ENERGY AND PHASE CHANGES
As we saw from the phase change diagram, a substance must absorb heat to
change from a solid to a liquid to a gas. It must lose heat to turn from gas to
liquid to solid. Heat, as you remember, is a form of energy. So, when you think
of phases and energy, remember that among the three phases, solid is lowest in
potential energy and gas is highest in potential energy.
One more thing: When ice melts, there is an increase in entropy, and when
liquids vaporize, entropy increases even more. Under certain conditions, the
increase in entropy (which the universe likes) is enough to overcome the
increase in energy (which it dislikes) and make a phase change spontaneous.
What are these conditions? Well, in the case of melting, if the substance is at a
temperature above the melting point, the phase change will be spontaneous. In
order for boiling to be spontaneous, the substance must be at a temperature
above the boiling point.
Review what we’ve discussed since the last set of questions, and then try the
following set. Answers can be found in Part III.