For instance, a system can be isothermally expanded by adding heat, or its pressure can be slowly decreased without the addition of heat. Yet the final pressure, temperature and volume are the same. When they are racked, the number of possible arrangements drops dramatically, and so too does their entropy. Entropy is an intrinsic state variable. In a completely isolated system, it can never decrease.
Disorder is a convenient concept for desktops and closets, but is too imprecise for physics.
We will therefore forget about it when we make use of entropy in Sections E and F . The temperature of a substance changes as heat energy is added to it. The specific heat of human tissue is .85. That of air is .23. For a given substance at a given pressure, phase changes occur at well-defined temperatures.