Question

What happens to the entropy when a gas undergoes expansion explain?

Answer 1

According to the second law, in any process in which thermally insulated system goes from one macrostate to another, the entropy tends to increase, i.e., DS>0. In particular during free expansion of a gas, which is a complicated irreversible process involving turbulence and gas nonuniformities of pressure and temperature (to the extend that these quantities can be defined at all for such a marked nonequilibrium situation) entropy increases.The formula DS=DQ/T is applicable only if a system undergoes a quasi-static process. To obtain DS you use a reversible pathway, where the system is not isolated.