Intermolecular Forces
Ever wondered why some substances melt easily whilst others need loads of heat? It's all down to intermolecular forces - the attractions between molecules that you'll need to master for your exams.
Induced dipole-dipole forces are the weakest type and occur in all molecules. Larger molecules like iodine have bigger electron clouds, creating stronger forces and higher boiling points. That's why straight-chain hydrocarbons have higher boiling points than branched ones - they can pack together more tightly.
Permanent dipole-dipole forces happen between polar molecules like HCl. These are stronger than induced forces because the molecules have permanent positive and negative ends. You can actually test for polar molecules by seeing if a liquid stream bends towards a charged rod!
Hydrogen bonding is the strongest intermolecular force and only occurs with nitrogen, oxygen, or fluorine. This explains why water has such unusual properties - hydrogen bonds make ice less dense than liquid water, which is why ice floats.
Quick Test Tip: Remember that when heating liquids, you're breaking intermolecular forces, not covalent bonds within molecules.
