Pure Substance and Two property Rule in Thermodynamics

By | March 19, 2019

Pure Substance

Let we first go through the definition of pure substance to understand its meaning.

“A pure substance is defined as one that is homogeneous and invariable in chemical composition throughout its mass”

To understand the definition let us first we understand the meaning of the word homogeneous.

The word homogeneous means that the substance should be in only one phase. It could be solid, liquid or gas. For e.g. A mixture of water and milk is a homogeneous mixture, while a mixture of water and sand is not a homogeneous mixture.

Examples of pure substance:

Atmospheric air, steam-water mixture and combustion products of a fuel are regarded as pure substance. It is very important while studying vapor power cycles and internal combustion engines.

Two property rule

One of the key advantage with the pure substance is that it follows ‘two property rule’. It means to determine state of any thermodynamic system we need only two thermodynamic properties, rest can be calculated with the help of various available formulae.

One of the well-known formula is ideal gas equation.

P*V = n*R*T


P: Pressure of pure substance

V: Volume of pure substance

n: Number of moles of pure substance

R: Universal gas constant, whose value is 8.314 kg m2 s-2 k-1 mol-1

T: Temperature of pure substance

In the above formula one can easily observe that if he (or she) knows the value of any two variables from P, V and T. He can easily derive the value of third one. Two property rule is very helpful in plotting graphs of various thermodynamic relations, P-V, P-T diagrams etc.

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