The Maxwell relationships are extremely useful for two reasons. First,all of them are generally applicable.They are not restricted to ideal gases,or even just gases.They apply to solid and liquid systems as well.Second, they express certain relationships in terms of variables that are easier to measure. For example, it might be difficult to measure entropy directly and determine how entropy varies with respect to volume at constant temperature.The maxwell relationship in equation 4.36 show that we don't have to measure it directly.If we measure the change in pressure with respect to temperature at constant volume,lll,we know ppp.