← Bonding, structure, properties and energy changes

Dissolving and solubility

"Like dissolves like" is the one-line summary, but full marks need the mechanism: which attractive forces form, and whether they can overcome what is being broken apart.

The method for any solubility question

  • Identify the TYPE of solid dissolving (ionic, molecular polar, or molecular non-polar) and the type of forces holding it together.
  • Identify what NEW attractive forces would form between the solute's particles and water (or another solvent).
  • Compare: if the new forces are strong enough to overcome the original ones, the substance dissolves; if not, it does not.
  • For an IONIC solid specifically, describe the mechanism: water's partial charges attract the ions individually, pulling them out of the lattice — a diagram showing hydrated ions is often explicitly required.
Worked example Why LiBr dissolves in water but not in cyclohexane Lithium bromide is an ionic solid. Explain why it dissolves in water but not in cyclohexane (a non-polar solvent). LiBr is held together by strong ionic bonds — the electrostatic attraction between Li⁺ and Br⁻ ions in a lattice. WATER: water molecules are polar, with a δ− oxygen and δ+ hydrogens. The δ− oxygen attracts Li⁺, and the δ+ hydrogens attract Br⁻, surrounding (hydrating) each ion individually. These ion-dipole attractions are strong enough to overcome the ionic bonds in the lattice, so LiBr dissolves. CYCLOHEXANE: a non-polar molecule with only weak London forces between its own molecules. It cannot form strong attractions with the charged Li⁺ and Br⁻ ions, so nothing exists that could overcome the ionic lattice — LiBr does not dissolve. ✓ The deciding factor is whether the SOLVENT can form attractions with the solute strong enough to overcome what already holds the solute together — water can (ion-dipole), cyclohexane cannot (only weak London forces available). ✕

"Like dissolves like" is a summary, not the explanation itself

Writing "like dissolves like, so it dissolves" states the rule without applying it. Full marks require naming the SPECIFIC attractive forces on both sides — what holds the solute together, and what new attraction forms with the solvent — and comparing their relative strength. The phrase is a useful memory aid, not a substitute for the mechanism.

Predicting solubility quickly from molecule type

  • Ionic solid + polar solvent (water) → usually SOLUBLE (ion-dipole attractions overcome the lattice).
  • Polar molecular solid (small alcohols, etc.) + water → usually SOLUBLE (hydrogen bonding or dipole-dipole with water).
  • Non-polar molecular solid (long hydrocarbons, iodine) + water → usually INSOLUBLE (water cannot form strong attractions with a non-polar substance).
  • Non-polar molecular solid + non-polar solvent (hexane, cyclohexane) → usually SOLUBLE (similar weak London forces on both sides, nothing strong to overcome).

Practise just this