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called fractional distillation (Fig. 21.1). The steps of fractional
distillation are as follows:
1. You heat the mixture of two or more substances (liquids)
with different boiling points to a high temperature. Heating is
usually done with high pressure steam to temperatures of about
1112 degrees Fahrenheit / 600 degrees Celsius.
2. The mixture boils, forming vapor (gases); most substances
go into the vapor phase.
3. The vapor enters the bottom of a long column (fractional
distillation column) that is filled with trays or plates. The trays
have many holes or bubble caps (like a loosened cap on a soda
bottle) in them to allow the vapor to pass through. They increase
the contact time between the vapor and the liquids in the column
and help to collect liquids that form at various heights in the
column. There is a temperature difference across the column (hot
at the bottom, cool at the top).
4. The vapor rises in the column.
5. As the vapor rises through the trays in the column,
it cools.
6. When a substance in the vapor reaches a height where the
temperature of the column is equal to that substance's boiling
point, it will condense to form a liquid. (The substance with the
lowest boiling point will condense at the highest point in the
column; substances with higher boiling points will condense lower
in the column.).
7. The trays collect the various liquid fractions.
8. The collected liquid fractions may pass to condensers,
which cool them further, and then go to storage tanks, or they
may go to other areas for further chemical processing.
Fractional distillation is useful for separating a mixture of
substances with narrow differences in boiling points, and is the
most important step in the refining process.

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