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Short answer
Fractional distillation separates liquids whose boiling points are close together by adding a fractionating column between the distilling flask and the condenser. The column gives vapor many repeated condense-and-vaporize steps in one run, so the distillate is enriched in the lower-boiling component far more than in a simple distillation. The usual rule of thumb is a boiling-point gap of less than 100 °C.
Definition and scope
Fractional distillation is a distillation in which a fractionating column sits between the distilling flask and the three-way adapter. It is used for two broad jobs: purifying reagents and reaction products in the laboratory, and separating complex mixtures such as crude oil into fractions that boil within similar ranges (LibreTexts, Nichols). In a refinery, lighter fractions leave near the top of the column and heavier ones lower down (Wikipedia, “Fractional distillation”).
This guide covers the concept and the equipment attributes that differ between columns. It does not give an operating procedure; follow your own laboratory SOP and the manufacturer’s instructions for the glassware you hold.
How the column separates a mixture
A simple distillation cannot purify much when the components’ boiling points are too close. The distillate of a simple distillation is always enriched in the lower-boiling compound, and each vaporization-condensation event in the column is treated as one more simple distillation. Each such event is called a theoretical plate (LibreTexts, Nichols).
The column does this through surface area. It contains indentations or a packing material; vapor temporarily condenses on these surfaces, and the heat of the distillation vaporizes the pooled liquid again (LibreTexts, Meredith College). More surface generally means more theoretical plates and a better ability to separate close-boiling components, but also more liquid held on the surfaces. The Nichols text notes that columns differ in the quantity of compound sequestered by wetting the column.
Column types compared
The two LibreTexts organic chemistry laboratory texts describe three common lab columns. The table restates what they say, row by row.
| Column | Surface area | Separating close-boiling components | Material recovery | Caveat stated in the source |
|---|---|---|---|---|
| Vigreux (glass indentations) | Least | Least capable | Highest | Suits separations that are not particularly difficult |
| Steel wool packing | Intermediate | Intermediate; depends on how tightly the wool is packed | Not stated | Cannot be used with corrosive vapors such as acid |
| Glass beads | High | A good choice for close-boiling components | Greatest loss of material | None stated |
Source: LibreTexts, Meredith College, Organic I Techniques 9.04, read 2026-10-29. The same source adds that the choice of column depends in part on availability and the task at hand.
Fractional, simple or rotary: choosing the technique
- Boiling-point gap under about 100 °C: the Nichols text says a column is needed, because simple distillation will not purify significantly.
- Easier separation where recovery matters: the Meredith text pairs the lowest-surface-area column (Vigreux) with the highest recovery.
- Hard separation of close-boiling liquids: the high-surface-area bead packing is the sourced fit, at the cost of the greatest material loss.
- Removing a solvent rather than separating components: that is a different job; see what a rotary evaporator is.
Where it matters for labs and supply decisions
When a laboratory specifies a fractionating column, the attributes the sources identify as differing are: column type (indentations or packing), packing material, surface area and theoretical plates, how much compound the column holds up by wetting, and chemical compatibility (steel wool and acidic vapors do not mix). Those are the points a requisition or SOP can state; the sources do not support any further ranking of products.
A column is one piece of a distillation assembly. For the surrounding glassware see types of laboratory glassware and the Erlenmeyer flask guide, and for heating equipment see the hot plate stirrer guide. If your concern is water purity rather than separation of organic liquids, the laboratory water purification system comparison covers distillation among water-purification methods.
Common misunderstandings
Does a taller or more heavily packed column always give a purer product?
Not without a cost. More surface area raises separating power, but the sources tie it to greater hold-up and material loss (bead packing), and tightly packed steel wool changes its effectiveness. Separating power and recovery trade off.
Can fractional distillation separate any liquid mixture?
No. An ethanol-water mixture forms an azeotrope of about 96% ethanol and 4% water that boils at about 78.2 °C; it cannot be fully separated by fractional distillation, and a method such as azeotropic distillation is needed (Wikipedia, “Fractional distillation”).
Is fractional distillation only a refinery process?
No. The Nichols text states it is used for both oil refining and purification of reagents and products.
FAQ
What is fractional distillation?
A distillation that uses a fractionating column to separate liquids with close boiling points through repeated vaporization-condensation steps, each called a theoretical plate.
What is the difference between simple and fractional distillation?
Fractional distillation adds a fractionating column between the distilling flask and the adapter. Per the Nichols text, this is the modification needed when the boiling-point difference is under 100 °C.
What is a theoretical plate?
One vaporization-condensation event in the column, comparable to a single simple distillation.
What fractionating column types exist?
Vigreux (glass indentations), steel wool packing and glass bead packing are described in the sources above; they differ in surface area, separating power and recovery.
Sources
- LibreTexts Chemistry, Nichols, Organic Chemistry Lab Techniques, 5.3 Fractional Distillation: chem.libretexts.org (read 2026-10-29).
- LibreTexts Chemistry, Meredith College, Organic I Techniques, 9.4 Fractional Distillation: chem.libretexts.org (read 2026-10-29).
- Wikipedia, “Fractional distillation” (secondary; used for the azeotrope example and refinery fraction order): en.wikipedia.org (read 2026-10-29).
