The fact that the bromine colour has disappeared only when mixed with an alkane and placed in sunlight suggests that the ultraviolet light is essential for the free radical substitution reaction to take place
The first step of the free-radical substitution reaction is the initiation step in which two free radicals are formed by sunlight
The second step of the free-radical substitution reaction is the propagation step in which the reaction grows in a chain type reaction
The free-radical substitution reaction gives a variety of products and not a pure halogenoalkane
The final step in the substitution reaction to form a single unreactive molecule
Make sure you practice and are able to write out these equations, especially the propagation steps which students frequently get wrong.It is quite common for students to incorrectly show a hydrogen radical produced in propagation, which does not happen:
CH3CH3+ Cl· → CH3CH2 Cl + H·
Do not fall into this trap!