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.
Another thing which could create doubt among students is that what is invariable chemical composition. For e.g. if we have a mixture of thoroughly mixed nitrogen and oxygen, then it is a pure substance, because it would have same chemical composition throughout.
But if we take a second example in which we have a mixture of oxygen and nitrogen, but the mixture is not uniform, means at one end density of nitrogen is more and at other end density of oxygen is more. Then this substance will not be called pure substance.
Examples of pure substance
Atmospheric air, steam-water mixture and combustion products of a fuel are regarded as pure substance. It is of utmost importance while studying vapor power cycles, internal combustion engines.
Two property rule
One of the key advantage with the pure substance is that it follows ‘two property rule’. Which means to determine state of any thermodynamic system we need only two thermodynamic properties, rest can be calculated with the help of various available formulas.
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 knows the value of ant two variables 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.