Thermodynamics: The branch of science that deals with the study of different forms of energy and the quantitative relationships between them.
System: Quantity of matter or a region of space which is under consideration in the analysis of a problem.
Surroundings: Anything outside the thermodynamic system is called the surroundings. The system is separated from the surroundings by the boundary. The boundary may be either fixed or moving.
Closed system: There is no mass transfer across the system boundary. Energy transfer may be there.
Open system: There may be both matter and energy transfer across the boundary of the system.
Isolated system: There is neither matter nor energy transfer across the boundary of the system.
State of the system and state variable: The state of a system means the conditions of the system. It is described in terms of certain observable properties which are called the state variables, for example, temperature (t), pressure (p), and volume (v).
State function: A physical quantity is a state function in the change in its value during the process depends only upon the initial state and final state of the system and does not depend on the path by which the change has been brought about.
Macroscopic system and its properties: If as system contains a large number of chemical species such as atoms, ions, and molecules, it is called macroscopic system. Extensive properties: These properties depend upon the quantity of matter contained in the system. Examples are; mass, volume, heat capacity, internal energy, enthalpy, entropy, Gibb's free energy. Intensive properties: These properties depend only upon the amount of the substance present in the system, for example, temperature, refractive index, density, surface tension, specific heat, freezing point, and boiling point.
Types of thermodynamic processes: We say that a thermodynamic process has occurred when the system changes from one state (initial) to another state (final).
Isothermal process: When the temperature of a system remains constant during a process, we call it isothermal. Heat may flow in or out of the system during an isothermal process.
Adiabatic process: No heat can flow from the system to the surroundings or vice versa.
Isochoric process: It is a process during which the volume of the system is kept constant.
Isobaric process: It is a process during which the pressure of the system is kept constant.
Reversible processes: A process which is carried out infinitesimally slowly so that all changes occurring in the direct process can be exactly reversed and the system remains almost in a state of equilibrium with the surroundings at every stage of the process.