We saw in Section 2.2 that a state function is a property that depends only on the current
state of a system and is independent of its history. The internal energy and enthalpy
are two examples of state functions. Physical quantities that do depend on the path
between two states are called path functions. Examples of path functions are the work
and the heating that are done when preparing a state. We do not speak of a system in
a particular state as possessing work or heat. In each case, the energy transferred as
work or heat relates to the path being taken between states, not the current state itself.