Cracking or pyrolysis is the process of decomposing higher molecular weight hydrocarbons (which are typically liquids) into smaller fragments or lower molecular weight hydrocarbons (which can be gases) by the application of heat . For example, the preparation of oil gas or petrol gas from liquid kerosene oil involves the principle of pyrolysis . Oxidation of hydrocarbons generally leads to combustion products ($\text{CO}_2$ and $\text{H}_2\text{O}$) or oxygen-containing functional groups . Distillation under reduced pressure is a separation and purification technique for high-boiling liquids . Hydrocarbons generally do not undergo hydrolysis.