Figure
|
Source of Figure
|
Figure 8a
|
en.wikipedia.org
|
Figure 8b
|
www.sabarscientific.net
|
Technique 5: Fractional distillation
Fractional
distillation is used to separate 2 or more miscible liquids. This technique
works on the basis of difference in boiling points.
Figure 8a:
Figure 8 shows the
apparatus used for fractional distillation. The fractionating column shown is
the key part of this apparatus. It is a glass tube containing shelves of glass
beads.
Figure 8b:
For example, consider
a mixture of water (boiling point = 100°C) and ethanol (boiling point = 78°C)
which need to be separated using this technique. The mixture is heated and at
78°C, ethanol in the mixture starts boiling. The ethanol vapours rise to the
fractionating column, lose their energy to the glass beads in the fractionating
column, condense and fall back in the mixture. This continues till the temperature
of the entire column rises to 78°C. At this temperature, the ethanol vapours
can rise to the top of the column and enter the Liebig condenser, which
condenses it to liquid ethanol. This ethanol is collected in a flask, from the
other end of the condenser.
Water vapours also
evaporate to the column, but since the temperature of the column is maintained
at 78°C, they condense and fall back in the mixture. The temperature of the
column remains at 78°C till all the ethanol has separated. After, all the ethanol
has separated, the temperature begins to rise. This rise is indicated by the
thermometer. At 100°C, water starts to boil, evaporate, reach the condenser,
condense and collected in a separate flask. This is distilled water.
However, ethanol
sample will contain samples of water and vice versa; that is 100% purification
is not possible.
Fractional
distillation is largely employed in industries to separate:
i.
Fractions
of oil from crude oil
ii.
Fractions
of air
iii.
Fermented
liquor
