What does this mean "The process of heating higher alkanes to sufficiently high temperatures in absence of oxygen, in order to obtain lower hydrocarbons is known as cracking or pyrolysis.
For example, n - hexadecane on heating cracks into n-octane and octene (an alkene)."

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Dear Student , 
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible.
Pyrolysis is the decomposition brought about by high temperatures.
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