Basic Concepts

Some common terms used in the study of thermodynamics are briefly explained below:–

1.    SYSTEM AND SURROUNDINGS

        The part of universe chosen for thermodynamic consideration (i.e. to study the effect of temperature, pressure etc.) is called a System.

        The remaining portion of universe excluding the system is called Surroundings.

2.    TYPES OF THE SYSTEM

        System can be classified into three types:–

    (i).    Open System:   

            A system which can exchange mass as well as energy with the surroundings is called Open System.

        For example: Calcination of lime stone in lime kiln.

                CaCO₃ CaO + CO₂­

            In this case, heat is supplied to the system while CO₂ gas escapes out into the atmosphere.

    (ii).    Closed System:       

            A system that can exchange energy with the surroundings and not mass is called Closed System.

        For example: If heating of calcium is done in a sealed tube, it decomposes into CaO and CO₂ gas, but CO₂ gas cannot escape out.

    (iii).    Isolated System:

            A system which can neither exchange heat nor the mass with surroundings is called Isolated System. A reaction carried out in a closed and well insulated vessel like thermos flask is an example of isolated system.

3.    EXTENSIVE AND INTENSIVE PROPERTIES

        The various physical properties of a system may be classified into two main types:

    (i).    Extensive Property:

             These are those properties which depend upon the quantity of matter contained in the system. The common examples of the properties are mass, volume, energy, heat capacity etc. The total value of extensive property is equal to the sum of values for the separate parts into which the system may be divided for the sake of convenience.

    (ii).    Intensive Properties:   

            These are those properties which depend only upon the nature of substance and are independent of the amount of substance present in the system. The common examples of these properties are temperature, pressure, refractive index, viscosity, density, surface tension, specific heat etc. It is because pressure and temperature are intensive properties, independent of the quantity of matter present in the system that they are frequently used as variables to describe the state of a system.