Even the simplification of matter into molecules, atoms, electrons, and so on, is too complex a picture for many problems of thermodynamics. Thermodynamics makes no hypotheses about the structure of the matter of the system. The volumes of the system considered are very large compared to molecular dimensions. The system is regarded as a continuum. The system is assumed to contain continuous distribution of matter. There are no voids and cavities. The pressure, temperature, density and other properties are the average values of action of many molecules and atoms. Such idealization is a must for solving most problems. The laws and concepts of thermodynamics are independent of structure of matter.
According to this concept there is minimum limit of volume upto which the property remain continuum. Below this volume, there is sudden change in the value of the property. Such a region is called region of discrete particles and the region for which the property are maintain is called region of continuum. The limiting volume up to which continuum properties are maintained is called continuum limit.
For Example: If we measure the density of a substance for a large volume (υ1), the value of density is (ρ1). If we go on reducing the volume by δv’, below which the ratio äm/äv deviates from its actual value and the value of äm/äv is either large or small.
Thus according to this concept the design could be defined as
ρ = lim δv-δv' [δm / δv]
=m/v
