Distillation: infer separation from volatility and a temperature record

MCAT content library / Chemistry & physics / Distillation and volatility

Use relative volatility and the head-temperature record to predict which component enriches first and whether a simple or fractionating column can achieve the needed separation.

About 16 minutes, including a two-component fractionation example and checks.

Distillation separates by vapor composition

When a liquid mixture is heated, the vapor above it can have a different composition from the liquid. If one component is more volatile, the vapor is generally enriched in that component relative to the boiling liquid. Condensing the vapor therefore creates a fraction whose composition differs from the original mixture.

Volatility is related to vapor pressure and boiling behavior under the specified conditions. The more volatile component typically has the higher vapor pressure and lower boiling point in a simple comparison.

Concept: the more volatile component enriches the vapor

Simple distillation performs essentially one vaporization–condensation separation step. It works well when one component is nonvolatile or when the volatile components differ enough in boiling behavior that the first vapor is strongly enriched in one component.

Fractional distillation adds a column that permits repeated partial vaporization and condensation. Each equilibrium-like stage can enrich the rising vapor in the more volatile component, improving separation when boiling points are closer.

A longer or more efficient fractionating column can improve resolution, but no real column produces infinite stages or perfect separation automatically.

Worked application: simple versus fractional distillation

An original mixture contains equal moles of liquid A and B. At 1 atm, pure A boils at 70°C and pure B at 90°C. Researchers compare simple distillation with a fractionating column and collect equal-volume fractions.

The fractionating column produces a much purer early A-rich fraction than simple distillation. As A is depleted from the boiling flask, the vapor becomes progressively richer in B and the head temperature rises toward B’s boiling region.

The 71°C first fraction is strongly enriched in A but is not pure A: composition measurement still reports 6% B. Temperature alone would not justify calling the fraction 100% pure.

Pattern: head temperature is evidence, not a purity certificate

The thermometer at the distillation head samples the vapor reaching the condenser. A relatively stable temperature near one component’s boiling region can indicate that component dominates the vapor, but temperature is influenced by composition, pressure, thermometer placement, and nonideal mixture behavior.

A purity claim therefore needs stronger evidence, such as a composition assay, refractive index, chromatography, or a suitably narrow boiling range under controlled pressure. A plateau is useful process evidence, not a guarantee of molecular purity.

Rule: azeotropes can limit ordinary distillation

Some nonideal liquid mixtures form an azeotrope whose vapor composition equals its liquid composition at a particular mixture ratio. At that composition, repeated ordinary distillation does not further enrich one component because vaporization no longer changes the composition.

If a passage states that a mixture forms an azeotrope, adding more theoretical plates cannot overcome that equilibrium limit by ordinary distillation alone. A different separation strategy or changed conditions may be needed.

This is another reason not to equate “fractional distillation” with “complete separation.” The achievable purity depends on vapor–liquid equilibrium as well as column performance.

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Continue learning

• Phase changes and phase diagrams
• Chromatography, retention, and separation
• Browse Chemistry & Physics MCAT lessons

Sources

1. AAMC. Content Category 5C.
2. OpenStax Chemistry 2e. Phase Transitions.