Question

Why is the reaction between methane and chlorine consisdered as a substitution reaction?

Answer 1

It is considered as an substition reaction because halogen substitute hydrocarbon molecule and form methane chloride and hydrochloric acid.
Ex-
Ch4+cl2=ch3cl+hcl

Answer 2

The reaction between methane and chlorine considered as a substitution reaction because one hydrogen atom methane is substituted by chlorine to form chloromethane. Hence, chlorine takes place of one hydrogen atom of methane.