Important Terms and Definitions of Thermodynamics

Important Terms and Definitions

1. System: A portion of universe which is under investigation, e.g., portion of test tube where reaction is taking place, is called system.

2. Surroundings: Everything else in the universe except system is called surroundings, e.g., except the portion of the test where reaction is taking place is surrounding, i.e. above and around, the test tube.

3. Open system: The system which can exchange matter and energy with the surrounding is called open system, e.g., a cup of tea is an open system because it will become cold as well as its taste will also change, Le., it is exchanging energy and matter with the surrounding.

4. Closed system: It can exchange energy hut not matter with the surroundings, e.g., tea placed in a closed kettle.

5. Isolated system: It can neither exchange heat nor matter with the surroundings, e.g., tea placed in thermos flask.

6. Homogeneous system: A system is homogeneous when it is completely uniform throughout, i.e. consists of one phase only, e.g., a pure solid, a pure liquid, a true solution, etc.

7. Heterogeneous system: A system is heterogeneous when it is not uniform throughout, i.e. it consists of two or more phases, e.g., a mixture of two solids or two or more immiscible liquids, a solid in contact with a liquid, a liquid in contact with its vapours.

8. Macroscopic system: When system consists of large number of atoms, molecules or ions, it is called macroscopic system.

9. Macroscopic properties: The properties associated with a macroscopic system, i.e. pressure, volume, temperature, density, composition, viscosity, surface tension, refractive index, colour, etc.

10. State of the system: A system is said to be in definite state when its macroscopic properties have definite values.

11. State variables: The macroscopic properties of a system are state variables since change in any of these properties, causes the system to change into another state.

12. Extensive property of a system: It is the property which depends upon amount of substance, e.g., mass, volume, pressure, internal energy, free energy, etc.

13. Intensive property: The property which does not depend upon amount of substance present in the system, e.g., temperature, density, viscosity, refractive index, surface tension, specific heat, etc.

14. Thermal equilibrium: A system is said to be in thermal equilibrium if no heat flows from one part of the system to another, i.e., when temperature remains constant throughout in all parts of the system.

15. Mechanical equilibrium: When no mechanical work is done by one part of the system on another part, the system is said to be in mechanical equilibrium. It is possible only when pressure remains the same throughout in all the portions of the system.

16. Isothermal process: The process in which temperature of the system remains constant at each stage of the process i.e., dT or ∆T = 0

17. Exothermic process: The process in which heat is given out to the surroundings is called exothermic process. In this process, products are more stable than reactants because they have lower energy.

18. Endothermic process: The process in which heat is absorbed by the system from the surroundings. In this process, products are less stable than reactants because they have higher energy.

19. Adiabatic process: The process in which no exchange of heat takes place between system and surroundings, i.e. dq = 0 or q = 0


a. In adiabatic expansion of an ideal gas, the internal energy of the system decreases and the temperature falls.

b. In adiabatic compression of an ideal gas, internal energy of the system increases and the temperature rises.

20. Isobaric process: The process which takes place at constant pressure is called an isobaric process, e.g., heating of water to its boiling point and its vaporization occurs at the same atmospheric pressure. Expansion of a gas in an open system is also an example of isobaric process in which dP or ∆P = 0

21. Cyclic process: In cyclic process, the system in a given state goes through a series of different processes, but in the end returns to its initial state. dE or ∆E = 0, dH or ∆H = 0

22. Isochoric process: The process which is carried out at constant volume is called isochoric process, i.e., dV or ∆V = O. e.g., ∆E, measurement at constant volume is isochoric process.

23. Reversible process: The process in which the change is carried out so slowly that the system and surroundings are always in equilibrium is known as reversible process. It never leads to completion.

24. State function: The thermodynamic property which depends upon initial and final state of the system d not on the means how the state is reached are state functions, e.g., E, H, S, G are state functions.

2 Responses to “Important Terms and Definitions of Thermodynamics”

  1. Anyewi George

    clear definitions


    Is there any app for it so that I can get direct information which is needed.

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