Explain how the properties of water make it an effective solvent for ionic compounds in living organisms.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Water is polar, with a partial negative charge (δ−) on the oxygen and partial positive charges (δ+) on the hydrogens. These partial charges attract the opposite charges of ionic compounds: the δ− oxygen is attracted to cations and the δ+ hydrogens to anions. Water molecules surround each ion, forming a hydration shell that separates the ions from the crystal lattice and keeps them in solution. This solvation allows ions such as Na⁺, Cl⁻ and mineral ions to be transported in blood, cytoplasm and across membranes, supporting biochemical reactions in living organisms.
Examiner tips
- Use the word ‘polar’ and describe δ− and δ+ charges; link attraction to cations and anions; mention hydration shell; explain biological relevance (transport, reactions).
Common mistakes
- Confusing water as non‑polar; omitting the hydration shell; failing to connect solvation to biological transport or reactions.
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 ionic compounds / the δ− oxygen is attracted to cations and the δ+ hydrogens are attracted to anions.
- Water molecules surround individual ions, forming a hydration shell, which separates ions from the crystal lattice and keeps them in solution.
- This allows ions (e.g. Na⁺, Cl⁻, mineral ions) to be transported in blood / cytoplasm / across membranes, supporting biochemical reactions in living organisms.
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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