What is the difference between distillation, distillation under reduced pressure and steam distillation ?

Explanation

Distillation involves convention of a liquid into vapours followed by condensation of the vapour thus produced by cooling to get the pure liquid while the non-volatile impurities remain in the flask. This method is commonly used for those liquid which the are sufficiently stable at their boiling points and contain non-volatile impurities.
Distillation under reduced pressure also involves conversion of a liquid into vapours by heating followed by condensation of the vapours thus produced by cooling but the pressure acting on the system is not atmospheric but is reduced by using a vacuum pump. Since the boiling point of a liquid decreases as the pressure acting on it is reduced, therefore, this method is used to purify such liquids which have high boiling liquids or liquids which decompose at or below their voiling points.
Steam distillation is comparable to distillation under reduced pressure (vacuum distillation) even though there is no reduction in the total pressure acting on the solution. Here, the mixture of organic liquid and water boils at a temperature when the sum of the vapour pressures of the organic liquid `(p_(1))` and that of water `(p_(2))` becomes equal to the atmospheric pressure (p), i.e., `p = p_(1) + p_(2)`
Since the vapour pressure of water around its boiling point is quite high and that of the liquid is quite low, therefore, the organic liquid will boil at a temperature much lower than its normal boiling point and hence its decomposition is avioded. Steam distillation is used to purify such liquids which are volatile in steam, insoluble in water, possess a vapour pressure of about 10-5 mm of Hg and contain non-volatile impurities.