Topic 1 of 7
Fuels, fractions and bioethanol
Follow carbon from a resource to useful products.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
Fuels, fractions and bioethanol
Follow carbon from a resource to useful products.
A hydrocarbon contains carbon and hydrogen only. Natural gas is mainly methane; crude oil is a mixture of hydrocarbons. Both are non-renewable because they form far more slowly than they are consumed. Burning a fuel releases energy, while the same hydrocarbon feedstock may also be valuable for manufacturing chemicals and plastics.
- Heat the oil
Many components vaporise.
- Vapour rises through the column
The column is hotter at the bottom and cooler at the top.
- Condense different fractions
Higher-boiling, generally larger molecules condense lower down; smaller, more volatile molecules travel higher.
- Collect useful mixtures
Each fraction contains hydrocarbons with a range of boiling points, not necessarily one pure compound.
As hydrocarbon molecules get larger, melting and boiling points generally rise overall, viscosity increases, and volatility decreases. Smaller fractions are useful fuels; larger fractions may be cracked into more sought-after smaller molecules. The amount supplied by fractional distillation need not match the demand for each fraction. Larger molecules generally have stronger intermolecular attractions, so more energy is needed to separate them and the liquid resists flow more. These are trends within comparable members, not a claim that every individual melting point rises smoothly.
Bioethanol can be made from sugars in a crop such as sugarcane and is renewable if the crop is regrown. Growing plants take in carbon dioxide, partly offsetting carbon dioxide released when the ethanol burns. Fossil fuels instead add carbon stored underground to the active carbon cycle. Biofuel is not automatically emission-free: cultivation, processing, transport and land-use change also affect its impact.