Water
Cambridge A-Level Biology (9700) · Unit 2: Biological molecules · 8 flashcards
Water is topic 2.4 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 2 — Biological molecules , alongside Testing for biological molecules, Carbohydrates and lipids and Proteins. In one line: A solvent is a substance that dissolves a solute, resulting in a solution. Water is an excellent solvent due to its polarity, which allows it to form hydrogen bonds with, and thus dissolve, many polar and ionic substances such as glucose and salts.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 8 flashcards — 3 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The term 'solvent' and explain why water is an effective solvent
A solvent is a substance that dissolves a solute, resulting in a solution. Water is an excellent solvent due to its polarity, which allows it to form hydrogen bonds with, and thus dissolve, many polar and ionic substances such as glucose and salts.
What the Cambridge 9700 syllabus says
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- explain how hydrogen bonding occurs between water molecules and relate the properties of water to its roles in living organisms, limited to solvent action, high specific heat capacity and latent heat of vaporisation
- www.cambridgeinternational.org/alevel
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Water
- › Remember: three polypeptide chains form one collagen molecule (triple helix). Molecules then arrange into fibrils, and fibrils into fibres.
- › Distinguish between the direct Benedict’s test and the non-reducing sugar test which requires prior acid hydrolysis and neutralization.
- › Amylose is unbranched with 1,4-glycosidic bonds only; amylopectin is branched and contains both 1,4 and 1,6-glycosidic bonds.
- › Recall that hydrolysis reactions use water to split molecules, whereas condensation reactions release water.
- › Understand that the non-reducing sugar test hydrolyzes disaccharides into monosaccharides, increasing the total concentration of reducing sugars detected.
Explain how hydrogen bonds form between water molecules.
Hydrogen bonds form due to the slight negative charge (δ-) on the oxygen atom and slight positive charge (δ+) on the hydrogen atoms within a water molecule. This polarity arises from oxygen's higher electronegativity. The δ+ hydrogen of one water molecule is attracted to the δ- oxygen of another, forming a hydrogen bond.
Define the term 'solvent' and explain why water is an effective solvent.
A solvent is a substance that dissolves a solute, resulting in a solution. Water is an excellent solvent due to its polarity, which allows it to form hydrogen bonds with, and thus dissolve, many polar and ionic substances such as glucose and salts.
Explain how water's solvent properties are important in living organisms.
Water's solvent properties allow for the transport of essential substances (
Define 'specific heat capacity' and explain why water has a high specific heat capacity.
Specific heat capacity is the amount of heat energy required to raise the temperature of 1 gram of a substance by 1 degree Celsius. Water has a high specific heat capacity because hydrogen bonds between water molecules require a lot of energy to break or weaken, limiting temperature increases.
Explain the biological significance of water's high specific heat capacity.
Water's high specific heat capacity helps maintain stable internal temperatures in organisms and aquatic environments. This prevents rapid temperature fluctuations that could harm biological processes.
Define 'latent heat of vaporisation' and explain why water has a high latent heat of vaporisation.
Latent heat of vaporisation is the amount of heat energy required to change 1 gram of a liquid into a gas at its boiling point. Water has a high latent heat of vaporisation due to the strong hydrogen bonds that must be broken to allow water molecules to escape as gas.
Explain how water's high latent heat of vaporisation is important for thermoregulation in living organisms.
Evaporation of water, such as sweat in mammals or transpiration in plants, requires a large amount of heat energy. This cools the organism as the heat is taken away from the body, providing an effective cooling mechanism.
Describe an example of how water's properties contribute to the transport of nutrients in plants.
Water acts as a solvent to dissolve mineral ions in the soil, which are then transported up the xylem vessels from the roots to the leaves through transpiration. Water's cohesive properties, due to hydrogen bonding, allow for the continuous water column needed for transpiration.
More topics in Unit 2 — Biological molecules
Water sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Water deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
How to study this Water deck
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