Pure Substance definition
A Pure Substance is defined as one that is homogeneous and invariable in chemical composition throughout its mass. The relative proportions of the chemical elements constituting the substance are also constant.
A pure substance is characterized by the following-

• Homogeneous in composition

• Homogeneous in chemical aggregation

• Invariable in chemical aggregation

Figure: System Comprising of Water, Steam and Gas

Homogeneous In Composition
Means that the composition of each part of the system is the same as the composition of every other part. Composition means the relative proportions of the chemical elements into which the sample can be analyzed.
For example system-1 as in figure comprising steam and water is homogeneous in composition. The chemical analysis would reveal that hydrogen and oxygen atoms are present in the ratio 2:1 whether the sample may be taken from the steam or from the water. The same is true for system-2 containing uncombined hydrogen and oxygen gas in the atomic ratio 2:1 in the upper part and water in the lower part. System-3 however is not homogeneous in composition as the hydrogen and oxygen gas are present in the ratio 1:1 in the upper part and water 2:1 in the lower part.

Homogeneous In Chemical Aggregation
Means that the chemical elements must be combined chemically in the same way in all parts of the system. System-1 satisfies this condition, whereas system-2 and system-3 does not satisfy this condition.

Invariable In Chemical Aggregation
Means that the state of the chemical combination of a system doesn’t change with time.

Examples for Pure Substance
Atmospheric air, steam, steam-water mixture, air-fuel mixture and combustion products of a fuel are regarded as pure substances. But the mixture of air and liquid in air is not a pure substance.