A spectacular reversible reaction

There are many ways of introducing reversible reactions, eg heating hydrated salts such as copper sulfate or cobalt chloride. Reactions in solution involving potassium dichromate or bismuth trichloride are normally controlled by pH, and an example of a simple reversible gas reaction involves copper sulfate with hydrogen chloride and ammonia. For this demonstration, I have developed this simple gas reaction by scaling it up and introducing a more dramatic colour change.