Explain how the polarity of water molecules enables water to act as a solvent for ionic compounds, and outline one consequence of this solvent property for living organisms.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Water molecules are polar, with a partial negative charge (δ−) on the oxygen atom and partial positive charges (δ+) on the hydrogen atoms.
The polarity of water allows the partial charges to attract oppositely charged ions of an ionic compound. When an ionic solid dissolves, water molecules surround each ion, forming a hydration shell that stabilises the ions and keeps them dispersed in solution.
A consequence of this solvent property is that dissolved ions such as Na⁺ in blood plasma or Ca²⁺ in xylem sap can be transported throughout the organism. This availability of ions is essential for processes such as nerve impulse transmission, enzyme activity and maintaining osmotic balance.
The polarity of water allows the partial charges to attract oppositely charged ions of an ionic compound. When an ionic solid dissolves, water molecules surround each ion, forming a hydration shell that stabilises the ions and keeps them dispersed in solution.
A consequence of this solvent property is that dissolved ions such as Na⁺ in blood plasma or Ca²⁺ in xylem sap can be transported throughout the organism. This availability of ions is essential for processes such as nerve impulse transmission, enzyme activity and maintaining osmotic balance.
Examiner tips
- Use the term ‘polar’ and specify δ− on O and δ+ on H. Show that water ‘attracts’ and ‘surrounds’ ions, forming hydration shells. Give a clear biological example (e.g. Na⁺ in blood).
Common mistakes
- Failing to mention the partial charges on water. Not describing the formation of hydration shells. Giving a non‑biological example or omitting the consequence for living organisms.
Mark scheme (4 marks)
- Water molecules are polar / have an uneven distribution of charge, with a partial negative charge (δ−) on the oxygen and partial positive charges (δ+) on the hydrogen atoms.
- The partial charges on water molecules attract the oppositely charged ions of an ionic compound, surrounding / dissociating the ions and holding them in solution (hydration shells form around each ion).
- One biological consequence: dissolved ions / minerals / nutrients can be transported in blood plasma, cytoplasm, or xylem sap (accept any specific example such as sodium ions in blood, mineral ions in xylem).
- Further consequence or elaboration: because ions/polar solutes remain dissolved they are available for metabolic reactions / enzyme activity / maintaining osmotic balance / nerve impulse transmission (accept any one distinct, relevant biological function linked to solvent property).
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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