Large hydrocarbons can be broken down into smaller molecules by a process of "cracking".
You need to do one of two things in order to crack a hydrocarbon: pass it over a hot catalyst or steam hydrocarbon vapours to a high temperature.
During cracking, smaller molecules are made and can be either alkanes (saturated hydrocarbons; C[n]H[2n+2]) or alkenes (unsaturated hydrocarbons; C[n]H[2n]).
Alkenes are more reactive than alkanes because they have double bonds. Alkenes, therefore, easily react with bromine water to change it from orange-brown to colourless.
Plastics are made of large molecules named polymers. Monomers are small molecules that make polymers. The reaction to create polymers with monomers is called "polymerisation".
During polymerisation, monomers lose the double bond and get a…