Breaking of higher hydrocarbons having high boiling points into a variety of lower hydrocarbons, which are more volatile (low boiling).

$C_{16} H_{34} \rightarrow_{\text{in the absence of air}} C_7 H_{16} + CH_2 = CH_2 + CH_4 $

Thermal Cracking:

Petroleum $\rightarrow ^ {700^0C}$ short chain H/C alkenes, alkynes

Catalytic Cracking:

High H/C $\rightarrow ^ {Al_2O_3 SiO_2 \ at \ 500^0C}$ $C_7, C_8$

Steam Cracking

H/C vapours + steam $\rightarrow$ $_{900^0C}$ unsaturated H/C