We have discussed that pressure, temperature, volume, internal energy, enthalpy and entropy are thermodynamic properties as applied to mechanical engineering problems. If we neglect the effect of internal energy due to gravity, observable motion, surface tension, electricity or magnetism, there are only two independent properties for a pure substance. If pressure and specific volume, for example, are fixed then all the other properties become fixed. Pressure, volume and temperature are often used to describe the states of a particular substance. A pure substance is one that is homogeneous and invariable in chemical aggregate. It may exist in -one or more phases, but the chemical composition remains the same in all phases. Liquid water, solid water (ice) or water vapour (steam) are pure substances. The mixtures of ice and water, and water and steam are pure substance. Atmospheric air is a pure substance so long it remains a gas but if there is a change of phase and we have a mixture of gaseous and liquid air, it cannot be considered as a pure substance since its phases are having different chemical compositions.