Table of Contents
In this article we will study the definition of pure substance and two property rule in detail.
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 of pure substance in thermodynamics, it is very important to understand the meaning of the word homogeneous.
The word homogeneous means that the substance should be in only one phase. It could be either only solid or only liquid or only gas. For example, A mixture of water and milk is a homogeneous mixture, while a mixture of water and sand is not a homogeneous mixture. The mixture of water and sand will be called heterogeneous mixture because it contains two phases (liquid and solid).
Examples of pure substance: Atmospheric air, steam-water mixture and combustion products of a fuel are considered as pure substance. It is very important while studying vapor power cycles and internal combustion engines.
Now let’s understand two property rule.
Two property rule
One of the key advantages of the pure substance is that it follows ‘two property rule’. Which means, to determine thermodynamic state of any pure substance we need only two thermodynamic states, third state can be calculated with the help of various available formulae.
One such formula is ideal gas equation (given below)
P*V = n*R*T
Where
P: Pressure
V: Volume
n: Number of moles
R: Universal gas constant, whose value is 8.314 kg m2 s-2 k-1 mol-1
T: Temperature
In the above formula we can easily observe that if we know the value of any two state variables from P, V and T. We can easily derive the value of third state variable. Two property rule is very helpful in plotting graphs of various thermodynamic relations P-V, P-T etc.
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