Can a mixture of H2 and Cl2 explode if a spark is added?
Chlorine can serve as an oxidizing agent, that is, as an acceptor of electrons from another element. The scenario described in your question would more likely produce an explosion of sorts if oxygen were present. Oxygen is the element by which what we would normally call "burning", or combustion, takes place. The chemical equation for the combustion of hydrogen is shown below:
H2 + O2 ---> H2O (subscripts)
While it is possible to have the chemical reaction take place between hydrogen and chlorine, it would not have the explosive reaction described in the question. The formulation of hydrochloric acid is a reaction very similar to this:
H2 + Cl2 ---> HCl