Electroylsis

Electrolysis is the process where compounds are broken down into similar substances when an electric current is passed through them.

Hydrochloric acid is made by reacting hydrogen and chlorine together to form hydrogen chloride. 

You can test a gas to see if it is chlorine by holding a piece of damp blue litmus paper in the mouth of the tube. If the gas is chlorine the paper will turn red and then turn white, as it is bleached.

You can also test a gas to see if it is hydrogen by holding a lighted splint in the mouth of a test tube containing the gas. If the gas is hydrogen, the mixture of hydrogen and air in the tube will explode with a squeaky 'pop'.

          (Photo from http://www.sciencephoto.com/media/435714/enlarge)