At this stage in the learning program it is a good time to ask yourself why, at a given temperature and pressure, a particular substance is a liquid rather than a gas. Think about what it means to convert a substance from a liquid into a gas. Heat must flow into the system, increasing the internal energy and performing work, enough energy must be added to overcome the short-range attractive forces between the molecules (intermolecular forces) as well as expand the system (pressure volume work).

Boiling occurs when the likelihood of heat becoming internal energy and work to carry out vaporization becomes equal to the likelihood of the reverse pathway. The boiling temperature is higher the stronger the intermolecular attractions (and the larger the molecules), because the stronger the electrostatic force of attraction, the more energy is required to separate the particles, the greater the electrostatic potential energy increase associated with vaporization.

In the terminology of chemical thermodynamics, strong intermolecular forces decrease the free energy of the liquid phase, i.e. they stabilize it. The three kinds of intermolecular force listed by increasing strength are van der Waals forces, dipole-dipole attractions, and hydrogen bonds. Alkanes are lower boiling than aldehydes, which are, in turn, lower boiling than alcohols.

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