvent action: what dissolves and what does not

Substance	Polar or non-polar?	Dissolves in water?	Reason
Na⁺ and Cl⁻ ions	charged	yes	Surrounded by oriented water molecules
Glucose	polar (–OH groups)	yes	Hydrogen bonds with water
Amino acids	polar / charged groups	yes	Hydrogen bonds and attraction to water dipoles
Triglyceride	non-polar	no	Cannot form hydrogen bonds with water
Phospholipid tails	non-polar	no	Tails turn away from water (head is hydrophilic)

Links: hydrophobic R groups folding into the centre of globular proteins (2.3) and phospholipid bilayers (2.2, Topic 4) both depend on water’s polarity.

Must-know distinctions

• Covalent bond vs hydrogen bond. O–H inside one molecule is covalent and strong. The attraction between molecules is a hydrogen bond and weak.
• Specific heat capacity vs latent heat of vaporisation. Specific heat capacity is about changing temperature of liquid water. Latent heat is about changing state from liquid to vapour with no temperature change.
• Stable vs warm. High specific heat capacity keeps temperature stable; it slows both heating and cooling.
• Evaporation cools; dripping does not. Only sweat that evaporates removes the latent heat.
• Polar vs non-polar. Polar and charged substances dissolve; non-polar ones do not, because they cannot form hydrogen bonds with water.
• Solvent vs reactant. Water dissolves substances (solvent action) and also takes part in hydrolysis (a reactant). For 2.4, the property asked for is solvent action.

Quick self-test

1. Which atom in a water molecule carries the δ− charge?
2. Between which two atoms does a hydrogen bond form?
3. State the maximum number of hydrogen bonds one water molecule can form.
4. Explain why sodium ions dissolve in water.
5. Give two substances transported dissolved in blood plasma.
6. Define specific heat capacity.
7. Calculate the energy needed to warm 300 g of water by 5 °C (c = 4.2 J g⁻¹ °C⁻¹).
8. 21 kJ is absorbed by 1 kg of water. Calculate the temperature rise (c = 4.2 J g⁻¹ °C⁻¹).
9. Explain why evaporation of water from a surface cools it.
10. A leaf loses 0.9 g of water by transpiration in an hour. Taking L = 2.4 kJ g⁻¹, calculate the energy removed.
11. Calculate the energy removed when 50 g of sweat evaporates (L = 2.4 kJ g⁻¹).
12. Why does a high specific heat capacity matter for enzymes in a fish?

Answers

1. Oxygen.
2. A δ+ hydrogen of one water molecule and a δ− oxygen of another.
3. Four.
4. The δ− oxygen ends of water molecules are attracted to Na⁺ and surround each ion, separating the ions.
5. Any two of: glucose, amino acids, ions (for example, Na⁺), urea.
6. The energy needed to raise the temperature of 1 kg (or 1 g) of a substance by 1 °C.
7. 300 × 4.2 × 5 = 6 300 J (6.3 kJ).
8. 21 000 ÷ (1 000 × 4.2) = 5.0 °C.
9. Molecules need energy to break hydrogen bonds and escape; the escaping molecules carry energy away, so the surface left behind loses energy and cools.
10. 0.9 × 2.4 = 2.16 kJ (2.2 kJ).
11. 50 × 2.4 = 120 kJ.
12. Water in and around the fish changes temperature slowly, so enzymes stay near their optimum and are not denatured by sudden heating.

Where marks are usually lost

• Calling the O–H bond inside a water molecule a hydrogen bond.
• Drawing a hydrogen bond from O to O or H to H, or leaving out δ+ and δ− labels.
• Saying water is polar without saying why (oxygen attracts electrons more; bent shape).
• Giving a property with no role, or a role with no property.
• Explaining specific heat capacity without mentioning energy used to break hydrogen bonds.
• Writing “sweat cools you” without saying energy is taken from the skin as water evaporates.
• Using cohesion or surface tension when the question is about the three 2.4 properties.
• Saying water “keeps organisms warm” instead of keeping temperature stable.
• Mixing g and kg, or J and kJ, in thermal calculations.
• Saying non-polar substances “repel” water instead of “cannot form hydrogen bonds with water”.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027, Cambridge University Press & Assessment – Topic 2, Biological molecules (section 2.4 Water).