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Saturday, 25 June 2016


We were discussing thermodynamic state, path,process and cycles in our previous post. We have also discussed various basic concepts of thermodynamics such as quasi static process, path function and point function, macroscopic and microscopic approach in thermodynamics and also thermodynamic equilibrium, intensive and extensive properties in our recent post.

Today we will see here the concept pure substance in thermodynamics.

Pure substance is basically defined as one substance which will have homogenous and invariable chemical composition throughout its mass even if there is any change in phase too. 

We can express the pure substance in simple words that if a substance will have fixed chemical composition and homogeneous throughout the mass then that substance will be considered as pure substance in the field of thermal engineering.

We must note it here that relative proportion of various chemical elements, containing the substance, must be constant throughout its mass. Mixture of various chemical elements will also be considered as pure substance if mixture is homogeneous throughout its mass. Atmospheric air is basically a mixture of several gases but will be considered as pure substance.

While mixture of air and liquid air will not be considered as pure substance because relative proportion of oxygen and nitrogen will be different in equilibrium condition in liquid and gaseous phases.

Pure water will be considered as pure substance as it will have only molecules of water i.e. H2O and a pure substance will have some important characteristics which will be used for identifying the substance.

Let us see the example of a pure substance i.e. water. Water will have following important characteristics as mentioned here.
  1. Colorless
  2. Tasteless
  3. Boiling point (1000C)
  4. Melting point (00C)
  5. Does not burn
  6. Density 1g/ml
Hence if we have a liquid having above characteristics, we can easily say that liquid will be most probably water.

Let us see few example of pure substance

  1. Carbon dioxide gas, CO2
  2. Water, H2O
  3. Nitrogen, N2
  4. Oxygen gas, O2
  5. Atmospheric air
  6. Salt (NaCl)
  7. Sugar
  8. Gold
  9. Helium, He
  10. Mixture of water and steam
We will see another topic i.e. “Heat and work energy transfer in thermodynamics” in our next post
Do you have suggestions? Please write in comment box


Engineering thermodynamics by P.K. Nag
Engineering thermodynamics by R. K. Rajput

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