Forces Inside Molecules vs Forces Between Molecules

0:00When water boils, what comes apart: the bonds inside each molecule, or the forces between molecules?

0:08Start with one water molecule, H two O: one oxygen atom joined to two hydrogen atoms.

0:16Each link is a covalent bond: a pair of electrons shared between the two atoms.

0:23Forces inside a molecule are called intramolecular forces. Intra means within, and these covalent bonds are strong.

0:33In liquid water, each molecule sits close to its neighbours.

0:38The attractions between neighbouring molecules are called intermolecular forces. Inter means between.

0:45In water, the strongest of these are hydrogen bonds, but even they are much weaker than covalent bonds.

0:53Boiling gives the molecules enough energy to overcome these attractions, so they spread far apart as steam.

1:01The covalent bonds inside each molecule do not break, so every molecule of steam is still H two O.

1:09Boiling takes about 41 kilojoules per mole of water. Breaking all its covalent bonds would take about 930 kilojoules per mole.

1:19So boiling is a physical change, with no new substance: H two O liquid becomes H two O gas. It does not make hydrogen and oxygen.

1:30Boiling overcomes the weak forces between molecules, while the strong covalent bonds inside them stay whole.