How Do We Know a Material Is Pure

Classically, pure organic compounds were defined as those that had the correct elemental composition (within a few tenths of 1% of that calculated) and had sharp melting points (mp) or constant boiling points (bp). However, your exposure to 

the original concept has been expanded to include materials without absorption in the visible region of the spectrum (color) and any two different phases, for example, gas-liquid and gas-solid, two immiscible liquids, etc. 

However, such techniques aside, large quantities of purified volatile materials were obtained, historically, by distillation through long columns packed with glass beads or other materials (to increase the surface area on which equilibrium between liquid and vapor could occur). Even today, such distillation columns are part of petroleum refining and most mixtures of fuels are prepared by remixing purified materials to obtain appropriate blends. 

The technique of fractional distillation is a practical application of Raoult s law.  

Consider an ideal solution containing two volatile components, jr (bpSO C) and y (bpllO°C), both at latm. The total vapor pressure above a solution of the two materials (P,) is equal to the sum of the partial vapor pressures of all components (Equation 1.1)  

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