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MacRoscopic Properties: The World We See

Phases of Matter and Intensive Properties

What molecular properties lead to higher boiling points?

There are a few factors that play important roles in boiling points of substances. The first is molecular weight: in general, heavier molecules have higher boiling points, which is pretty straightforward considering how we defined boiling point above (heavier molecules take more energy to move from the liquid to the gas phase).

All other properties that affect boiling point deal with intermolecular forces, or interactions between molecules. Think of this like the affinity of one molecule of a substance to be attracted to, or stick to, another molecule. Noncovalent bonds, like ionic or hydrogen bonds, significantly increase boiling point. Why? Because to move to the vapor phase the molecules typically have to break these interactions. Dipole interactions and Van der Waals forces have similar effects (see “Atoms and Molecules”), but these interactions are weaker, so the effect they have on boiling points is smaller. Finally, branching of the carbon backbone of a molecule is also frequently touted as a factor leading to lower boiling points; while this is true, it is really the weakening of Van der Waals forces at work here too.



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