In order that a single process be both isothermal and occur at constant pressure: A)one must use an ideal gas B)such a process is impossible C)a change of phase is essential D)one may use any real gas such as N₂ E)one must use a solid

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A change of phase, such as a gas turning into a liquid or a solid, is necessary for a process to be both isothermal (occurring at constant temperature) and occur at constant pressure. This is because during a change of phase, the energy (in the form of heat) is used to break or form intermolecular attractions, rather than to increase the temperature or do work. As a result, the pressure remains constant during the phase change.

In Order That a Single Process Be Both Isothermal and Occur