Ch 4 · 2.4 Water · AS & A Level Biology 9700

Topic 4 of 4 in Biological molecules — AS & A Level Biology 9700 — 58 lessons.

Introduction

Water makes up the majority of the mass of most living cells and is not a passive filler but an active participant in almost every biological process. Its unusual properties, cohesion, solvent power and thermal stability, all follow from one structural feature: the polarity of the water molecule and the hydrogen bonds this allows between neighbouring molecules. (Syllabus section 2.4; the syllabus examines solvent action, specific heat capacity and latent heat here. Cohesion is examined with transport in plants, section 7.2.)

Key ideas — The dipole nature of water and hydrogen bonding

A water molecule is polar: oxygen is more electronegative than hydrogen, so it pulls the shared electrons in each O–H bond towards itself, giving the oxygen atom a slight negative charge (δ⁻) and each hydrogen atom a slight positive charge (δ⁺). This uneven charge distribution makes water a dipole. The δ⁺ hydrogen of one water molecule is weakly attracted to the δ⁻ oxygen of a neighbouring molecule, forming a hydrogen bond. Each hydrogen bond is individually weak and transient, but water molecules form vast, constantly re-forming networks of hydrogen bonds, giving water many of its distinctive properties.

🧑‍🏫 Teacher's explanation

Imagine a group of people dancing in a crowded room. Each person has a slightly stronger attraction to the person's hands than to their feet. Even though the individual grip between two people is quite loose and can be broken easily, because everyone is holding hands in a massive, interconnected web, the crowd moves as a single, cohesive unit. This is how water behaves.

📖 Definitions

🔍 Worked example — Easy

Explain why water is described as a polar molecule.

🔍 Worked example — Standard

Relate the property of high specific heat capacity to the hydrogen bonding in water.

🔍 Worked example — Exam-style

Calculate the amount of energy required to raise the temperature of 500 g of water from 20circC to 30circC, given the specific heat capacity of water is 4.18 J g^-1circC^-1.

⚠️ Common errors

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🎯 Exam question

Key ideas — Cohesion and surface tension

Cohesion is the mutual attraction between water molecules caused by hydrogen bonding, which allows water to be pulled as a continuous, unbroken column, for example up the narrow xylem vessels of a plant during transpiration. At the surface of water, molecules are hydrogen-bonded more strongly to the molecules beside and below them than to the air above, pulling the surface layer taut and creating surface tension. This supports small organisms that live or move on the water surface and allows water to form curved menisci in narrow tubes.

🧑‍🏫 Teacher's explanation

Imagine a crowded dance floor where everyone is holding hands very tightly. If one person is pulled, everyone else in the chain feels the tug because they are all linked. This is how water molecules behave. Because water is a polar molecule, the slightly positive hydrogen atoms of one molecule are attracted to the slightly negative oxygen atoms of another. This 'hand-holding' is called a hydrogen bond.

📖 Definitions

🔍 Worked example — Easy

Identify the type of bond responsible for the cohesion between water molecules.

🔍 Worked example — Standard

Explain why water forms a curved surface (meniscus) in a narrow glass tube.

🔍 Worked example — Exam-style

A plant is undergoing transpiration. Describe how the properties of water allow it to transport water from the roots to the leaves.

⚠️ Common errors

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🎯 Exam question

Key ideas — Water as a solvent

Water's polarity makes it an excellent solvent for other polar and ionic substances: the δ⁺ and δ⁻ regions of water molecules surround and separate charged ions or polar molecules, keeping them in solution. This property is essential for transport in living organisms, since dissolved glucose, amino acids, mineral ions, urea and respiratory gases can all be carried in blood plasma, tissue fluid, xylem sap and phloem sap. Metabolic reactions also require reactants to be dissolved in water so they can diffuse and collide within the cell.

🧑‍🏫 Teacher's explanation

Imagine a crowded dance floor where everyone is holding hands. In water, the molecules are like dancers who are slightly 'sticky' because one side of them is slightly positive and the other is slightly negative. This 'stickiness' is called hydrogen bonding. Because water molecules are constantly moving and pulling on each other, they can easily get between other substances.

📖 Definitions

🔍 Worked example — Easy

Describe the role of water's polarity in its function as a solvent.

🔍 Worked example — Standard

Explain why water is an essential medium for metabolic reactions in a cell.

🔍 Worked example — Exam-style

A student observes that salt (NaCl) dissolves in water but not in a non-polar solvent like hexane. Using your knowledge of water's structure, explain this observation.

⚠️ Common errors

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🎯 Exam question

Key ideas — Specific heat capacity and latent heat of vaporisation

Because so much energy is absorbed in breaking hydrogen bonds, water has a high specific heat capacity: a large amount of heat energy is needed to raise its temperature, so the temperature of cells, tissues and aquatic habitats fluctuates only slowly even as the external environment changes, protecting enzyme-catalysed reactions from disruptive temperature swings. Water also has a high latent heat of vaporisation, meaning a large amount of energy is required to convert liquid water into vapour, again because hydrogen bonds must be broken. This makes evaporation, as in sweating in mammals or transpiration in plants, an effective cooling mechanism, since a small loss of water removes a disproportionately large amount of heat energy from the body.

🧑‍🏫 Teacher's explanation

Imagine you are trying to pull a group of people apart who are all holding hands very tightly. Each person represents a water molecule, and the 'hand-holding' represents the hydrogen bonds. To move the group or separate them, you have to exert a massive amount of energy to break those connections. This is why water is so good at absorbing heat without changing temperature quickly.

📖 Definitions

🔍 Worked example — Easy

Explain why water is a good solvent for many biological molecules.

🔍 Worked example — Standard

A 500 ext g sample of water is heated from 20^° extC to 30^° extC. Calculate the energy required if the specific heat capacity of water is 4.2 ext J/g^° extC.

🔍 Worked example — Exam-style

Compare the role of high specific heat capacity and high latent heat of vaporisation in maintaining homeostasis in mammals.

⚠️ Common errors

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🎯 Exam question

Key ideas — Linking properties to biological roles

Every biological role of water traces back to hydrogen bonding between polar molecules: cohesion depends on it directly, solvent action depends on the same polarity that permits hydrogen bonding, and both specific heat capacity and latent heat of vaporisation are high because hydrogen bonds must be broken before temperature can rise or evaporation can occur. Understanding this single underlying cause allows each named property to be explained from first principles rather than memorised in isolation.

🧑‍🏫 Teacher's explanation

Imagine a crowded dance floor where everyone is holding hands. Even if people are moving around, those handshakes (bonds) keep the group together. In water, these 'handshakes' are hydrogen bonds. Because water is a polar molecule, it has a slight positive charge on the hydrogen atoms and a slight negative charge on the oxygen atom. This electrical attraction is what creates the bond.

Ch 4 · 2.4 Water · AS & A Level Biology 9700

All chapters in Biological molecules