Properties are described completely when the states of a system is in thermodynamic equilibrium. How ca11 we describe the intermediate stnt2s of a system along the path of a process since sthe process takes place olliy when the system is in equilibriw11? We can justify the above question by the following example. Let us take a piston-cylinder mechanism as shown in Fig. 1.7. The weight W is made up of several small weights like W1, W2, … , W q. If we remove the weights one by one slowly, then the gas inside the cylinder undergoes infinitesimal change of state. All the states in between will be deviated from equilibrium infinitesimal. This intermediate processes are called quasistatic process or quasi-equilibrium process.
However, if tl1e total weight W is removed suddenly, the piston will get a jerk and strike the stoppers. It will con1e to equilibrium after many oscillations. But in this case, it is not possible to describe the intermediate states or paths. Such a process is a non-equilibrium process.
Thus in quasi-static process, there is only an infinitesimal change in properties of substance from its equilibrium.
In several cases, one property remains constant during a process. The prefix iso is used to describe such a process. If temperature remains constant, the process is called isothermal process. If pressure remains constant, the process is known as isobaric or isopiestic process and if volume is constant, it is called isochoric process