The state is the condition of the system at an instant of time as described or measured by its properties. It is a complete description of a system in terms of its properties. At a given state all the properties of a system have fixed values.

The requirement to specify the property to fix the state:

1. All the properties values should be uniform throughout the system.

2. They should not change continuously Any operation in which one or more of the properties of a system changes is called a change of state. The process occurs when the system undergoes a change in state or in an energy transfer at a steady state.

Simply when system intercuts with surrounding, a process takes place.

The series of the state through which a system passes during a process is called the path of the process. with time.