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Fractional distillation is a process to separate mixtures of volatile liquids and miscible liquids.

It is commonly used to separate various components (difference in boiling point is less than 25oC) of a mixture depending upon their boiling points.

### Principle Involved

If a liquid is heated in closed container, some molecules present at the surface of the liquid evaporate into the space above the liquid.

After vaporization, some of these molecules present in the vapor condense back into the liquid.

The space above the liquid is occupied by molecules of vaporized liquid, the vapor pressure above the liquid (vapor pressure of a liquid) rises until it reaches a certain value.

When the pressure stabilizes, the rates of evaporation and condensation become equal and the system reaches equilibrium.

Increase in temperature increases equilibrium vapor pressure of a liquid.  As the temperature of a liquid rises, more molecules get vaporized. Consequently, the equilibrium vapor pressure increases.

When the vapor pressure of a liquid is equal to the pressure of the atmosphere above the liquid, it boils. The normal boiling point of a substance is defined as the temperature at which the vapor pressure of that substance equals standard atmospheric pressure, 760 mmHg. If the barometric pressure is less than 760 mmHg, the temperature at which a substance boils will be less than the normal boiling point.

To compare observed boiling point to the one reported in literature, it is often required to compensate for differences in pressure. The following formula provides a good estimate to correct an observed boiling point to a normal boiling point.

∆t = 0.00012 (760 – p) (t + 273)

Where, ∆t = is the correction in degrees centigrade to be added to the observed boiling point t at barometric pressure p (in mmHg).

A basic method can also be used for estimating boiling points near 760 mm Hg i.e. the observed boiling point will differ from the normal boiling point by 0.5° for every 10mm Hg difference in pressure. Miscible liquids mix well together. These mixtures obey Dalton’s law of partial pressure, according to which, the vapor pressure above a mixture is equal to the sum of vapor pressures of the individual components.

PTotal = PA + PB

Where, PA and PB are the partial pressures of components A and B, respectively. The difference in the behavior of the two types of mixtures on distillation is due to differences in their partial pressures.

In a mixture of miscible substances, partial pressure of a component depends on the vapor pressure of pure component and relative amount of the component.

### Raoult’s law

According to Raoult’s law, the partial pressure of a component in an ideal solution is equal to the vapor pressure of the pure component multiplied by its mole fraction.

The mole fraction of component A and B, XA and XB respectively, which is defined as

XA = number of moles of A / number of moles of A + number of moles of B

As the vapor pressures of pure components increase with temperature, the vapor pressure of each molecule above mixture also increases.

When the vapor pressure of the mixture reaches 760 mmHg, it starts boiling. For ideal solutions, i.e., solutions that obey Raoult’s law, boiling point of the mixture will be between boiling points of the pure components.

Increasing surface area increases the purity of the distillate due to increase in distillation process.

As vapor travels up the column it condenses on surface and then re-evaporates. The process repeats causing vapor to purify at each step.

Units for measuring how many times vapor enters the cycle of condensation and re-evaporation are called the theoretical plates.

## Apparatus Used Figure 2. Laboratory setup for fractional distillation with a fractionating column

Distillation flask is a round-bottom flask and is sometimes referred to as “the pot”. The liquid to be distilled must fill the distillation flask to 1/2 to 2/3 of its capacity.

For even boiling of liquid, boiling chips or magnetic stirrer is added to the liquid before heating it.

The position of a thermometer is adjusted so that the bulb below the adapter side arm is connected to the condenser.

### Condenser

Condenser cools the vapor to liquid and directs this condensate to the receiving flask. The most common type of condenser used is the water-jacketed type.

### Distillation Column

Fractionating column is a piece of glassware used to separate vaporized mixtures of liquid compounds with close volatilities. These columns help to separate the mixture by allowing the mixed vapors to cool, condense and vaporize again. Columns can be filled with glass or plastic beads. Filled columns improve the separation between the liquids being distilled.