Solubility and 'like dissolves like'
A substance dissolves when attractions between solute and solvent are about as strong as the attractions it must break. Polar solvents dissolve polar and ionic solutes. Nonpolar solvents dissolve nonpolar solutes.
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Analogy
Oil and water don't mix, like two groups at a party who have nothing in common. Oil and petrol mix easily.
Example
NaCl in water: dissolves (ion-dipole)I₂ in hexane: dissolves (London)NaCl in hexane: does notOil in water: does not
In practice
Salt dissolves in water but not in hexane, and grease dissolves in hexane but not in water.
Practice
Flashcards
- Why doesn't NaCl dissolve in hexane?Hexane is nonpolar and cannot attract the ions strongly enough to break the crystal.
Questions people ask
Why does temperature affect solid and gas solubility differently?
Most solids dissolve better when heated, because dissolving usually absorbs heat. Gases dissolve less when heated, because added heat gives gas molecules the energy to escape. That is why a warm fizzy drink goes flat faster than a cold one.
What does ion-dipole attraction mean?
When salt dissolves in water, the partly negative oxygen ends of water molecules surround the positive ions, and the partly positive hydrogen ends surround the negative ions. These attractions between ions and polar molecules are called ion-dipole forces, and they pay the energy cost of breaking up the crystal.