I received a question from one of my students of a high school (17 years old) with reference to a graph of a Clayperon-plane. Suppose that it is the one shown in the figure.
From state $A \to B$ is evidently an isobar (with volume expansion); and from $B\to C$ an isochore.
Are the transformations from $C\to D$ and from $D \to A$ hypothetical or not real transformations referring to ideal gases or to real gases or are they approximations of particular thermodynamic transformations that I am not known to except for adiabatic, isothermal, isochore and isobaric? What is the best justification that I can give my students of a high school about transformations such as those going from $C$ to $D$ and from $D$ to $A$?