We have set forth on the fact that volume, temperature, internal energy, with regards to mechanical engineering problems is thermodynamic properties. If we ignore the impact of internal energy due to observable motion, magnetism, electricity or surface tension for a pure substance there tends to be for a pure substance only two independent properties. If pressure or specific volume tends to be fixed, then all is in order and this coupled with temperature is often used to describe the state of a particular substance. A pure substance is always homogenous in nature and there is no change with regards to the chemical composition part. It is known to exist in more than one phase ,but the point that needs to be noted is that the chemical composition is intact in all these situations. Ice( solid water), liquid water or steam ( water vapor) are same in all these cases. When you mix water and steam or for the matter water and ice you get a pure substance on all counts. In the case of atmospheric air, it is considered to be a pure substance as long as it remains a gas, but if there is a change in the phase and we have a mixture of gaseous and liquid air, then in no way it can be considered to be a pure substance. The reason on why it cannot be considered is that since it is in various phases the chemical composition also tends to vary in a lot of ways.
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