Explain how the polarity of water molecules enables water to act as a solvent for ionic compounds.

IB DP Biology Higher Level (2023 syllabus) — A1.1 Water · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Water is polar, with a δ− on the oxygen and δ+ on the hydrogens.

The δ− oxygen is attracted to cations and the δ+ hydrogens to anions.

Water molecules orient around each ion, forming a hydration shell that separates the ions.

These electrostatic attractions overcome the lattice energy, allowing the ionic compound to dissociate and dissolve.

Examiner tips

Use the word ‘polar’ and specify δ− and δ+ charges; link attraction to cations/anions; mention hydration shell; explain that attraction overcomes lattice energy.
Keep the answer concise – 4 short sentences matching the 4 mark points.

Common mistakes

Confusing the role of water as a solvent with the role of ions; writing that water ‘breaks’ the ionic bonds instead of ‘overcomes’ the lattice energy.
Omitting the polarity of water or the orientation of water molecules around ions.

Mark scheme (4 marks)

Water molecules are polar / have an uneven distribution of charge, with a partial negative charge (δ−) on the oxygen atom and partial positive charges (δ+) on the hydrogen atoms.
The δ− oxygen atoms are attracted to the positive ions (cations) and the δ+ hydrogen atoms are attracted to the negative ions (anions) of the ionic compound.
Water molecules surround / orientate around the individual ions, forming a hydration shell, which separates the ions from each other.
The electrostatic attractions between water molecules and ions overcome the lattice / ionic bonds holding the compound together, allowing the compound to dissociate and dissolve.

Key terms in this question

polarity