Lecture - 4 ( Macroscopic and microscopic approach, pure substance )

Macroscopic approach

In this approach we consider a certain quantity of matter without taking into account the events occurring at molecular level. This comes under classical thermodynamics. The analysis of macroscopic system require simple mathematical formula. Few properties are needed to describe such a system. The values of the properties of system are there average values. For example consider a sample of gas contains in a closed cylinder. The pressure of the gas is the average value of pressure exerted by the millions of individual molecules.
This approach can be applied if the continuum concept is valid. According to Continuum concept the matter is treated as continuously distributed without any void or gap present between the molecules. Continuum concept is valid if the mean free path is much less than the system dimensions. Now, mean free path is defined as the average distance travelled by a molecule between two successive collision.

Microscopic approach

This approach considers that the system is made up of a very large numbers of discrete particles known as molecules. These molecules have different velocities and energies. The values of these energies are constantly changing with time. So here individual molicular behaviour is studied because as the matter is rare average properties become meaningless. The behaviour of the system is found by using statistical method as the number of molecules are very large. So advanced statistical and mathematical methods are needed to explain the changes in the system and also large number of variables are needed to describe such a system, therefore this approach is complicated.

Pure substance

A substance is said to be a pure substance if it is of homogeneous chemical composition throughout. it is not necessary that a pure substance have to be a single chemical element or compound. a mixture of various chemical elements are compounds also qualifies as a pure substance as long as the mixture is homogeneous.
Listed below are some examples of pure substance - 
A closed container filled with Nitrogen gas
 Mixture of water and water vapour

 Listed below are some example of impure substance - 
Mixture of mustard oil and water
Mixture of gaseous air and liquid air. since air is a mixture of gases and different gases will have different condensation temperature therefore when liquefied the amount of substance in liquid and vapour phase will be different. That's why this mixture is impure.


The mixture of liquids behaving as pure substance is called AZEOTROPIC mixture. Mixture of ethyl alcohol and water is azeotropic mixture. They behave as pure substance because of molecular level hydroxyl bond formation. They boils at same temperature also. 

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