Crude oil is a mixture of hydrocarbons separated by fractional distillation (by boiling point). Short chains: low boiling point, volatile, easy to ignite, low viscosity. Long chains: opposite.
Alkanes: CₙH2ₙ₊2, saturated, single bonds. Alkenes: CₙH2ₙ, unsaturated, C=C double bond (tested with bromine water → decolourises).
Cracking: breaks long chains into shorter alkanes + alkenes. Thermal cracking (high temp/pressure) or catalytic cracking (catalyst).
Complete combustion: hydrocarbon + O2 → CO2 + H2O. Incomplete combustion (when oxygen supply is limited): produces CO (toxic) and/or soot (carbon).