Vaporization: The crude oil is first heated to high temperatures (around ) until most of it turns into a vapor before entering the bottom of the column.
Ascension and Condensation: As the vapors rise through the column, they cool down. When a specific hydrocarbon reaches a level where the temperature is just below its boiling point, it condenses into a liquid.
Collection: The condensed liquids are collected on trays at various heights and tapped off, while the smallest molecules with the lowest boiling points remain as gases and exit at the very top.
| Property | Top of Column (Small Molecules) | Bottom of Column (Large Molecules) |
|---|---|---|
| Boiling Point | Low | High |
| State at Room Temp | Gas or volatile liquid | Thick liquid or solid |
| Chain Length | Short (e.g., to ) | Long (e.g., ) |
| Viscosity | Low (Runny) | High (Thick/Sticky) |
Identify the Bond Type: Examiners frequently test whether students know that fractional distillation breaks intermolecular forces, not covalent bonds. Always specify that the molecules themselves remain intact.
Gradient Direction: Ensure you correctly identify that the column is hot at the bottom and cool at the top. Reversing this is a common error that leads to incorrect predictions of where fractions condense.
Sulfur Impurities: Remember that crude oil often contains sulfur compounds. When these fractions are burned as fuel, they produce sulfur dioxide (), which is a primary contributor to acid rain.
