Cambridge International AS & A Level Biology 9700: Water – Study Guide

This study guide teaches section 2.4 Water from Topic 2, Biological molecules, in the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. Section 2.4 has a single learning outcome, and this page covers all of it. This is AS Level content: it is examined on Paper 1 (Multiple Choice) and Paper 2 (AS Level Structured Questions), and Paper 4 can draw on it because A Level questions require knowledge of the AS content. Calculators are allowed in 9700 examinations.

For an overview of all of Topic 2, read the Biological molecules study guide. Afterwards, use the Water revision notes for recall and the Water practice questions to test yourself.

What this section covers

Outcome	What you must be able to do
2.4.1 (first part)	Explain how hydrogen bonding occurs between water molecules
2.4.1 (second part)	Relate the properties of water to its roles in living organisms, limited to: solvent action, high specific heat capacity, latent heat of vaporisation

The words “limited to” matter: for 2.4 you need exactly three properties. Water potential (Topic 4), cohesion-tension in the xylem (Topic 7) and water in blood and tissue fluid (Topic 8) build on this section.

Why water is polar

A water molecule, H₂O, has one oxygen atom joined to two hydrogen atoms by covalent bonds. Each bond is a pair of shared electrons.

Oxygen attracts the shared electrons more strongly than hydrogen does. So the electrons spend more time nearer the oxygen. This gives:

• the oxygen atom a small negative charge, written δ− (delta negative)
• each hydrogen atom a small positive charge, written δ+ (delta positive)

The molecule is not straight. The two hydrogen atoms sit on one side of the oxygen at an angle of about 104.5°. Because of this bent shape, one side of the molecule (the oxygen side) is slightly negative and the other side (the hydrogen side) is slightly positive. A molecule with an uneven spread of charge like this is called polar, and it is said to have a dipole.

How hydrogen bonds form between water molecules

Opposite charges attract. The δ− oxygen of one water molecule is attracted to a δ+ hydrogen of a neighbouring water molecule. This attraction is a hydrogen bond.

Points you must be able to explain:

1. A hydrogen bond forms between molecules. The O–H bonds inside a molecule are covalent.
2. Each hydrogen bond is weak compared with a covalent bond.
3. Each water molecule can form up to four hydrogen bonds: two through its two δ+ hydrogens and two through its δ− oxygen.
4. In liquid water, hydrogen bonds break and re-form all the time. At any moment there are very many of them, so together they have a large effect.

When you draw it, label δ+ and δ− on the atoms and use a dotted line for the hydrogen bond, running from an H on one molecule to an O on the other.

Every property in this section comes back to hydrogen bonding. Learn the chain: polar molecule → hydrogen bonds between molecules → property → role in an organism.

Property 1: solvent action

Water is a very good solvent for polar molecules and ions.

Ions. When a salt such as sodium chloride is put into water, water molecules gather around each ion:

• around a positive ion (Na⁺), water molecules turn their δ− oxygen towards the ion
• around a negative ion (Cl⁻), water molecules turn their δ+ hydrogens towards the ion

The ions become surrounded by water molecules and separate from each other. They are now dissolved.

Polar molecules. Glucose, amino acids and urea are polar. They have groups such as –OH or –NH₂ that can form hydrogen bonds with water. So they dissolve.

Non-polar molecules. Triglycerides are non-polar and hydrophobic. They cannot form hydrogen bonds with water, so they do not dissolve. Water molecules hydrogen bond to each other instead and push the non-polar molecules together. This same idea explains why the hydrophobic tails of phospholipids point away from water (2.2) and why hydrophobic R groups end up inside a globular protein (2.3).

Roles of solvent action in living organisms

• Transport. Substances dissolved in water can be carried in a moving stream. Glucose, amino acids, ions and urea are transported dissolved in blood plasma. Mineral ions travel in the xylem dissolved in water, and sucrose travels dissolved in the phloem.
• A medium for reactions. Metabolic reactions take place in solution in the cytoplasm. Molecules must be dissolved to move around and collide with enzymes.
• Removal of waste. Urea is excreted dissolved in water in urine.
• Reactant. Water also takes part in reactions. Hydrolysis uses a water molecule to break glycosidic, ester and peptide bonds (2.2 and 2.3).

Worked example 1: dissolving

Question. Explain why glucose dissolves in water but a triglyceride does not. [3 marks]

Answer.

• Glucose has many –OH groups, so it is polar.
• The –OH groups form hydrogen bonds with water molecules, so glucose dissolves.
• A triglyceride is non-polar (its fatty acid chains are hydrocarbon), so it cannot form hydrogen bonds with water and does not dissolve.

Each point names a molecular feature and links it to hydrogen bonding.

Property 2: high specific heat capacity

Specific heat capacity is the energy needed to raise the temperature of 1 kg (or 1 g) of a substance by 1 °C.

For liquid water it is about 4.2 kJ per kg per °C (4.18 J g⁻¹ °C⁻¹ at 25 °C). This is high.

Why it is high

Temperature is a measure of how fast molecules are moving (their kinetic energy). When water is heated, much of the energy is used to break hydrogen bonds between the molecules. Only the rest makes the molecules move faster. So a lot of energy must be absorbed before the temperature rises by 1 °C. In the same way, water must lose a lot of energy before its temperature falls.

