Solubility Explained: Like Dissolves Like
Whether something dissolves comes down to one energy question: do the new attractions between solute and solvent release enough energy to overcome the ones you break? Here is how to compare the forces and decide, the way the exam wants it.
A tug-of-war of attractions
Dissolving means breaking attractions and making new ones. Compare three: the solute-solute attraction, the solvent-solvent attraction, and the new solute-solvent attraction. It dissolves only if the new solute-solvent attraction releases enough energy to overcome the solute-solute and solvent-solvent attractions you had to break.
Like dissolves like
Match the intermolecular forces. Non-polar solutes (id-id) dissolve in non-polar organic solvents; polar and hydrogen-bonding solutes dissolve in water. So Br2 (id-id) dissolves in CCl4 (id-id), and H2O2 (hydrogen bonding) in water, but ethane does not dissolve in water: weak id-id cannot release enough energy to overcome water's strong hydrogen bonds.
Ionic solids: ion-dipole vs the lattice
An ionic solid dissolves in water when the ion-dipole attractions between its ions and water release enough energy to overcome the strong electrostatic attraction in the lattice. NaCl dissolves; AlF3 does not, because the small, highly charged Al3+ makes its lattice attraction too strong for ion-dipole to overcome.
How to phrase the answer
Name the new solute-solvent force, say it releases sufficient (or insufficient) energy to overcome the named forces in the solute and the solvent, then conclude soluble or insoluble. Stick to the H2 named forces: id-id, hydrogen bonding, ion-dipole.
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