Cracking

Summary: Breaking long-chain alkanes into shorter, more useful alkanes and alkenes using heat and a catalyst. Tags: igcse chemistry definition organic-chem Created: 2026-07-14 Last Updated: 2026-07-14

Cracking is a thermal decomposition reaction in which large, long-chain hydrocarbon molecules (typically alkanes from the heavier, high-boiling fractions of crude oil) are broken down into a mixture of smaller, more useful hydrocarbon molecules. The reaction exists in two main forms: catalytic cracking, which uses an aluminium oxide (Al₂O₃) catalyst at approximately 550°C, and thermal cracking (also known as steam cracking), which operates at a much higher temperature of around 700°C without a catalyst. The economic driver for cracking is the mismatch between the supply of crude oil fractions and market demand: fractional distillation of crude oil yields a surplus of long-chain, high-boiling alkanes (used mainly as fuel oil and bitumen) which are in relatively low demand, while there is a shortage of short-chain alkanes (needed for petrol) and a vast chemical industry demand for alkenes as feedstocks for polymer production, ethanol synthesis, and other petrochemical processes. The products of cracking are therefore short-chain alkanes (saturated) and alkenes (unsaturated, containing a C=C double bond), with the presence of alkenes confirmed by the bromine water test — a positive test is indicated by the orange-brown bromine water turning colourless as the alkene undergoes an addition reaction. In the IGCSE exam, students must be able to write a balanced equation for the cracking of a given long-chain alkane into specified products (recognising that one product is an alkane and one is an alkene), describe the conditions for catalytic cracking, and explain the economic argument that cracking converts low-demand, low-value fractions into high-demand, high-value products.

Key term — IGCSE Chemistry (0620/0971) glossary.