# Why are Andrews' isotherms divided into three fragments at low temperature and low pressure?

In college, I studied that for an ideal gas $$PV = nRT$$ , that is, at constant temperature and constant number of moles, $$P \propto \frac {1}{V}$$. Thus, graph between $$P$$ and $$V$$ is a rectangular hyperbola when $$dT =0$$ and $$dn=0$$. But in my textbook there is this topic called 'Andrews' isotherms' wherein they have given some $$P$$ vs $$V$$ curves for gases and at low temperatures and low pressures the graphs seem to be divided into three fragments. How is this possible? Shouldn't the curve be a rectangular hyperbola? How is it possible? Shouldn't ideal gases obey $$PV = constant$$ at all temperatures and pressure. That is literally the definition of ideal gas : A gas that obeys ideal gas equation at all temperatures and pressure. I also studied that real gases show deviation from ideal gases. But when we draw $$P$$ vs $$V$$ curves of a real gas, they deviate from the $$P$$ vs $$V$$ curves of ideal gases slightly at high pressures and low temperatures. They almost remain rectagular hyperbolas. But the $$P$$ vs $$V$$ curves of real gases don't literally break into three fragements like Andrews' isotherms. The curves in Andrews' isotherms don't even come close to rectangular hyperbolas. What type of curve is being shown in Andrews' isotherms? How are these curves disobeying Boyle's law to such a great extent, not even coming close to a rectangular hyperbola?

• Is the figure in this Physics SE question what you mean? If so, the answer is that these fragmented curves are cases where the the substance is transitioning from a liquid to a gas.
– Tyberius
Commented Apr 16, 2021 at 15:49