Roles in living organisms

• Stable aquatic habitats. Seas, lakes and ponds change temperature slowly, even when air temperature changes a lot between day and night or between seasons. Organisms living in water face a more stable temperature.
• Stable body temperature. Cells and bodies are mostly water. Their temperature changes slowly when heat is gained or lost. This keeps conditions steady for enzymes, which work best within a narrow temperature range and can be denatured if they get too hot.

Worked example 2: a calculation

You may be given data and asked to use energy = mass × specific heat capacity × temperature change.

Question. A garden pond holds 2 000 kg of water. On a sunny day it absorbs 50 MJ of energy from the Sun. Take the specific heat capacity of water as 4.2 J g⁻¹ °C⁻¹. Calculate the rise in temperature, and suggest how this would differ for a liquid with half the specific heat capacity of water.

Working.

mass        = 2 000 kg = 2 000 000 g
energy      = 50 MJ = 50 000 000 J
temp change = energy / (mass x c)
            = 50 000 000 / (2 000 000 x 4.2)
            = 5.95 °C  (about 6.0 °C)

With half the specific heat capacity, the same energy gives twice the rise: about 11.9 °C. Organisms living in such a liquid would face much larger daily swings.

Check your units first. Mass in grams must go with J g⁻¹ °C⁻¹; mass in kilograms must go with J kg⁻¹ °C⁻¹.

Property 3: high latent heat of vaporisation

Latent heat of vaporisation is the energy needed to change a liquid into a vapour (gas) without a change in temperature. For water it is very high: about 2 257 J for every gram at 100 °C, and nearer 2 400 J g⁻¹ at body temperature.

Why it is high

For a water molecule to escape from the liquid surface as vapour, all the hydrogen bonds holding it to its neighbours must be broken. That takes a lot of energy. The molecules that escape are the ones with the most energy. They take that energy away with them, so the liquid left behind (and the surface it is on) is cooled.

Roles in living organisms

• Sweating in mammals. Sweat spreads over the skin. As water in sweat evaporates, it absorbs energy from the skin and cools the body.
• Panting. Some mammals, such as dogs, evaporate water from the moist surfaces of the mouth and airways to lose heat.
• Transpiration in plants. Water evaporates from the cell walls of mesophyll cells inside the leaf, and the vapour diffuses out through stomata. The evaporation cools the leaf, which helps stop leaf enzymes overheating in strong sunlight.

Because the value is so high, a lot of heat is lost for only a small loss of water.

Worked example 3: comparing the two thermal properties

Question. Use the values 4.18 J g⁻¹ °C⁻¹ (specific heat capacity) and 2 257 J g⁻¹ (latent heat of vaporisation) to compare the energy removed by evaporating 1 g of water with the energy removed by cooling 1 g of water by 1 °C.

Working. 2 257 ÷ 4.18 = 540 (to 3 significant figures).

Answer. Evaporating 1 g of water removes about 540 times as much energy as cooling 1 g of water by 1 °C. This is why evaporation is such an effective way for an organism to lose heat.

Worked example 4: sweat

Question. During a long run, a person must lose 1 500 kJ of heat by evaporation of sweat. Taking the latent heat of vaporisation of water as 2.4 kJ g⁻¹, calculate the mass of sweat that must evaporate.

Working. mass = energy ÷ latent heat = 1 500 ÷ 2.4 = 625 g.

Only sweat that evaporates cools the body. Sweat that drips off the skin takes very little heat with it, so the person usually has to produce more than 625 g.

In a calculation, use the value the question gives you.

Putting it together: a model answer

Question. Explain how hydrogen bonding between water molecules allows water to have roles in living organisms. [6 marks]

Model answer.

1. Oxygen attracts the shared electrons more than hydrogen, so water molecules are polar (δ− O, δ+ H).
2. Hydrogen bonds form between the δ− O of one molecule and the δ+ H of another.
3. Polar water molecules surround ions and form hydrogen bonds with polar molecules, so water is a good solvent; for example, glucose is transported dissolved in blood plasma.
4. Energy is needed to break many hydrogen bonds before temperature rises, so water has a high specific heat capacity.
5. So aquatic habitats and cells change temperature slowly, keeping enzymes near their optimum.
6. Many hydrogen bonds must be broken for water to evaporate, so it has a high latent heat of vaporisation; evaporation of sweat or transpiration cools the organism.

Each property gets its own hydrogen-bond reason and its own named role.

Common errors

• Calling the bond between O and H within a water molecule a hydrogen bond. It is a covalent bond.
• Drawing a hydrogen bond from O to O, or from H to H. It always joins a δ+ H to a δ− O.
• Writing “water has a high boiling point” or “water is cohesive” when the question asks for the 2.4 properties. Those are not the three properties this outcome is limited to.
• Saying “water keeps organisms warm”. High specific heat capacity keeps temperature stable, in both directions.
• Explaining evaporative cooling with “water absorbs heat” alone. You must say that energy is used to break hydrogen bonds and that the escaping molecules carry energy away.
• Saying a non-polar substance “repels water”. Explain that it cannot form hydrogen bonds with water.
• Listing a property with no organism or role, or a role with no property.

Next steps

• Recall: Water revision notes
• Test yourself: Water practice questions
• Related sections: Carbohydrates and lipids study guide and the whole-topic Biological molecules practice questions
• Course hub: Cambridge A Level Biology
• Printable checklist: 9700 checklist
• Check your gaps: Free 9700 AS diagnostic

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).