What is substance in thermodynamics ?

In thermodynamics a working substance is defined as a fluid (liquid or gas) in which energy can be stored, removed or transferred through.

Example Air in IC engine, steam in boiler, water in hydraulic turbine.

Pure substance

A pure substance needs to be –

1.  Homogeneous in composition (i.e. single chemical species)

      2.  Homogeneous and invariable in chemical aggregation (i.e. chemically stable)

   Please Read : 

•       What is Thermodynamics and its objectives 
•       What is the first law of thermodynamics