Chemical Thermodynamics: System and Surrounding, State of System (For CBSE, ICSE, IAS, NET, NRA 2022)

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System and Surrounding

  • System is the part of the physical universe, which is under study, while the rest of the universe is surroundings.
  • For Example, If we are studying the reaction of two substances A and B kept in a beaker, the reaction mixture of A and B is a system and the beaker and the room where it is kept are surroundings as shown in figure.
System and Surrounding

Isolated System

  • Isolated system is a system which can exchange neither matter nor energy with the surroundings .
  • For example, hot tea/milk (let us call it a system) kept in a stoppered thermos flask which is made of perfect insulating material, then there would be no exchange of matter or energy between the system and the surroundings. Such a system is called an isolated system.

Closed System

  • Closed system is a system which can exchange energy but not matter with the surroundings.
  • For example, hot tea/milk in a stoppered stainless-steel flask, it will not remain hot after some time. Here energy is lost to the surroundings through the steel walls, but due to stopper, the matter will not be lost. Such a system is called closed system.

Open System

  • Open system is a system which can exchange both energy and matter with surroundings.
  • For example, hot tea/milk in an open stainless-steel flask or thermos flask , then some matter will also be lost due to evaporation along with energy. Such a system is called an open system.

State of System

  • State of a system is described by its measurable properties.
  • For example, the state of a gas by specifying its pressure, volume, temperature etc.
  • These variable properties are called state variables or state functions.
  • State function depend only on the initial and final state of the system and not on the path taken by the system during the change.
  • Change in state of a system is defined by giving the initial and the final state of the system.
State of System

Properties of the System

The properties if the system is divided into two main types:

Extensive Property

Extensive Property (variable) is one whose value depends upon the size of the system. For example, volume, weight, heat, etc.

Intensive Property

Intensive Property (variable) is one whose value is independent of the size of the system. For example, temperature, pressure, refractive index , viscosity, density, surface tension, etc.


The method of bringing about a change in state is called process.

Types of Processes

Isothermal Process

  • When the temperature of the system remains constant during various operations, then the process is said to be isothermal.
  • This is attain either by removing heat from the system or by supplying heat to the system.
  • For example, Ice melts at 273 K and 1 atm pressure . The temperature does not change as long as the process of melting goes on.

Adiabatic Process

  • In an adiabatic process there is no exchange of heat between the system and surroundings. Thus, in adiabatic processes there is always a change in temperature.
  • For example, If an acid is mixed with a base in a closed thermos flask, the heat evolved is retained by the system.

Reversible Process

  • Reversible processes are those processes in which the changes are carried out so slowly that the system and surroundings are always in equilibrium.
  • For example, a liquid in equilibrium with its vapour in a cylinder closed by a frictionless piston, and placed in a constant temperature bath as shown in fig.
Reversible Process
  • The external pressure on the piston is increased by an infinitesimally small amount, the vapours will condense, but the condensation will occur so slowly that the heat evolved will be taken up by the temperature bath.
  • The temperature of the system will not rise, and the pressure above this liquid will remain constant. Although condensation of the vapor is taking place, the system at every instant is in the state of equilibrium.
  • Similarly, If the external pressure is made just smaller than the vapour pressure, the liquid will vaporize extremely slowly, and again temperature and pressure will remain constant.

Irreversible Process

  • In Irreversible process, rapid evaporation or condensation by the sudden decrease or increase of the external pressure and lead to non-uniformity in temperature and pressure within the system and equilibrium is disturb.
  • Such processes are called as irreversible processes.

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