question_answer 131)

  A liquid with high boiling point decomposes on simple distillation but it can be steam distilled for its purification. Explain how is it possible?  

Answer:

  Steam distillation: This type of distillation is essentially a co-distillation with water and is carried out when a solid or liquid, practically insoluble in water, is volatile with steam, possess a vapour pressure of about 10-15 mm of mercury but the impurities are non-volatile. The principle of steam distillation is based on Dalton?s law of partial pressures. Let \[{{p}_{1}}\] and \[{{p}_{2}}\] be the vapour pressures of water vapour and the organic liquid at the distillation temperature. The liquid boils when total pressure is equal to the atmospheric pressure p, i.e., \[p={{p}_{1}}+{{p}_{2}}\]                 or                                            \[{{p}_{2}}=p-{{p}_{1}}\]   This process is used in the purification of compounds such as chlorotoluenes, aniline and nitrobenzene. It is also employed in the isolation of essential oils from flowers.