Joule-Thomson expansion (also known as the throttling process) involves the expansion of a gas from a high-pressure region to a low-pressure region through a porous plug or restricted valve under adiabatic conditions ($q = 0$) . According to the first law of thermodynamics, for such a process, the enthalpy ($H = U + pV$) remains constant from the initial to the final state . Therefore, the process is described as isoenthalpic. While the sources provided discuss Joule's experimental work on internal energy and the definition of enthalpy as a state function, the specific classification of the Joule-Thomson expansion as isoenthalpic is a standard principle in chemical thermodynamics .