Crude Oil & Biofuels — study notes

Petrol, jet fuel and the tar on your road all start life as the exact same underground goo. One raw material, dozens of finished products — and the trick behind that split is a single physical property most students never stop to question.

Splitting One Mixture Into Many Fuels

Crude oil isn't a single substance; it's a jumble of countless hydrocarbon molecules (built only from carbon and hydrogen) tangled together but never chemically bonded to one another. Because nothing is bonded, no chemical reaction is needed to pull the jumble apart — a purely physical trick does the job, and that trick is fractional distillation.

Every hydrocarbon has its own boiling point, and chain length decides it: short chains slip apart easily and boil off at low temperatures, while long chains cling together and need serious heat before they'll turn to vapour. Refineries exploit exactly this. Crude oil is heated until it vaporises, then fed into a column that runs hot at the base and cool near the top. Each hydrocarbon rides upward as gas until the column has cooled past its own boiling point — at that height it turns liquid again and gets drawn off. Short, light molecules therefore settle out near the top; long, heavy ones drop out low down, close to the furnace.

Don't picture heavy blobs "sinking" — everything inside is gas the whole way up. Position is decided purely by when a molecule gets cold enough to condense, never by weight alone.

Worked Example — Complete Combustion of Propane

1. Propane's formula is C₃H₈ — only carbon and hydrogen atoms, so it counts as a hydrocarbon and the standard burning pattern applies.
2. Complete combustion always needs a generous oxygen supply as the other reactant.
3. Every hydrocarbon's carbon ends up as CO₂ and every hydrogen ends up as H₂O, giving: propane + oxygen → carbon dioxide + water.

Refineries, biofuels, and how to argue the environmental case for each fuel type — plus the audio walkthrough and worksheet — are waiting for you in the full lesson below.