Methane is the simplest alkane. Describe what happens when methane reacts with chlorine in the presence of ultraviolet light, and explain why this type of reaction is described as substitution.

Ultraviolet light is required to initiate the reaction by breaking the C–H bond in methane, forming a methyl radical and a hydrogen chloride radical. The methyl radical then reacts with a chlorine molecule, replacing one hydrogen atom with a chlorine atom. The product is chloromethane (CH₃Cl). This reaction is called a substitution because a hydrogen atom is replaced by a chlorine atom rather than a new group being added across a bond.