Water — AQA A-Level Biology
Test yourself on Water with AQA A-Level practice questions.
7 days Premium · Then free forever · No card, no charge
Water explained
Water is polar, forming hydrogen bonds between molecules.
Read the full explanation
This gives it five key biological properties. 1) It is a metabolite in condensation and hydrolysis reactions. 2) It is an important solvent for metabolic reactions, as polar molecules dissolve easily. 3) It has a high specific heat capacity, buffering temperature changes because energy breaks hydrogen bonds rather than raising kinetic energy, protecting enzymes from denaturation. 4) It has a large latent heat of vaporisation, providing a cooling effect (e.g. sweating) with little water loss. 5) Strong cohesion between molecules supports continuous water columns in plant xylem under tension, and produces surface tension where water meets air, supporting small organisms.
Your focus
- Explain how hydrogen bonding between water molecules allows a continuous column of water to be pulled up a xylem vessel.
- Relate the high specific heat capacity of water to the stability of enzyme activity in a cell or an aquatic habitat.
- Give one reaction in which water is a reactant and one in which it is a product, naming each type of reaction.
Water exam tips
Marking Points
- one mark for cohesion between water molecules, caused by hydrogen bonds, supporting an unbroken column of water in the xylem
- one mark for a high specific heat capacity buffering changes in temperature, so enzymes are not denatured
- one mark for water acting as a metabolite in a named condensation or hydrolysis reaction
- one mark for water acting as a solvent allowing metabolic reactions to occur
- one mark for a large latent heat of vaporisation providing a cooling effect through evaporation with little water loss
Examiner Tips
- 💡In xylem questions, write cohesion, continuous column and tension in the same answer; a correct cohesion-tension statement is creditworthy even if the individual points are loosely worded.
- 💡Match the property to the context asked for; listing all five properties when one is asked for wastes time and risks contradiction.
- 💡Every property answer should end with a biological consequence, not just the physics.
Common Mistakes
- saying water molecules are held together by hydrogen bonds within the molecule, when the bonds inside a molecule are covalent and hydrogen bonds form between molecules
- explaining cohesion as water sticking to the walls of the xylem, which is adhesion
- calling water a solvent without saying it is polar, or claiming that it dissolves non-polar lipids
- confusing latent heat of vaporisation with specific heat capacity when explaining sweating